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Review

Progress and Prospects of Research on Physical Soil Crust

by
Huiyun Xu
1,2,
Xuchao Zhu
1,3,* and
Meixia Mi
2
1
State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China
2
College of Urban and Rural Construction, Shanxi Agricultural University, Taigu 030801, China
3
University of Chinese Academy of Sciences, Beijing 100049, China
*
Author to whom correspondence should be addressed.
Soil Syst. 2025, 9(1), 23; https://doi.org/10.3390/soilsystems9010023
Submission received: 11 December 2024 / Revised: 25 February 2025 / Accepted: 25 February 2025 / Published: 4 March 2025
Browse Figures
Figure 1
<p>Methods for measuring the thickness of physical crust. (<b>a</b>). Vernier calipers. (<b>b</b>). Observation of microscope sections. (<b>c</b>). CT scanning and porosity thresholding.</p> "> Figure 2
<p>Four stages of quantifying physical crust. (<b>a</b>). Qualitative description. (<b>b</b>). Semi-quantitative representation. (<b>c</b>). Preliminary quantification. (<b>d</b>). Quantitative expression.</p> "> Figure 3
<p>Simulation of the processes and trends in the development of physical crust in Quaternary red clay in southern China.</p> ">
Review Reports Versions Notes

Abstract

:
Physical soil crust (PSC) is a dense structural layer formed on the surface of bare or very low-cover land due to raindrop splashes or runoff. The formation of crust changes the properties of the soil and strongly affects water infiltration and runoff and sediment production processes on slopes. The irrational use of soil and water resources and frequent human production activity under the influence of urbanization increase the possibility of inducing erosion. Studying the formation and structural characteristics of PSC to predict terrestrial hydrological processes and improve models for predicting erosion is very important. Many studies of PSC have been carried out in China and abroad, but they are mainly unilateral discussions of the basic properties and characteristics of crust and its effects on runoff and sediment yield on slopes. Studies systematically analyzing and synthesizing the progress of crust research, however, are lacking. By reading the literature and analyzing the developmental history of PSC, we provide a comprehensive review of the following: (1) the meaning, main types, and classification of PSC, (2) the mechanism of formation and the characteristics and dynamic development of crust, (3) the factors affecting the formation of crust, including natural and anthropogenic factors and comprehensive effects, and (4) the development and formation of crust in the soil environment, i.e., hydrological processes and erosion. We also summarize the potential directions for future research on PSC: (1) studying the dynamics of soil structure during the development of crust, (2) developing an objective and standardized quantitative method for studying crust formation, (3) using models of erosion influenced by crust development, (4) improving the scale of the degree of crust development and structural characteristics, and (5) rationalizing the management of crust to optimize land structure and increase crop yield.

1. Introduction

The soil crust is a dense layer ranging in thickness from a few millimeters to a few centimeters formed by physical, chemical, or biological action on the topsoil [1,2] and is a common natural phenomenon in arid and semi-arid regions [3]. Crusts can be broadly categorized into biological and physical crusts based on the presence or absence of organisms on their surfaces (Table 1). Biological crust is a composite of organisms formed by cryptogamic plants such as bacteria, algae, fungi, lichens, and bryophytes bonded with particles on the soil surface. Biological crust is an important component of the ecological landscape in arid and semi-arid regions [4]. A physical crust is a dense layer formed on the surface of the soil after rain, which is an important stage of erosion caused by the splash of raindrops. It has been found that the physical properties of crusted soils can change greatly, regardless of whether or not the crust is mature such as an increase in bulk density, a decrease in the number of macropores, and a decrease in porosity [5], which prevents water from penetrating further downward, contributing to an increase in surface runoff, altering the hydrological processes of the soil, increasing the likelihood of inducing erosion, and affecting agricultural production and the livelihoods of the local people [6,7].
Physical soil crust (PSC), commonly known as soil crust, has been recognized by most soil scientists [8]. The research on crust began in the 1930s, and the researchers mainly carried out experiments with simulated rain, took photographs of the surface soil at different periods of rain, observed the surface morphology and crusted area in the photographs, and determined the degree of development of crust based on theoretical knowledge and practical experience. They also made slices of the crust and analyzed the development and mechanism of formation of the crust by observing the trend of changes in the internal structural characteristics of the soil using an electron microscope [9,10]. The development of science and technology had promoted the progress of research methods by the early 21st century, and the research methods for studying physical crust had become increasingly mature. Studies used software to fit the equation parameters that affected the development of crust, such as soil structure or rainfall, used conceptual models to simulate surface runoff, calculated hydrodynamic parameters, and predicted the loss and erosion of soil based on the results of previous studies of the characteristics and mechanisms of crust [11,12].
The development of physical crust, however, is a dynamic process. The impact of raindrops and the filling of soil pores with fine particles during the formation of topsoil crust smooths the soil surface, which constantly changes the internal structural characteristics of the soil. The increase in bulk density due to the action of the continuous compaction of raindrops greatly strengthens the shear strength of the soil, improves its resistance to corrosion, and affects the various processes of slope erosion. Visual observation alone cannot objectively and comprehensively quantify the trend of the dynamic changes to crust thickness and structural characteristics. Studies have recently found that X-ray computed tomography (CT) scanning can objectively extract indices of soil pores, such as the pore number, pore size and porosity, etc. Scanning crusted soil samples under different durations of rain can objectively quantify pore structure, analyze the characteristics of pore structure under different crust morphologies, study the influence of crust development on the internal pore structure, and further explore the influence of the development of surface crust on water infiltration and slope erosion. Armenise et al. [13] carried out tests with simulated rain on three soils in the United Kingdom and analyzed crusted samples using CT scanning, proposing that crust thickness could be objectively quantified based on pore thresholds, clarifying the development of crust, and achieving a new breakthrough in the objective quantification of crust thickness. Zhu et al. [14] carried out a test with simulated rain in the southern red soil area of China to verify the feasibility of this method. Few studies, however, have synthesized the dynamics of physical-crust development in recent decades. As a special soil subsurface [15], physical crust affects the process of runoff and sediment yield, and its influence on erosion has been controversial. An in-depth analysis of the dynamic development of physical crust and an overview of the progress of research into crust are, therefore, urgently needed.
The objective of this study was to review the development of PSC by reviewing the literature and summarizing the progress of the research into PSC, including summarizing the concepts and classifications of physical crust, analyzing the mechanism of its formation, clarifying the factors affecting its formation and impact on the environment, and proposing some directions for the future study of physical crust based on the deficiencies of previous research. We also propose some directions for future research on PSC based on the shortcomings of the previous studies to provide theoretical support and a scientific basis for the prediction of soil hydrological processes and the alleviation of erosion.

