Red soil

Red soil is a type of soil that typically develops in warm, temperate, and humid climates and comprises approximately 13% of Earth's soils.^[1] It contains thin organic and organic-mineral layers of highly leached soil resting on a red layer of alluvium. Red soils contain large amounts of clay and are generally derived from the weathering of ancient crystalline and metamorphic rock. They are named after their rich red color, varying from reddish brown to reddish yellow due to their high iron content.^[2] Red soil can be good or poor growing soil depending on how it is managed. It is usually low in nutrients and humus and can be difficult to cultivate due to its low water holding capacity; however, the fertility of these soils can be optimized with liming and other farming techniques.^[1]

Red soils are an important resource because they make up such a large portion of farmland on the earth. In countries such as China, India, and Greece, where there are large amounts of red soil, understanding the soil's properties is crucial to successful agriculture. Red soil properties vary across regions and may require different management practices to achieve the best results.^[3]

Red soils include multiple soil types (e.g. ultisols, alfisols, oxisols) that are classified as red soil when they develop a distinct reddish color, which can vary from reddish brown to reddish yellow due to their high iron content.^[1] In general, red soils possess some characteristics of a good growing soil. They are generally acidic soils, which can be positive for agriculture but, in this case, often cause a lack of sufficient nutrients. These soils are also prone to frequent drought in drier regions.^[1]

Red soils are generally derived from the weathering of crystalline and metamorphic rock in areas of high rainfall. Red soil contains large amounts of clay and thin organic and organic-mineral layers of highly leached soil resting on an alluvium red layer.^[2] The composition and agricultural properties of red soil vary across regions. One type of red soil may be considered infertile in one region but nutrient-rich in another.^[1] There is not a singular composition that classifies all red soils.

Red soils are typically difficult for crop cultivation because high leaching leads to low water holding capacity, low nutrients, low organic matter (humus), and acidification.^[1] Fluctuations in the concentration of iron within red soil are found to have significant implications on its fertility and growth properties. The fertility of red soils can be improved with various farming techniques.^[1]

The soil liming process helps raise the pH of acidic soils. Because red soils are generally acidic, liming is a valuable farming technique that allows crops intolerant of acidic environments to thrive in red soil. However, modern research suggests that liming may have long-term environmental consequences on the soil. The systematic overflow of water through the soil gets into the organic matter of relevant sophgate of the soil.^[1]

Red soils are often deficient in nitrogen, which limits the growth properties of most red soil. Phosphorus and potassium can also become limited after the land has been harvested repeatedly. Nutrient application techniques introduce more of these lacking nutrients to the soil and allow them to restore chemicals that have diminished over time.^[1]

Another management practice that can be used to improve the fertility of red soil is incorporating organic matter into the soil. Some strategies employed to practice this include the use of organic manure and establishing proper tillage systems for the land.^[1]

The rotation of crops grown in red soil can significantly help to limit some of the compositional issues mentioned previously. Crop rotation helps to increase the content of organic matter, minimize nitrogen deficiency, and help avoid pests that damage the crops.^[1]

Red soil resources are estimated to cover 102 million hectare (1,020,000 square kilometers) of land in tropical and subtropical regions of China. The primary areas of distribution are Hainan, Guangdong, and Yunnan among other agricultural regions.^[4]

Red soils have a significant role in agriculture in Greece as well. They fall into two groups: residual soil forming in place from parent rock and soil forming in deep sedimentary deposits. The residual red soils in Greece tend to be less than a meter in depth and form on sloping hillsides. Like other red soils in the Mediterranean, they tend to form in limestone. The red soils that form in deep sediments are widespread in the lowlands of Greece, occurring on gently sloping terrain. Taxonomically, the Greek red soils belong to Rhodoxeralfs (red alfisol), Palexeralfs (well-aged alfisols), Xerochrepts (xeric inceptisol), and Orthents.^[5]

Red soils denote the most extensive soil group of India, covering an area of about 350,000 km² (10.6% of India's area) across the peninsula. India is rich with red soils in their southern, eastern, and northern regions. There, the soil appears yellow in its hydrated form.^[6] This soil, also known as the omnibus group, have been developed over Archaean granite, gneiss and other crystalline rocks, the sedimentaries of the Cuddapah and Vindhayan basins and mixed Dharwarian group of rocks. In the uplands of India, the red soils are thin, poor and gravelly, sandy, or stony and porous, light-colored soils on which food crops like bajra can be grown. In contrast, on the lower plains and valleys, they are rich, deep, dark-colored fertile loam which, under irrigation, can produce excellent crops like cotton, wheat, pulses, tobacco, jowar, linseed, millet, potatoes and fruits.

Prince Edward Island is famous for its iron-rich red soil.^[7]

• Ultisol, or red clay soil
• Latosol, or tropical red earth
• Inceptisol, or a soil order