Fresh water

In most countries where irrigation is applied, irrigation constitutes a major part of freshwater consumption. The efficiency of irrigation (percentage of water taken up by crops and total water applied) ranges between 50% and 90%, depending to a large extent on the type of irrigation. With drip irrigation, where the water drips through plastic types to individual plants, the irrigation efficiency is in the range of 90% whereas in furrow irrigation it is about 50%. The problem is not only the consumption of water but also the quantities of salt brought on the field with the water under arid conditions which finally results in soil salinity and thus soils become infertile. Already in ancient times, in North Africa and the Euphrates/Tigris lowlands, large areas became saline through irrigation and went out of crop production. The drip irrigation cannot be applied to small crops such as cereals. These crops, however, are grown on a large area. Recently a new subsoil irrigation system was described which is based on irrigation techniques from ancient Persia. The clay types used are imbedded into the soil and have a porous structure which releases water into the soil according to the water requirement of plants. The moist zone around the pipe does not reach the soil surface. Hence no evaporation of water occurs and the risk of soil salinization is reduced to a minimum. The water-use efficiency (kg wheat grain/m3 irrigation water) of the furrow irrigation system was on average 85 and for the subsoil irrigation 250.

High Na+ concentration in soils affects soil structure and the uptake of other cationic species, particularly K+ Hence plants suffer from K deficiency which particularly depresses protein synthesis and thus growth. Field experiments carried out under arid conditions in Turkey showed that the negative effect of saline water on the leaf growth of satsuma mandarins was efficiently counteracted by high K fertilizer rates.

See also: Growth Constraints: Michaelis-Menten Equation and Liebig's Law; Nitrogen; Nitrogen Cycle.

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