2. Concept and Classification of Physical Crust

2.1. Definition of Physical Crust

Research on soil crust mainly focused on the mechanisms of crust formation and the factors that influence it. Duley [16] found that the effect of raindrops striking the soil caused fine particles in the runoff to fill the pore spaces, leaving the soil surface in a gradually compacted state, forming a relatively smooth, dense, and impermeable layer after settling. He used the term “seal” to define this special phenomenon on the soil surface and indicated that the sealing of the surface was not due to the increase in fine material, but to the dense structure formed by fine particles surrounding large particles. Experiments after the 1950s conducted with simulated rain to investigate the effect of raindrops on the soil surface indicated that the formation of a “lamellar crust” on the surface by the effect of raindrop strikes was similar to the “skin seal”. The term “crust” has since been widely adopted by the academic community to describe this phenomenon, leading to a new understanding of surface crust. Specifically, crust is a thin layer formed when aggregates on the soil surface are dispersed into fine particles by the impact of raindrops, which then infiltrate into the pores of the underlying soil by leaching, and the fine particles on the surface are eventually gradually compacted [17]. More relevant experiments were conducted in the 21st century, and scholars developed a more mature understanding of physical crust. PSC is a structurally dense layer formed by raindrops striking the soil surface, breaking up the aggregates, and filling the pores with fine particles after spattering, thereby compacting the soil and depositing suspended sediments.

2.2. Types of Physical Crust

Studies of physical crust were conducted at the beginning of the 20th century, but the types of crust were not clearly defined due to the insufficiency of observational techniques (Table 2). Chen [10] analyzed the structural sections of loessial crust using an electron microscope and classified physical crust into two types based on the mechanism of formation and structural characteristics: structural and depositional crusts. This classification has been recognized by many scholars. Researchers later gained a deeper understanding of the two types of PSC based on this classification, combined with different experimental methods and observations, and clearly defined their attributes and characteristics [18,19,20]. Structural crust, a layer with low permeability usually located at higher topographic levels, is formed by rearranging and combining macroaggregates after they are dispersed into fine particles under the impact of raindrops [21,22]. Its development is controlled by a variety of factors, including soil properties, characteristics of rain, and flow conditions. Structural crust can be further categorized into four subgroups based on how it is formed: slaking, infilling, coalescing, and sieving crusts [23]. Depositional crust is formed by the deposition of sediment particles as the velocity of the runoff slows under the action of microtopography or plant interception, decreasing the capacity to transport sand and leading to the accumulation of particles on the soil surface in relatively low terrain [24,25]. The development of depositional crust depends not only on the dynamic conditions of the runoff, but also on the physical and chemical conditions of the sediments. Depositional crust can, therefore, also be divided into two subgroups: runoff depositional crust and still depositional crust. By studying the mechanisms of formation of sandy and loamy soils, however, Valentin and Bresson [26] suggested that soil crust could be classified into three categories: structural crust, depositional crust, and erosion crust. Using microscopic observation and an analysis of soil structure, they found that structural crust gradually developed into erosional crust by the continuous impact of raindrops and the consequent splash erosion on the slaking and sieving crusts. This type of crust, consisting of a single hard, thin, and smooth surface layer enriched in fine particles, develops further from structural crust.
The degree of crust development can be better differentiated by distinguishing between the micromorphological types of crust. In addition to structural, depositional, and erosion crusts, two other types of pavement crusts were identified in northern Niger by Valentin [29] based on the microstructures of the types of layers that constitute crust: (1) “filtration pavement”, also designated as “three-layered” crust, consisting of vertical particle sieving, and (2) “erosion pavement”, which consists of an exposed plasmic layer after the sandy layers of the erosion pavement are removed by wind or runoff. Zhu et al. [30] divided physical crust into “surface crust” and “subsurface crust” based on the position of the soil in the profile. Crust that forms within 1–2 mm of the surface is the surface crust. Surface particles are dispersed and not displaced downward with the runoff, and the power generated by the runoff on the surface cannot move them, so they are gradually compacted or deposited by the raindrop strikes. The subsurface crust is a thin layer 3–5 mm below the surface formed by the accumulation of fine particles when the structure of the soil is loose and sufficient internal pore spaces allow the fine particles to move downward but not deeper than 5 mm.
Many scholars, however, believe that the morphological characteristics, developmental mechanisms, slope position, and environmental responses of structural and depositional crusts differ greatly, which can be used to identify the influence of physical crust on soil hydrological processes [31,32,33,34]. Current studies, therefore, classify PSC into structural and depositional crusts based on their mechanisms of formation [35], to explore the effects of the property characteristics of different types of crust on the runoff and sediment process, and to identify the responses of crust development and slope erosion.

3. Mechanism of Formation of Physical Crust

3.1. Mechanism of Crust Formation

PSC is common in nature, and the mechanism of formation of soil crust is constantly supplemented and improved as our understanding of them increases. Duley [16] believed that crust was formed due to the impact of raindrops, which dispersed soil particles and filled the pores with fine particles. McIntyre [17] later proposed that the formation of crust not only involved the percussion and compaction by raindrops but was also accompanied by the leaching of dispersed particles. Soil crust generally forms two layers under the action of raindrops: the first layer is the topsoil crust, which is mainly a thin layer of about 1 mm in thickness formed by the compaction by raindrop strikes, and the second layer is the “washed-in” region, which is formed by the leaching of fine particles into pores due to infiltration from the surface, forming a layer of lower porosity [36]. Onofiok [37] observed that raindrop percussion and the deposition of suspended particles were also mechanisms of crust formation by making slices of crust under different durations of rain. Agassi et al. [38] summarized the mechanisms of crust formation in two categories in their study of rates of infiltration: (1) physicomechanical action, mainly manifested by raindrop percussion on the surface, leading to the physical disruption of aggregates and the reorganization of the surface soil by constant compaction, and (2) physicochemical effects from processes such as the dispersion of aggregates and cation-exchange reactions, where fine particles fill the pores during transport and deposition. Both effects contribute to the formation of a thin crusted layer on the surface, characterized by a light-colored upper layer, low permeability, and high runoff, strongly affecting soil hydrological processes [39].
In summary, the mechanism of formation of crust mainly includes the physical destruction of aggregates by the impact of raindrops, the downward movement of fine particles, the compaction of the surface by raindrops, the chemical dispersion of clay, the filling of pores, and the deposition and rearrangement of particles. Different types of soil texture and differences in environmental conditions can affect the mechanism of crust formation. The loess soil on the Loess Plateau in China, where rain is common and mostly heavy, was formed by deposition from weathering and has low organic matter content, weak cohesivity, and low aggregate stability. Physical crust is easily formed on the surface under the influence of continuous compaction by raindrops [40]. The purple-soil region in southwestern China is prone to the disruption of large aggregates into fine particles under the influence of rain, due to the relatively large proportion of sand and silt particles, which promotes the formation of surface crust. In contrast, the red-soil region in southern China, characterized by high clay content and low aggregate stability, is susceptible to the effect of the monsoon climate. Fine particles continuously fill pores to form a layer of crust under the combined effects of raindrop impacts and the chemical dispersion of clay. The black-soil region in the north, however, maintains a relatively good soil structure and is less prone to crusting due to its unique temperature conditions [41,42]. We thus need to discriminate between the mechanisms of development of physical crust based on the current environment of the experiment and the experimental results due to the complexity of the factors affecting crust formation and the interaction and influence of different mechanisms of formation [43].

