Plant growth may be aflfected indirectly through 

 the influence of water quality on soil. For example, 

 the adsorption by the soil of sodium from water 

 will result in a dispersion of the clay fraction. 

 This decreases soil permeability and often results 

 in a surface crust formation which deters seed 

 germination and emergence. Soils irrigated with 

 highly saline water will tend to be flocculated and 

 have relatively high rates of infiltration (23). A 

 change to waters of sufficiently lower salt content 

 will reduce soil permeability and rates of infiltra- 

 tion by dispersion of the clay fraction in the soil. 

 This hazard increases when combined with high 

 sodium content in the water. Much depends upon 

 whether a given irrigation water is used continu- 

 ously or occasionally. 



If irrigations are applied frequently enough, 

 and with sufficient extra amounts to leach salts 

 from the root zone to maintain a favorable growth 

 environment, irrigation water with relatively high 

 salt concentrations may be used indefinitely. The 

 degree to which a saline water can be used to 

 irrigate a given soil is closely related to the drain- 

 ability of that soil. 



Other irrigation water quality considerations 

 may affect plant growth. Temperature of the wa- 

 ter, if excessively high or low, and its resultant 

 effect on the soil temperature in the root zone, 

 could depress plant growth. Soil aeration and oxy- 

 gen availability may be a factor deterring plant 

 growth if water having high BOD values is used 

 although no specific information is available. 



Osmotic Effects 



The effect of salinity, or total dissolved solids, 

 on the osmotic pressure of the soil solution is one 

 of the most important water quality considerations. 

 This relates to the availability of water for plant 

 consumption. Plants have been observed to wilt 

 in fields apparently having adequate water content. 

 This is usually the result of high soil salinity creat- 

 ing a physiological drought condition. Specifically, 

 the ability of a plant to extract water from a soil 

 is determined by the following relationship: 



TSS = MS-I-SS 



In this equation, the total soil suction (TSS) repre- 

 sents force with which water in the soil is withheld 

 from plant uptake. In simplified form, this factor 

 is the sum of the matric suction (MS), or the 

 physical attraction of soil for water, and the solute 

 suction (SS), or the osmotic pressure of the soil 

 water. 



As the water content of the soil decreases due 

 to evapotranspiration, the water film surrounding 

 the soil particles becomes thinner and the remain- 



ing water is held with increasingly greater force 

 (MS). Since only pure water is lost to the atmos- 

 phere during evapotranspiration, the salt concen- 

 tration of soil solution (SS) increases rapidly dur- 

 ing the drying process (97). Since the matric 

 suction of a soil increases exponentially on drying, 

 the combined effects of these two factors can pro- 

 duce critical conditions with regard to soil water 

 availability. 



The dissolved solids or saline content of the soil 

 solution results from natural dissolution of soil 

 minerals and primarily from that added as irri- 

 gation water or fertilizers. Water moves downward 

 primarily through gravitational and capillary forces 

 until it approaches a state where further movement 

 is slow; then moves back toward the surface as a 

 result of evapotranspiration. With adequate leach- 

 ing, however, excess water passes through the root 

 zone carrying the salt towards the ground water. 

 Soil salinity in the root zone will vary between 

 irrigations, but may, under certain circumstances, 

 present a stable pattern over long periods of time. 



Plant growth is related to salinity level of the 

 soil solution within the root zone. In assessing the 

 problem, criteria must be developed for assessing- 

 the salinity level of the soil solution. It is most 

 difficult to extract the soil solution from a moist 

 soil within the range of water content available 

 to plants. It has been demonstrated, however, that 

 salinity levels of the soil solution and their resultant 

 effects upon plant growth may be correlated with 

 salinity levels of soil moisture at saturation. The 

 quantity of water held in the soil between field 

 capacity and the wilting point varies considerably 

 from relatively low values for sandy soils to high 

 values for soils high in clay content. The U.S. 

 Salinity Laboratory developed the concept of the 

 saturation extract to meet this need (181). This 

 involves the addition of demineralized water to a 

 soil sample to a point at which the soil paste glis- 

 tens as it reflects light and flows slightly when the 

 container is tipped. The amount of water added is 

 reasonably related to the soil texture. For many 

 soils, the water content of the soil paste is roughly 

 twice that of the soil at field capacity and four 

 times that at the wilting point. This water content 

 is called the saturation percentage. When the satu- 

 rated paste is filtered, the resultant solution is re- 

 ferred to as the saturation extract. The salt con- 

 tent of the saturation extract does not give an exact 

 indication of salinity in the soil solution under field 

 conditions because soil structure has been de- 

 stroyed, nor does it give a true picture of salinity 

 gradients within the soil resulting from water ex- 

 traction by roots. Although not truly depicting 

 salinity in the immediate root environment, it does 



147 



