Soil degradation is the weakening of organic soil due to chemical, environmental or human influence. Frequent flooding, lack of water, cattle rearing and the application of pesticides and herbicides for soil reduces the soil's ability to produce fertile growth and reduces the topsoil layer needed for plant life.
Strong winds on dry soil cause the topsoil layer to blow away, reducing fertility in the land. This is often after long periods of drought which reduces the strength of the soil. Wind erosion is reduced with the planting of cover materials such as trees and shrubs and rotating crop land to reduce the agricultural degradation of the soil. Water erosion is caused by four factors, says the United Nations: rainfall, soil type, slope gradient and soil use/vegetation cover. When soil is dry and there is an intense rain, the water causes the topsoil to run off. Soil type, the types of vegetation blocking the water flow and the slope the soil is on determines how much soil loss there is.
High salt content in soil will also cause soil fertility to degrade. As salt level rises, soil pH is affected, reducing a plant's ability to grow. Saline groundwater that rises up into the soil level due to deforestation, reducing the amount of rainwater entering ground water areas, leaves a salty layer which becomes a plant dead zone. Reduction of crops in these areas causes further topsoil erosion due to a lack of vegetative cover.
Soil acidification is the increase of a soil's hydrogen ions in comparison to the more basic soil ions says the ACS Garden website. A soil pH below 7.0 is called acidic. When the acidity is very low, plants cannot grow. Acidic soil is naturally occurring, but the application of fertilizers that leave acidic residues in the soil compounds the situation.
Compaction is the reduction of a soils pores due to surface activity on the soil. Regular human or animal footfall, as well as large farming vehicles, reduces the soil's ability to drain properly, causing run off of water during heavy rains which removes the top soil. Deep plowing reduces compaction says Global Change, as well as the regular addition of organic soil amendments.
Chemical residue is know to reduce the fertility of soil, although most modern agricultural practices reduces the likelihood of severe land degradation due to chemical application. Long-term accumulation of the same chemical product may increase the toxicity of the soil which reduces a plant's ability to survive. Poisoning of farm workers and animals due to excessive chemicals in the soil is also likely when chemical residue increases.