3.2. Characteristics of Crust

The formation of PSC is also the process of dispersion, reorganization, and redistribution of surface particles, which is accompanied by changes in the properties of the crusted soil, including the apparent morphology, basic physical properties, hydraulic properties, and structural properties. Apparent morphology is the most intuitive state of PSC, and the morphological characteristics are usually observed using “photographing + visualization” and “slicing + microscopy”. The upper soil layer is lighter in color and relatively smooth in texture when the soil surface is crusted, the fine particles are separate but are tightly packed in a dense layer, and the surface of uncrusted soil is rough, and the particles are relatively loose [44]. Onofiok [37], Hu et al. [45], Norton [46], and others have found that macroaggregates in the surface soil gradually dispersed and the size of the particles became smaller as the number of small pores increased, and the number of large pores decreased with a change in the duration of rain, unlike areas without crust. The apparent morphology directly represents the external morphology of the surface soil and establishes the basis for further analysis of the basic properties of crust.
The basic attributes of PSC include thickness, roughness, bulk density, and hardness. Thickness is an important indicator of the degree of crust development, which normally does not exceed 10 mm, but sometimes it can accumulate in the surface crust under the influence of rain or other factors to exceed 10 mm [2]. Crust is thin and friable, and the current methods for quantifying its thickness include three main methods: direct measurement using vernier calipers [47], observation of microscope sections [48], and CT scanning combined with the pore-threshold method [13] (Figure 1). A dense protective layer, i.e., a physical crust, is formed on the surface under the influence of raindrop strikes and the rearrangement of particles and gradually thickens with the duration of rain [21]. Soil roughness indicates the degree of fluctuation of micro-landforms and the change in surface micro-morphological structure during erosion. Methods for measuring roughness mainly include the needle method, the chain method, and the use of stereo cameras [49]. The development of physical crust affects surface runoff and roughness. A decrease in roughness will reduce the detachment of particles caused by raindrops and the shear effect of water flow on the surface, affecting surface storage capacity, the routing and velocity of water flow, and the rate of runoff [50]. Wu et al. [51] indicated that the formation of PSC reduced surface roughness, leading to faster water flow and lower shear stress of runoff and ultimately reduced erosion. Bulk density is an effective indicator of the development of crust. The coating method is the most effective method for testing the bulk density of soil or its aggregates, as stipulated by the United States Soil Survey Office [52]. Bulk density is an effective indicator to characterize changes in pore conditions, the capacity of water infiltration, and the resistance to erosion of the soil surface. Bu et al. [24] and Wang and Wang [53] measured the bulk density of three types of soil crust and the properties of physical crusts of straw checkerboards, respectively. Both studies found that bulk density decreased as crust thickness increased; bulk density tended to stabilize when the crust gradually matured. Hardness indicates the strength of a crust and represents the resistance to erosion, which depends on the depth of surface runoff and the degree of erosion and is often measured by shear strength. The shear strength of soil mainly depends on the cone penetrometer method. The development of crust helps to increase the strength of the soil surface and reduces the capacity of soil detachment [54]. Shear strength is a critical factor affecting erosion, and it is not a static value but changes with the amount of soil moisture and compactness. Bu et al. [55] found that the shear strength of purple soil gradually increased with the duration of rain and the continuous development of crust.
The dispersion and reorganization of particles during the development of surface crust alter its internal structural characteristics, directly affecting the hydraulic properties of the crusted soil, e.g., water conductivity, water content, and rate of infiltration. Common methods for determining hydraulic properties include direct laboratory measurement, model calibration, and fitting equation parameters. Jiang et al. [56] used the weight method to measure water content for different types of crust and found that water content decreased with depth, with the highest content in the surface soil layer, i.e., the crusted area, having the highest water content. This experiment indicated that the formation of surface crust affected the infiltration of water, reducing the rate of infiltration in the surface soil. Bahddou et al. [57] and Fohrer et al. [5] established hydraulic parameter models based on soil properties to analyze the trend of changes to the infiltration of water under the influence of crust. The results indicated that the rate of infiltration was higher in the initial stage of crust development but gradually decreased with changes in the duration of rain, leading to the initiation of runoff on the surface. Changes in soil properties strongly affect the infiltration and evaporation of water. Fitting hydraulic parameter equations using soil properties can be used to predict hydrological trends, but the accuracy of simulations may be low due to differences in the method of measurement [39]. Hardie and Almajmaie [58] and Wu et al. [59] suggested that optimizing the methods for measuring and estimating crust hydrological properties could improve the accuracy of simulations of the infiltration and movement of water in a “crop-soil-moisture” model.
Soil pores are the primary channels for the transport of water and gases. Pore structure differs greatly between crusted and uncrusted soil [14]. Pore metrics encompass both two-dimensional (2D) and three-dimensional (3D) indicators. Two-dimensional indicators primarily refer to pore number, porosity, circularity, and equivalent diameter. The 3D characteristics of pores primarily refer to fractal dimension, anisotropy, surface area, and the density of connectivity. Two primary methods are used for determining pore metrics: (1) preparing slices of crust samples and visually assessing changes in microstructure using a polarizing microscope and (2) using a Polyvinyl Chloride (PVC) cutting ring to obtain crust samples, acquiring images by CT scanning, and then extracting the metrics using software. Hu et al. [48] observed slices of three types of crust, including loess, red soil, and yellow-brown soil, under a microscope and found that the pores in the soil crust layer were significantly smaller than those in the uncrusted area below. Pires [60] used CT scanning to analyze the trends in porosity at various depths in Brazilian coffee fields after crust formation. The results indicated that porosity was significantly lower in the upper than the lower layers after crust formation, which was attributed to the compaction of the soil surface caused by raindrop impact.
We conducted experiments with simulated rain in July 2023 at the Jiangxi Yingtan Red Soil Ecological Experimental Station using CT scanning to obtain 2D and 3D pore metrics of crusted Quaternary red clay under different durations of rain. The results indicated that the 2D metrics generally indicated a trend of gradually decreasing pore number, porosity, and equivalent diameter and a gradual increase in circularity. In contrast, the 3D metrics indicated a trend of the fractal dimension initially increasing and then stabilizing, and the pore surface area and the density of connectivity initially increased and then decreased. Each metric differed to varying degrees across different stages of crust development. Armenise et al. [13] proposed the use of porosity to quantify crust thickness, achieving a breakthrough in the objective quantification of crust thickness. Pore structure is a crucial characteristic of crust properties, and analyzing changes in the pore structure of crust can provide insights into crust development, enabling predictions of soil hydrological processes and erosion.

