|Soils, Crops and Fertilizer Use: A Field Manual for Development Workers (Peace Corps, 1986, 338 p.)|
|Chapter 12: Salinity and alkalinity problems|
· Osmotic Effect of Salts: Soluble salts in the soil water reduce the ability of plants to absorb water through their root hair membranes (a process called osmosis). If the salt concentration is high enough, water actually starts moving out of the plant roots back into the soil, and the plant may soon die; this is called plasmolysis. At lower salt levels, plants may suffer leaf tip burn, stunting, and defoliation. Germinating seeds and young seedlings are the most sensitive to this osmotic effect. As shown in Table 12-2, crops vary considerably in their salinity tolerance.
· Effect of Sodium: Sodic soils harm plant growth mainly through the toxic effect of sodium itself, the high alkalinity (pH 8.5-10), and the toxicity of the bicarbonate ion with which the sodium is often associated. Germinating seeds and young seedlings are the most sensitive.
· Boron Toxicity: Most irrigation water contains boron which becomes toxic above 1-2 parts per million. Boron is not easily leached from the soil. Irrigation water with a high boron content may limit farming to boron tolerant crops. As shown in Table 12-3, crops vary considerably in their tolerance to boron.
· Rainfall-induced injury: If high evaporation and lack of sufficient leaching allow a high level of salts to accumulate at the soil surface over the weeks, an unseasonal, light rain shower cay move these salts only as far as the crop root zone and cause injury. This is mainly a problem on peat soils when sub-irrigation is used. (Sub-irrigation consists of running water down wide canals through the field to raise the water table enough to irrigate plants by upward capillary movement; It's commonly used on peat soils, which tend to have high water tables.)