Soil and land degradation is caused by human mismanagement of soils, mostly due to agricultural activities. A report on the global assessment of soil degradation states that ‘the earth’s soils are being washed away, rendered sterile or contaminated with toxic chemicals at a rate that cannot be sustained’ Today almost a quarter of the world’s farmland is affected by serious degradation, up from 15% two decades ago. The Food and Agriculture Organisation (FAO) study indicates the world may currently be losing about 1% (50,000 sq kms) of its farmland annually due to a combination of degradation, urban sprawl, mining, recreation, toxic pollution and rising sea levels. 
Poor management degrades soil Soil does not have to be washed or blown away for its productivity to be lowered. Through improper soil and water management, many soils are losing the minerals and organic matter that make them fertile, and in most cases, these materials are not being replaced nearly as fast as they are being depleted, they may be lost for ever. Excessive cultivation, for example, can wreck the structure of some soils so that they are no longer capable of holding enough moisture for growing plants. Salinization, or the accumulation of salts in the topsoil, can also have a deletrious effect on soil productivity and crop yields. In extreme cases, damage from salinization is so great that it is technically unfeasible or totally uneconomic to reverse the process.  In general, irrigation induced salinity occurs when the irrigation water percolating through the soil exceeds the dispersal capacity of underground aquifers and drainage systems. Ground water containing dissolved salts rises and once within a metre of the soil surface, growth of the least salt tolerant plants is impaired.  While salinization is occasionally the result of natural soil-forming processes, it occurs most frequently in irrigated soils, where it is worsened by the high salt content of irrigation water. Salt-affected soils are found on every continent and nearly 7 percent of the land area of the world is affected. Salinization is a serious problem in Australia, the Soviet Union, and the United States, and it is critical in countries of north Africa and the Near East.  Frequently, soil waterlogging is the precursor of salinity. 
Soils degrade through waterlogging and loss of nutrients Waterlogged soils … deter agriculture in many countries, even in parts of the world where an excess of water is not usually thought of as a problem. Waterlogging interferes with agriculture in many countries; in Egypt, for example, where about one-third of the Nile Delta has a water table only 80 centimetres below the surface. Other countries with waterlogging from high water tables and runoff include Iran, Iraq, Somalia, parts of Syria, and Pakistan. Soil can also become degraded through loss of nutrients – chiefly nitrogen, phosphorus, and potassium – if these are not replenished to maintain soil fertility. Besides being lost through erosion, nutrients are also depleted by the crops themselves, particularly if the same crops are grown on the same land year after year. And in the humid tropics, many nutrients are leached during the intense rainstorms, especially on unprotected land. Without question, farming all over the world is removing more nutrients from the soil than are being put back. Soil compaction is still another destroyer of the soil. Sometimes it results from repeated passes over the same field with heavy machinery, particularly when the field is wet. It can also result from the hooves of grazing animals pounding down the soil too often in the same area, as they do around the only waterhole for miles. Compaction is not easy to correct.
Some countries poison soils Other forms of soil degradation occur in the more developed countries, but are rarely of concern to the developing ones – so far. Farmland is not only paved over by urbanization but is occasionally poisoned with chemicals.There are heavy metal impurities in fertilizers with cadmium being of greatest concern because it is the one that moves most readily from soils to edible food crops.  Some apple orchards sprayed with arsenic compounds in the 1930s were reported as still unproductive 30 years later. In recent years, there has been a general movement in many developed countries against using the more persistent insecticides, including a chemical group that includes DDT and chlordane.  Acidification of soils in yet another form of damage initiated by conventinal agriculture’s utilisation of fertilizers and mono-crop cultivation. Acidification occurs when anions such as nitrate (NO3 – ) from nitrogenous fertilisers and the nitrate-producing process in legume based pastures, leach through the soil profile accompanied by positively charged cations (eg, K+, Ca++) leaving behind an excess of positively charged hydrogen ions (H+) (the more hydrogen ions, the lower the pH = the more acidic). Changes in soil acidity can occur quite quickly but as plants grow readily between pH 5-8 it can take decades before declining productivity becomes obvious. In some soils, once the pH falls to 4 the fine fractions are mobilised in the soil profile leaving infertile coarse sands at the surface. Other causes of acid soils include … the removal of plant products that tend to be alkaline. The easiest solution to the problem of acid soils is to add lime but this is expensive and does not deal with the underlying causes.  Today a more serious problem in several highly industrialized countries is the indiscriminate dumping of chemical wastes, some of which are extremely toxic to plants, animals, and man, and the growing use of sewage sludge, some of which contains dangerous heavy metals which can be taken up by plants. For a developing nation, however, such problems are at present insignificant compared with the growing threat to their agricultural productivity from erosion, salinization, waterlogging, and general loss of fertility.  One last form of land degradation – associated with the coast – is acid sulfate soils. Naturally occurring, iron-sulphides underlie large areas of Australia’s low-lying coastal areas. When these normally waterlogged soils are exposed to air by drainage or drought, the iron sulphides oxidise producing acids which can then leach into marine or estuarine environments killing vegetation, fish and other organisms. 
2. http://www.fao.org/docrep/t0389e/t0389e02.htm (1990)
3. http://www.australiancollaboration.com.au/pdf/FactSheets/Land-degradation-FactSheet.pdf (post 2010)