3.3. Crust Development

PSC is constantly undergoing dynamic changes due to factors such as rainfall and runoff. The formation of crust alters the structural characteristics of soil (e.g., porosity, water content, and bulk density), directly affecting the infiltration and evaporation of water. Understanding how crust develops can therefore effectively allow us to monitor the trends of the changes in soil properties and identify the response of crust development to slope erosion. Research on crust development progresses through four stages depending on the methods of observation and quantification: qualitative description, semiquantitative representation, preliminary quantification, and quantitative expression (Figure 2).
Both domestic and international researchers initially relied primarily on photography and visual inspection to qualitatively describe the changing morphological characteristics of the soil surface and the development of physical crust. They believed that surface particles were splashed and subsequently reorganized under the influence of rain or runoff, gradually forming a crust on the uncrusted surface. Farres [9] semiquantitatively described the development of physical crust by creating soil sections at different time intervals and observing them with an electron microscope. He summarized the development into four stages based on the developmental status of the crust: an initial stage, a preparatory developmental stage, a rapid developmental stage, and the formation of a typical crust. Onofiok [37] used a scanning electron microscope to observe crust development for three types of soil in the United States of America, dividing it into four processes based on the dynamic changes in soil particles: raindrop impact and slaking, movement of detached clay and silt particles into pores, deposition of suspended particles, and compaction of surface particles. Boiffin [61] discovered two major processes that occur during the development of physical crust based on the typical morphological characteristics of the structural evolution of the soil surface: (1) the formation of structural crust, which seals the soil surface, and (2) the development of depositional crust. Both photography and the microscopic observation of prepared slices, however, involve visual inspection, and the presence of subjective factors can reduce the accuracy of a judgment. Objectively extracting soil indicators and quantitatively describing the development of physical crust are, therefore, urgent problems that need to be addressed.
With the advance of technology, researchers have recently begun to experiment with using changes in specific properties of crust, such as hardness, thickness, and shear force, during a rain event as indicators to measure the degree of development of physical crust. The water content of the underlying soil changes as the surface soil is compacted by raindrop impact and eroded by runoff; the formation of physical crust increases the hardness of the surface soil and alters its shear resistance. Changes in the characteristics of soil properties, therefore, provide a good indication of the development of crust [41,46,62]. Cai and Lu [63] concluded that the development of PSC undergoes two processes during formation during 30 min of rain, exhibiting a periodic alternation of “formation-destruction-reformation”. Bu et al. [24] indirectly calculated the shear strength of crust samples by measuring their water content under different durations of rain. They found that the shear strength of purple soil and loess first increased and then stabilized within 30 min of rain, indicating the formation of physical crust on the soil surface during this period, with the crust gradually maturing. PSC is highly susceptible to fragmentation, but detecting the content and hardness of soil water requires inserting instruments into the soil, which can easily damage undisturbed samples. Differences in soil types, stages of crust development, and sampling methods can also introduce deviations in the measurements of these indicators. Selecting appropriate crust indicators can, therefore, improve the accuracy of quantifying crust development [64].
The thickness of crust directly represents its degree of development, typically ranging from a few millimeters to several centimeters. Early studies used vernier calipers to directly measure the thickness of physical crust, considering it a process where fine particles accumulate on the soil surface over time, forming from nothing under the impact of raindrops. Zhou et al. [65] quantified crust thickness by making crust slices and concluded that thickness mostly ranged from 0.3 to 0.8 mm and categorized the development of physical crust into four phases based on the trend of thickness: (i) initial wetting, (ii) aggregate disruption by raindrop impact, (iii) fragment transport by splash, overland flow, and rearrangement, and (iv) compaction by raindrop impact. This method of observation, however, still suffers from interference by subjective factors, and an important problem at present is the lack of a standardized method for determining thickness. The application of CT scanning in soil science has recently provided a new approach for objectively quantifying crust properties due to its advantages of being nondestructive, sensitive, and capable of rapidly acquiring information about soil structure [60]. Armenise et al. [13] first proposed the use of CT technology to scan and analyze pore thresholds for quantifying crust thickness. Zhu et al. [14] and Zhu et al. [66] used this method to extract and quantify the pore characteristics of two typical red soils in southern China under the influence of crusting, measured the thickness of physical crust for the red soils, and demonstrated the feasibility of this method. We have, therefore, found that the method of “CT scanning + porosity threshold” can objectively and accurately quantify thickness, serving as an important indicator of crust development. In August 2023, we conducted experiments at the Yingtan Red Soil Experimental Station in Jiangxi Province, focusing on Quaternary red clay, a typical red soil in southern China. The rainfall intensity was set to 30 mm/h, representing the conventional local rainfall conditions. Using PVC cutting rings, two typical crusted soil samples, respectively, were collected at 1, 10, 30, 60, and 90 min of rainfall. Industrial CT scanning technology was employed to extract pore metrics of the crusted soil under different rainfall durations. By quantifying crust thickness, we fitted parametric equations for describing the trends of crust development (Figure 3). We believe that the development of physical crust in red clay can be characterized by an initial stage of particle splashing and pore filling, a stage of soil compaction by raindrops, a later stable stage, and a final stage of erosion. This characterization achieves the goal of quantitatively describing the development of crust based on its thickness.
The construction of spectral models has provided a novel method for objectively quantifying the development of physical crust. Studies have found that the development of crust can be predicted using spectral reflectance to quantify soil properties such as water content and roughness [67]. Agassi [68] used experiments with simulated rain to determine that crust thickness increases with the cumulative energy of raindrops, but that infrared reflectance decreases, confirming that changes in spectral values can indicate changes in the structural properties of soil. de Jong et al. [69] reported that spectral differences could be used to map the distribution of different types of land, distinguish between land properties, and plan land use in a reasonable manner. Model simulations can dynamically monitor the development of crust, but the selection of structural properties can affect the accuracy of the simulations, thereby influencing the accuracy of experimental results. Determining appropriate parameters to simulate, optimizing models, and improving simulation accuracy are, therefore, effective ways to reasonably quantify the development of physical crust.

4. Factors Influencing Crust Formation

Soil physical crusts are dense structural layers formed on the surface of bare or sparsely covered soil due to external forces such as raindrop impact and runoff erosion. The formation of crusts significantly alters the physical, chemical, and biological properties of the soil, thereby affecting hydrological processes such as infiltration, runoff, and evaporation, ultimately having profound impacts on soil productivity, ecosystem functions, and soil-water conservation capabilities. Existing research indicates that crusts are a product of soil erosion, and their formation is influenced by a combination of natural and anthropogenic factors. Specifically, regional differences in climatic conditions (e.g., rainfall intensity, frequency, and distribution), topographic factors (e.g., slope gradient, aspect, and elevation), soil properties (e.g., texture, structure, and organic matter content), and human activities (e.g., farming practices, land use types, and management measures) all significantly impact the development of soil crusts. Furthermore, the interactions of multiple factors (e.g., the coupling of tillage and rainfall and the superposition of soil properties and slope gradient) can either exacerbate or mitigate crust formation. Therefore, by systematically reviewing the research on soil crusts, it is possible to explore the key factors influencing crust formation and their interaction mechanisms from three perspectives: natural factors, anthropogenic factors, and other comprehensive factors (e.g., tillage and rainfall; soil and slope gradient). This will provide a scientific basis for soil erosion control, sustainable land management, and ecological restoration.

