WATER UNDER SALINE CONDITIONS 223 
Progress is being made on methods for measuring and expressing 
the effect of saline conditions on plants. Reliable information is 
accumulating on the salt tolerance of various crops. This informa- 
tion is important as a guide for leaching operations and makes it 
possible to continue agricultural production with salt tolerant 
crops under conditions where it may not be feasible to maintain 
low levels of soil salinity. 
Application of Irrigation Water 
The method of application of irrigation water is important. At 
low salinity, furrow irrigation can be used. At higher levels of 
salinity, the requirements for land preparation are more exacting. 
The shape of the ridge and the position of the seed line with respect 
to the water line in the furrow may require consideration. At high 
salinity, flood irrigation must be practiced. 
Excess soluble salts can be removed from soil only by the action 
of water moving downward through the soil profile. Surface flush- 
ing operations for salt removal are generally ineffective. Some- 
times a trend toward salt accumulation can be reversed by minor 
changes in the method of irrigation or the amount of irrigation 
water applied. In extreme cases, ponding of water on the soil 
surface tor long periods is required. In all cases, adequate drainage 
is a controlling factor. 
Principles governing a field leaching operation are simple and 
straightforward. Soil tests can be used to indicate the need. The 
process involves covering the soil surface until sufficient water has 
passed through the soil to remove the soluble salt. The time re- 
quired will depend on soil properties and drainage conditions. If 
the exchangeable sodium is high, chemical amendments should be 
applied. Soil tests are available for indicating the soluble calcium 
required to reduce the exchangeable sodium to permissible levels. 
Leaching Requirement 
When the salinity of the irrigation water is high, leaching should 
be accepted as one of the controlling factors in farm management 
operations. Although soil salinity tests can be used as a guide, it is 
helpful to have an estimate of the amount of water to be allocated 
for leaching purposes. The leaching requirement is the fraction of 
