depth. Under conditions where precipitation of 

 salts and rainfall may be neglected, the salt con- 

 tent of irrigation water will increase to higher con- 

 centrations in the soil solution without change in 

 relative composition. The SAR increases in pro- 

 portion to the square root of the concentration; 

 therefore, the SAR applicable for calculating equi- 

 librium ESP in the upper root zone may be as- 

 sumed to be two to three times that of the irriga- 

 tion water. 



Many attempts have been made to predict ca- 

 tion exchange reactions in soils {45, 51, 80, 181). 

 Some of these have been used to predict the degree 

 to which sodium will be adsorbed by a soil from a 

 water of given quality. Many variables can influ- 

 ence the cation equilibria attained in the soil. These 

 include the relative proportions of cations and 

 anions in the water added and those present in the 

 soil, the presence of slightly soluble constituents 

 such as lime and gypsum, clay mineral types pres- 

 ent, and the salt concentrating action of evapo- 

 transpiration. For this reason, field studies are 

 needed to support predictive relations developed 

 under laboratory conditions. 



BOD and Soil Aeration 



The need for adequate available oxygen in the 

 soil for optimum plant growth is well recognized. 

 To meet the oxygen requirement of the plant, soil 

 structure (porosity) and soil water contents must 

 be adequate to permit good aeration. Conditions 

 that develop immediately following irrigation are 

 not clearly understood. 



Soil aeration and oxygen availability normally 

 present no problem on well-structured soils with 

 good quality water. Where drainage is poor, oxy- 

 gen may become limiting. Utilization of waters 

 having high BOD or COD values could aggravate 

 the condition by further depleting available oxygen 

 and produce reducing conditions in the soil. Aside 

 from detrimental effects of oxygen deficiency for 

 plant growth, reduction of elements such as iron 

 and manganese to the more soluble divalent forms 

 may create toxic conditions. Other biological and 

 chemical equilibria may also be affected. 



There is very little information regarding the 

 effect of using irrigation waters with high BOD 

 values on plant growth. Between source of con- 

 tamination and point of irrigation, considerable 

 reduction in BOD value may result. Sprinkler 

 irrigation may further reduce the BOD value of 

 water. Infiltration into well-drained soils can also 

 decrease the BOD value of the water without 

 seriously depleting the oxygen available for plant 

 growth. 



Where irrigation is used for disposal of waste 

 effluents with high BOD, lack of oxygen and 

 reducing conditions could easily become signifi- 

 cant factors for plant growth. However, if amounts 

 of water applied do not greatly exceed crop re- 

 quirements, it is probable the crop will not be 

 adversely affected. 



Suspended Solids 



Large quantities of suspended solids in irrigation 

 water can affect irrigation in many ways. In sur- 

 face irrigation, suspended solids can interfere with 

 the flow of water in conveyance systems and struc- 

 tures. Deposition of sediment not only reduces the 

 capacity of these systems to carry and distribute 

 water, but can also decrease reservoir storage ca- 

 pacity. For sprinkler irrigation, suspended mineral 

 solids may cause undue wear on irrigation pumps 

 and sprinkler nozzles as well as possibly plugging 

 up the latter, thereby reducing irrigation efficiency. 



Soils are specifically affected by deposition of 

 these suspended solids, especially when they con- 

 sist primarily of clays or colloidal material. These 

 cause crust formations which reduce seedling emer- 

 gence. In addition, these crusts reduce infiltration 

 thereby reducing irrigation efficiency and hinder- 

 ing the leaching of saline soils. The scouring action 

 of sediment in streams has also been found to affect 

 soils adversely by contributing to the dissolution 

 and increase of salts in some areas {130). 



Conversely, sediment high in silt may improve 

 the texture, consistence, and water-holding capac- 

 ity of a sandy soil. An example of this beneficial 

 effect has occurred by irrigation from the silt-laden 

 waters of the Virgin River in Southwestern Utah, 

 where silt loams have been deposited over loamy 

 sands to depths of 1 foot or more over a period of 

 many years. 



166 