4.1. Natural Factors

4.1.1. Soil

The inherent conditions and basic physical properties of soil are the most direct factors influencing the formation of physical crust (Table 3). Some studies have found that most soils can form physical crust under constant compaction by raindrops, but different soil types and properties in specific environments have different degrees of difficulty for the formation of physical crust [70].
Soil texture, water content, organic matter content, and the quantity and stability of aggregates are the main factors influencing the development of surface soil crust. Soil texture, which refers to the particle composition of soil, directly determines whether crust can develop on the soil surface, with clay a necessary component for crust formation. Crust can easily form when the clay content is between 10 and 30% [73]. The stability of aggregates determines the degree of development of crust and the rate of its formation. Nciizah and Wakindiki [74] argued that the dispersion of aggregates and clay particles under the impact of raindrops altered the particle composition of the soil and increased clay content and the dispersion of particles during crust formation and, thereby, promoted the development of crust. The degree of crust development under various rainfall patterns differed greatly among soils of different textures and mineral compositions. Soils dominated by mineral stones were more susceptible to the formation of crust under the action of raindrops, and the crust was stronger. Improving the stability of aggregates can, therefore, slow the rate of crust formation [75]. The content of exchangeable sodium ions and the types of minerals in the soil also affect the development of crust. Tang et al. [76] found that chemical mechanisms were the primary drivers of crust formation in soils with a high exchangeable sodium percentage (ESP), whereas the physical impact of raindrops played a dominant role in soils with a low ESP value.
Differences in soil properties can affect the rate of development of crust, and the formation of crust can affect the soil properties. During rain, topsoil is affected by the striking effect of raindrops, aggregates are destroyed, and large particles are dispersed into fine particles to fill the internal pores, after which they are continuously compacted to form a dense structural layer [35]. Exploring the influence of different soil properties on crust development could, therefore, clarify the trend of soil properties during crust development and be used to predict erosion. Reasonable farming practices and methods of irrigation can prevent the degradation of land and alleviate crust development. Hu et al. [77] reported that spraying soil conditioners (polyvinyl alcohol or polyacrylamide) on the surface could improve soil structure, increase the stability of aggregates, and inhibit crust development.

4.1.2. Rain

Rainwater is the driving force of erosion, and the kinetic energy of raindrops is the primary factor driving the formation of crusts. Raindrops can disperse fine particles, forming the material basis for filling pores. The impact of raindrops and their compaction of the soil surface, however, make the soil structure more compact, increasing the likelihood of inducing the formation of a physical crust [45,65]. Chen et al. [27] conducted experiments with simulated rain to investigate the effect of raindrops on crust development, and the results indicated that raindrop kinetic energy could disrupt aggregates, decompose particles, increase the content of fine particles, alter particle composition, and promote the formation of physical crust. Some studies have indicated that soil surfaces exposed to rain were highly susceptible to the formation of physical crust because a portion of the rainwater directly infiltrates the soil interior, causing particles to leach; rainwater that does not quickly infiltrate forms a thin layer of water flow on the soil surface, transporting fine particles from the surface soil, causing them to undergo lateral displacement or depositing them in low-lying areas, ultimately leading to the development of physical crust [62,78].
The kinetic energy of the raindrops and the runoff formed by the convergence of rainwater during rainfall are crucial conditions for the development of PSC. The compacting action of raindrop impacts, the transporting action of runoff, and the wetting and dispersion of particles are all indispensable factors for driving the development of crust [64]. Bu et al. [79] found that the main forces driving the formation of loess crust were the combined effects of compaction by raindrops and the wetting and dispersion of particles, both of which were indispensable. Hu et al. [45] reported that the continuous impact by raindrops caused particles to disperse. Some of these particles filled soil pores, and others were displaced by runoff, ultimately leading to the rearrangement of surface particles into a dense thin layer. The duration and intensity of rain also affect the development of crust. The duration of rain alters the timing of surface runoff, and the intensity of rain directly influences the magnitude of the kinetic energy of the raindrops. Experiments have demonstrated that longer rains and higher raindrop kinetic energies lead to higher rates of crust development [71,80].

4.1.3. Landform

Crust is a product of erosion, and its formation is affected by many factors. In addition to soil texture and rainfall, topography is an important factor affecting crust development. Slope is a key topographic factor influencing the formation and development of physical crust. An increase in slope gradient reduces the effect of the impact of raindrops and slows the rate of crust formation [81]. Liu and Jiang [82] found that the force of impact of raindrops on a slope could be resolved into a normal compressive force perpendicular to the slope and a shear force directed downslope. The compressive force of the impact of raindrops decreases as the slope gradient increases, and the strength of the crust decreases accordingly. Variations in topography can also affect many processes occurring at the soil surface, including the infiltration of water, the velocity of runoff and its direction of flow, the loss of soil, and deposition, which in turn affect the development of physical crust and may even directly lead to different types of crust [83]. Bresson [26] and Chen et al. [18] both indicated that PSC that developed at different topographic heights had different structural characteristics. Areas at higher elevations experience stronger raindrops and are prone to forming structural crust, and low-lying areas tend to form depositional crust due to particle deposition.

4.1.4. Vegetation

Vegetation influences the formation of physical crust mainly by three mechanisms: weakening the force of the impact of raindrops with surface leaves, intercepting runoff by bottom roots and stems, and improving physicochemical properties by the growth of roots [26,84]. Zhao et al. [85] reported that the construction of artificial vegetation could improve the physicochemical properties of soil, promote the formation of crust on sandy land, and increase crust thickness and hardness as forests aged, increasing surface stability and decreasing wind erosion. Vegetation cover may also weaken the impact of raindrops on topsoil and slow the velocity of runoff. Maintaining vegetation cover on the soil surface should, therefore, be an effective method to conserve moisture, reduce erosion, and mitigate crusting [65]. Li et al. [86], however, indicated that plant roots promoted the formation of aggregates and increased crust thickness and hardness, even though vegetation led to significant differences in cohesion between particles and crust thickness.

4.2. Human Factors

In addition to natural factors, human activities such as the cultivation of land, the irrigation of farmland, and afforestation also influence the formation of physical crust. Common tillage practices include contour tillage, manual digging, and straight-line slope tillage. Different methods of agricultural tillage directly alter the roughness of the soil surface, causing changes in terrain relief. This activity leads to the formation of structural crust on higher areas solely due to the impact of raindrops, but low-lying areas experience both raindrop impact and runoff erosion, leading to the formation of depositional crust. Robinson [87] found that soil crusting did not occur in woodland or shrubland after 30 min of simulated rain, but raindrop impact on agricultural land and wasteland led to the formation of structural and depositional crusts, respectively. The formation of crust alters the structural characteristics of the soil, further influencing the hydrological conditions of the land [88]. Unreasonable practices of irrigation and the destruction of natural vegetation by humans expose farmland and forest soils, increasing the area of raindrop impact during rain, weakening the stability of aggregates and promoting the formation of crust. Maintaining vegetation cover and adopting reasonable tillage practices can, therefore, effectively inhibit the formation of PSC.

4.3. Combined Influence

The formation of PSC is influenced by multiple factors, each with its own process and pathway of effect. The formation of most of the physical crust in nature, however, is due to a combination of multiple factors. Lu et al. [3] found that crust formed on the surfaces of four types of soil under various rain intensities. Crust strength increased with rain intensity when soil moisture content was <30%, indicating that rain was a dominant factor in crust formation. Wu [88] conducted a multi-factor analysis to investigate the impacts of rainfall intensity and tillage practices, as well as slope and tillage practices, on crust formation and sediment yield on hillslopes. The study found significant differences in crust development due to individual factors, with little correlation between tillage practices and either rain or slope factors. Physical crust develops under the combined action of multiple mechanisms. The complexity of its formation leads to many influencing factors, which interact and influence each other. Quantitatively analyzing the impact of a single factor on crust development therefore remains a challenge. Future research should focus on determining the magnitude of the effect of individual factors during crust development and their contribution ratio to crust formation, accurately quantifying the influence of different factors on crust development.

