The term alkaline soil has been rather loosely applied to all soils containing sufficient amounts of soluble salts to cause injury to plant life. When correctly used, the term alkaline refers only to those soils that have a high pH value, usually through the presence of alkaline sodium salts, particularly sodium carbonate. Saline soils are not necessarily very alkaline, this depends on the type of salt. Soil alkalinity tends to be associated with low rainfall: soluble bases released by weathering are not leached from the soil and consequently the soil becomes more alkaline. The fertility of such soils is usually variable and often quite low. Strongly alkaline soils are sticky, impervious to water, and unfavourable to agriculture.
Alkaline soils are widely distributed in the drier areas of the world. A great deal of foodstuff is produced on soils where salts may be a problem. In regions of low rainfall, salts accumulate where drainage is poor. The tolerance of various plants to soluble salts in soils differs greatly. Among the less tolerant agricultural crops are beans, peas, clover, vetch, oats and peaches. The presence of high concentrations of salts limits the intake of water by plant roots. Yields are reduced approximately in proportion to the osmotic pressure of the nutrient solution. Concentrations of more than 0.2% of salts in a soil will harm crop yields. Alkaline salts are toxic at even lower concentrations. Many irrigation projects have failed because the soil has become alkaline through lack of proper drainage facilities or failure to use enough water to move excess salts down and out of the soil. Others have failed because of the high concentration of sodium in the irrigation water.