5. Environmental Impacts of PSC

5.1. Hydrological Processes

PSC is a dense structural layer formed by the reorganization and redistribution of fine particles in the topsoil. The formation of crust affects the characteristics of the soil properties and hydrological processes, mainly the infiltration of rainwater, surface evaporation, surface runoff, and water content [23].
The infiltration of water refers to the process by which water flows through pores into the interior of the soil, and the pore structure of the soil directly affects the rate of infiltration. Surface particles are disrupted and rearranged during the formation of physical crust, with fine particles filling the pores near the soil surface, altering pore size and leading to a substantially lower rate of infiltration on crusted than uncrusted soil [89]. Both Fox et al. [32] and Feng et al. [27] observed and analyzed the pore characteristics of crusted soil samples at different stages of development by creating sections and using CT scanning, respectively. Their findings consistently indicated that crusted areas had lower porosity and fewer large pores, which slowed the speed of water flow within the soil, leading to lower rates of infiltration. Wu et al. [34] used the Kostiakov model, Horton model, and Jiang Dingsheng model to simulate the soil moisture infiltration process on loess slopes and found that the effect of slower infiltration varied among the types of crust; the effect was as much as 37.13% larger for structural than depositional crust. Crust develops continuously with the strikes of raindrops, and water no longer penetrates downward but will directly pool on the surface to form runoff when the intensity of rain exceeds the infiltration capacity of the soil, or the internal soil reaches saturation. The infiltration of water into soil during crust development therefore mainly occurs in three stages: an early stage when the rate of infiltration is higher, an intermediate stage when the rate of infiltration gradually decreases, and a late stage when the rate of infiltration gradually tends to stabilize [90].
Studies have found that the presence of a physical crust on the surface not only hinders infiltration but also advances the time of generation of surface runoff and increases runoff volume [28]. Crust thickness and the generation of runoff are generally positively correlated. Kidron [91] verified this correlation by fitting a linear equation to the relationship between crust thickness and surface runoff under different durations of rain. Wu [85] found that changes in the angle of hillslopes affected the time of runoff generation, with runoff occurring earlier as the angle increased. The difference in time for generating runoff between crusted and uncrusted slopes, however, gradually decreased. The development of physical crust leads to increased runoff volume and shear force and deeper runoff, strongly affecting soil hydrological processes [92,93].
Field and laboratory experiments have been conducted to better quantify the impact of crust development on infiltration and surface runoff, to predict soil hydrological processes, to measure hydraulic parameters at different stages of crust development, and to construct models of infiltration (such as the Green-Ampt, Philip, and Smith–Parlange models) to simulate soil hydrological processes. Some studies have simulated the infiltration of water and the generation of hillslope runoff during crust formation by fitting various hydraulic parameters in equations [94,95]. Accurately measuring hydraulic properties, however, remains a challenge in practical operations, and inaccuracies in measurements directly lead to insufficient precision in model simulations [12,58]. The introduction of CT scanning has provided an objective method to quantify pore structure, which directly affects the transport of air and water and alters infiltration and evaporation. Hydrological processes can be reasonably predicted based on pore properties. Ju et al. [96] and Xu and Hu [97] have experimentally supported the hypothesis that incorporating information about soil structure into models can improve the accuracy and precision of simulations, offering new insights for reasonable predictions of hydrological processes.
In addition to affecting infiltration and runoff, the development of PSC also influences the evaporation of surface water and the internal water content. Xing and Dong [98] found that soil water content under physical crust cover consistently tended to increase with depth. The impact of crusting on the water cycle has been an ongoing controversy due to factors such as the properties of soil and rain. Some people believe that crust development promotes evaporation, but others hold the opposite view, arguing that the presence of crust inhibits evaporation. Qiao and Xu [89] argued that the formation and development of crust fundamentally altered surface conditions, and that physical crust mainly promoted evaporation under the influence of rainfall and rain intensity. Li et al. [99], in exploring the impact of different types of crusts under Haloxylon ammodendron plantations on precipitation infiltration dynamics, believed that crust development increased the contents of fine particles and organic matter, increased the interception of precipitation, and promoted evaporation in the crust developmental layer but restricted the infiltration of precipitation and inhibited evaporation in deep soil.

5.2. Erosion

Soil erosion is a worldwide environmental problem that directly affects the quality of arable land, ecological security, and the development of human society. Soil properties such as mechanical composition, organic matter content, and structure determine the erodibility of soil [100]. The formation of physical crust alters characteristics, affects environmental and hydrological processes, and increases the possibility of erosion [101].
The rate of erosion, resistance to erosion, and shear strength are effective indicators for evaluating erodibility. Internal factors such as texture, structural properties, aggregate stability, and bulk density are critical in influencing the rate [102]. Li [103] believed that the formation of PSC altered structural properties, leading to increased bulk density and hardened topsoil, which increased resistance to erosion and decreased the occurrence of erosion. Cheng and Cai [104] argued that splash erosion under the same duration of rain was mainly determined by aggregate stability. Differences in the structural composition of topsoil crust of different textures directly led to variations in soil stability. Improving aggregate stability could, therefore, effectively inhibit erosion [105]. The impact of crust development on erosion is also closely associated with changes in soil shear strength and the runoff shear force. The dynamic relationship between soil shear strength and the runoff shear force under the influence of cumulative rainfall and runoff scouring determines the response of crust development to erosion. The formation of physical crust during the initial stage advances the time for the generation of surface runoff, causing dispersed fine particles, which are hit by raindrops, to fail to fill pores in a timely manner and move downward with the runoff. Soil shear strength at this time is less than the shear force of the runoff, and scouring by the flow of water increases the production of sediment, promoting erosion. The presence of crust, however, increases shear strength, and erosion due to raindrop splash which reduces the shear force of runoff as the crust gradually matures under prolonged rain, thereby inhibiting erosion [106].
External factors such as rainfall, slope, and soil roughness directly influence the rate of crust development, altering the yields of water and sediment on the soil surface and leading to different effects of erosion. The rate of crust development differs greatly under different intensities and durations of rain. Studies have found that crusting promoted the generation of runoff on slopes, inhibited the production of sediment, and slowed erosion as the duration of rain increased [93]. Slope is a dominant factor affecting the generation of runoff. Gao et al. [107] found that the influence of crusting on slope erosion in black soil was closely correlated with slope gradient. An increase in slope gradient advanced the time for the generation of runoff, causing fine particles carried by runoff to displace and affect sediment yield. Physical crust, as a dense thin layer on the soil surface, determines the roughness of soil, which represents the undulations of the surface and influences the yields of water and sediment and thus erosion [83]. Surface roughness not only increases resistance to runoff and decreases flow velocity, the sediment carrying capacity of runoff, and the yields of water and sediment but also promotes the convergence of slope runoff, increases the scouring ability of runoff, and exacerbates erosion [108,109].

5.3. Other Effects

Physical crust is widespread in arid and semi-arid regions and represents a distinctive micro-landscape feature of deserts. Temperature, soil, and moisture are the main factors affecting crop growth and seed germination. The development of topsoil crust strongly alters the properties of soil, influencing the infiltration and evaporation of water and subsequently affecting crop growth and seed germination. In exploring the causes and effects of soil crust on sloping farmland, Fan [110] found that crust development notably affected the rate of wheat emergence in farmland, inhibited plant growth, and reduced crop yield. Wu et al. [111] and Jiang et al. [112] indicated that the presence of crust affected the infiltration and redistribution of water and that different types of crust had varying capabilities of retaining water, directly leading to differences in the rate of seed germination. In agricultural production, to address the adverse effects of soil crust on crop emergence, measures such as selecting crop varieties with strong emergence capabilities and optimizing planting methods like ridge planting can improve seedling emergence rates. Additionally, soil management techniques, including manual tillage, surface mulching, and the application of soil conditioners, can inhibit the formation of crust, enhance water infiltration and soil moisture content, and improve soil structure, thereby creating favorable conditions for seed germination and crop growth, ultimately leading to increased crop yields [113]. Research has also found that as a dense structural layer on the soil surface, topsoil crust, influenced by factors such as rainfall, not only affects runoff and sediment production on slopes but also alters the loss of soil nutrients, becoming one of the important factors contributing to the degradation of the quality of sloped land. Hu [114] analyzed the loss of soil nutrients from slopes with and without crust in the Hengduan Mountains of southwestern China under different rain intensities. The results indicated that the loss of phosphorus and potassium in runoff and sediments from slopes during rain mainly occurred for dissolved and extracted forms, respectively. The structural layer formed by crust development can stabilize the structure of topsoil, reduce the loss of soil nutrients, and thus delay the degradation of soil quality.

6. Deficiencies and Prospects

6.1. Increase Research on the Dynamics of Soil Structure During Crust Development

Topsoil crust is formed gradually under the action of scouring by runoff or strikes by raindrops, and the development of physical crust alters the characteristics of pores. Studies have found that porosity was significantly lower in crusted than uncrusted areas, leading to reduced infiltration and increased surface runoff, which affects runoff and sediment processes. The development of physical crust, however, is a dynamic process, in which the properties and structure of soil change dynamically as the crust forms. Research on the structural properties of soil after crusting has mostly focused on observing changes in pore size on microscope slides or conducting laboratory experiments to calculate changes in soil-water content and bulk density, lacking dynamic monitoring of other information about pore structure, leading to an unknown state of the changes in structural information throughout the entire development of crust.
Both photographic observation and the preparation of microscope slides are limited to analyzing the 2D structural characteristics of soil, which cannot quantify the changing trends in the 3D structural characteristics of crusted soil. Armenise et al. [13], Zhu et al. [14], and Feng et al. [27] have objectively extracted and quantified changes in the 2D and 3D structural characteristics of pores under the influence of crusting using CT scanning and constructed 3D models of crusted soil columns using visualization software, providing new insights for dynamically quantifying the structural information. We could, therefore, attempt to use CT scanning combined with visualization software in future research to fully explore the structural properties of crust, identify the pore structural characteristics of crust at different stages of development, objectively quantify the dynamic trends of the properties and structure of soil during crust development, and construct 3D visualization models of crusted soil, providing a theoretical basis for reasonably predicting the impact of crust development on soil hydrological processes and erosion.

6.2. Develop Objective and Standardized Quantitative Methods for Studying Crust Formation

PSC is a dense structural layer formed on the soil surface after particles are disrupted and reorganized, and crust thickness is an important index to evaluate the degree of crust development. Three methods are currently used to quantify crust: (1) direct measurement using vernier calipers, (2) observation of microscope sections, and (3) CT scanning combined with the pore-threshold method. Direct measurement using vernier calipers first requires the destruction of the crust before measurements can be obtained. The observation of microscope sections requires making longitudinal slices of soil that has formed a crust, observing the pore characteristics using an electron microscope, judging the area of crust formation, and then finally calculating the thickness of the crust. All of these methods require the destruction of the original crust samples, and visual observation can easily lead to subjective errors, affecting data accuracy. Armenise et al. [13] analyzed the pore indices of crusted soil using CT scanning and objectively quantified the thickness indices of different samples of crusted soil based on the pore threshold, which was a new breakthrough in the objective quantification of crust thickness. To quantify the crust thickness by this method, however, it is necessary to ensure that the crust thickness of soil samples is not more than 10 mm. The establishment of a standardized method for the objective quantification of crust thickness and the comparability of the results of the data at different developmental stages are therefore urgent issues. Whether or not crust thickness can be used as an objective criterion to classify the stage of crust development is also a key concern for us in the future, because thickness is an effective indicator for evaluating the degree of crust development.

6.3. Build Erosion Models That Account for the Impact of Crust Development

The formation of crust on the soil surface alters soil biological activity, affecting the stability of soil structure and the susceptibility to erosion [115]. Dynamic models linking physical crusting to erosion have not yet been established due to the complexity of crust development. Some scholars have calculated erodibility based on soil properties, developed transfer functions, and constructed models of erosion. Errors in testing soil parameters, however, often lead to low accuracy, and the introduction of fixed parameters cannot dynamically simulate the response to erosion of crust at different stages of development [116]. Pore structure is a fundamental attribute of soil, which directly affects the infiltration and evaporation of water and changes in pore structure that accompany the disruption and rearrangement of particles, indirectly leading to changes in other properties such as bulk density and aggregate stability. Monitoring the dynamic changes in pore structure could, therefore, effectively be used to predict the formation of physical crust and represent the impact of crust development on erosion. The application of CT scanning in the field of soil science provides an objective method to quantify pore structure [117]. Pore indices can be objectively extracted using CT scanning, and dynamic models of the effect of crust development on erosion can be established based on the structural characteristics of soil. These models can analyze the effect of crust at different developmental stages on erosion, clarify the relationship between crust properties and erosion, predict erosion, and provide a foundation and basis for managing erosion and the rational deployment of soil and water conservation measures.

6.4. Elevate the Use of Crust Developmental Stages and Structural Characteristics to Broader Scales

Physical crust gradually forms on the soil surface along with the fragmentation, dispersion, and reorganization of particles [117]. The crust changes the structural characteristics of soil properties and affects internal paths of water flow. Physical crust is common in arid and semi-arid regions, but the current research on crust is mostly focused on the Loess Plateau, where the research object is mainly loess, and fewer studies are focusing on crust development and the influence mechanism of other types of soils, which leads to insufficient in-depth studies at different watershed scales. Liu [43] reported that different types of soils in China tested using simulated rain produced crust under specific conditions of rain, and that the structural characteristics of the crust varied greatly depending on the region and climatic factors, which affect the development of physical crust and soil hydrological processes. Exploring the mechanisms of the formation and development of crust in different soil types can, therefore, effectively guide the deployment of regional soil and water conservation measures. The introduction of remote-sensing technology and unmanned aerial vehicles has provided an effective way to accurately obtain data for soil resources. The use of remotely sensed data can expand the research area from the Loess Plateau to all regions of China with water erosion, increasing the range of soil types and focusing on point scales. Detailed research can be conducted on crust development and changes to its attributes, clarifying the effect of crust development in different regions and soil types on local hydrological processes and mechanisms of erosion. This research would provide scientific evidence and theoretical references for constructing models for prediction soil infiltration and erosion in different regions, increasing the accuracy of the results from model simulations.

6.5. Implement Reasonable Strategies of Management to Address Physical Crusting in Agricultural Land

Physical crust is a dense structural layer on the soil surface, and the formation of crust breaks the connection between the external environment and the soil, leading to a decrease in the ability of pores to transport water and air, reducing seedling emergence and crop yield and affecting human production and life [110]. The rational ploughing of fields or loosening the deep soil layers can effectively reduce the degree of soil compactness, reduce bulk density, improve porosity, and eliminate the surface dense layer formed by the splashing of raindrops, which can reduce the formation of crust on farmland and improve crop yield [118]. The reasonable use of ameliorators of ponding on soil, though, can effectively inhibit the production of crust. Polyacrylamide is a highly polymerized soil amendment that can improve the structural characteristics of soil, increase the stability of agglomerates, and resist the striking effect of raindrops to inhibit crust development [77]. A cover of natural humus or litter can weaken the impact of raindrops, slow the separation and disruption of particles, and inhibit the development of crust. Reasonably deploying measures for conservating soil and water can effectively manage physical crust, increase crop yield, improve the ecological environment, and delay the erosion of soil.

7. Summary and Conclusions

Physical soil crust refers to a dense structural layer formed on bare or sparsely covered topsoil due to the impacts of raindrops and runoff erosion, with a thickness ranging from a few millimeters to several centimeters. Research on crust began in the 1930s and gained widespread attention in the 1980s, covering topics such as the concepts and types of PSC, the mechanisms of formation of physical crust, factors influencing crust formation, and the environmental impacts of crust development. Physical crust is influenced by rainfall and runoff, making its formation quite complex. The development of topsoil crust alters soil properties, reduces the rate of infiltration of water, leads to increased surface runoff, and affects hydrological processes and slope erosion. Future research on physical crust should, therefore, focus on the following: (1) dynamically quantifying the changes in soil structural properties during crust development, (2) establishing objective and quantitative methods for measuring the thickness of physical crust based on the changing characteristics of soil properties during crust development, (3) constructing dynamic models to assess the impact of crust development on soil erosion, (4) expanding the scale of research to explore crust development and the structural characteristics of soils in different regions, and (5) adopting protective tillage or applying soil amendments to agricultural land to reduce the formation of crust, improve farmland conditions, and increase crop yield. Government policy support, scientific and technological guidance, interdisciplinary communication, and the sharing of academic information can help us better conduct experiments, dynamically monitor the development of physical crust, reasonably predict soil hydrological processes, and provide scientific evidence for reducing soil erosion.

Author Contributions

H.X.: Investigation, Data curation, Writing—original draft; X.Z.: Investigation, Writing—review and editing; M.M.: Writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA0440202), the Natural Science Foundation of Jiangsu Province, China (BK20220163), the Jiangxi Province Natural Science Foundation (20224BAB203031), and the Youth Innovation Promotion Association of the Chinese Academy of Sciences (2023327). The APC was funded by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA0440202).

Data Availability Statement

Data in this study are all available.

Acknowledgments

We would like to thank everyone who contributed to this review.

Conflicts of Interest

The authors declare no conflict of interest.

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Soilsystems 09 00023 g001
Figure 1. Methods for measuring the thickness of physical crust. (a). Vernier calipers. (b). Observation of microscope sections. (c). CT scanning and porosity thresholding.
Figure 1. Methods for measuring the thickness of physical crust. (a). Vernier calipers. (b). Observation of microscope sections. (c). CT scanning and porosity thresholding.
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Figure 2. Four stages of quantifying physical crust. (a). Qualitative description. (b). Semi-quantitative representation. (c). Preliminary quantification. (d). Quantitative expression.
Figure 2. Four stages of quantifying physical crust. (a). Qualitative description. (b). Semi-quantitative representation. (c). Preliminary quantification. (d). Quantitative expression.
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Figure 3. Simulation of the processes and trends in the development of physical crust in Quaternary red clay in southern China.
Figure 3. Simulation of the processes and trends in the development of physical crust in Quaternary red clay in southern China.
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Table 1. The main classification of soil crust.
Table 1. The main classification of soil crust.
Basis of Classification TypesMain Mechanism of FormationCommon Features
Whether there are organisms present on the soil surfaceYesBiological crustComposite of organisms formed by cryptogamic plants bonded with particles on the soil surfaceIncrease in bulk density; decrease in the number of macropores and porosity
NoPhysical soil crustStrike splash of raindrops during rainfall
Table 2. Published methods for classifying physical crust.
Table 2. Published methods for classifying physical crust.
Main Representative ReferencesBasis of Classification Types
Lu et al. [3], Yeom and Sjoblom [7] Y. Chen et al. [10], Chen et al. [18], Feng et al. [27]Mechanism of formation, structural characteristicsStructural and depositional crusts
Bullard et al. [21], Bresson, C.V. and Bresson, L.-M. [26], Malam Issa [28]Mechanism of formationStructural, depositional crust, and erosional crusts
Valentin [29] Microstructure, formation, propertiesFiltration pavement, erosion pavement, structural crust, and depositional crust
Zhu et al. [30]Distribution positionSurface and subsurface crusts
Table 3. Published analyses of the influence of soil factors on crust development.
Table 3. Published analyses of the influence of soil factors on crust development.
ReferenceSoil TypesPrimary AttributeSecondary AttributeAction PathwayPhase of Influence
Armenise et al. [13]Silty clay loam, sandy silty loam, sandy loamTextureOrganic matter content, aggregate stabilityRaindrop strikeSoil particles dispersed, aggregates broken
Fang et al. [19]Sandy soilTexture—Wind sorting particles, settling—
Bu et al. [24]Black soil; loess; purple soilTextureAggregate stabilityRaindrop strikeSoil particles dispersed, aggregates broken, leaching
Fan et al. [42]Black soil; loessTextureAggregate stabilityRaindrop strike, rain wettingSoil particles dispersed, aggregates broken
Han et al. [62]Red soilAggregate stabilitySoil-water content, bulk densityRain wettingSlaking of aggregates
Zhou et al. [65]Red clay soilAggregate size—Raindrop splashSoil particles dispersed, fill pores and deposit
Yan et al. [71]Fine-sand lossTextureOrganic matter contentRaindrop compactionSoil particles dispersed
Bedaiwy [72]Sandy loam; clay soilTexture—Rain wetting, raindrop compactionPhysicochemical dispersion of soil clay particles
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Xu, H.; Zhu, X.; Mi, M. Progress and Prospects of Research on Physical Soil Crust. Soil Syst. 2025, 9, 23. https://doi.org/10.3390/soilsystems9010023

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Xu H, Zhu X, Mi M. Progress and Prospects of Research on Physical Soil Crust. Soil Systems. 2025; 9(1):23. https://doi.org/10.3390/soilsystems9010023

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Xu, Huiyun, Xuchao Zhu, and Meixia Mi. 2025. "Progress and Prospects of Research on Physical Soil Crust" Soil Systems 9, no. 1: 23. https://doi.org/10.3390/soilsystems9010023

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Xu, H., Zhu, X., & Mi, M. (2025). Progress and Prospects of Research on Physical Soil Crust. Soil Systems, 9(1), 23. https://doi.org/10.3390/soilsystems9010023

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