at least two consecutive samples examined per 

 month during the irrigation season and any one 

 sample examined in any 1 month shall not exceed 

 a coliform group density of more than 20,000 per 

 100 milliliters, or a fecal coliform density of more 

 than 4,000 per 100 milliliters. 



For the control of plant pathogens, guidelines 

 for irrigation water are best framed in terms of 

 preventive measures rather than by assay pro- 

 cedures. 



Ph 



Acidity or alkalinity as such in irrigation water 

 is seldom directly detrimental to crop growth. 

 Normally, water with pH values of 4.5 to 9.0 

 should not present any insurmountable problems, 

 but a range of 5.5 to 8.5 would be more desirable 

 (p. 155). 



Temperature 



Excessively high or low temperatures in irriga- 

 tion water may aifect crop growth and yields (pp. 

 157, 160). A desirable range of water tempera- 

 tures is from 55 to 85 F. 



Suspended Solids 



Sediment and suspended solids may be detri- 

 mental in irrigation water because of their effect 

 on irrigation structures and equipment and on the 

 soil to which the water is applied. No guidelines 

 are available to establish standards for either 

 particle size or quantity, (pp. 163, 175) 



Pesticides 



On the basis of the limited information avail- 

 able, levels of herbicides at which crop injury has 

 been observed are shown in Table IV-4. There 

 is little evidence to indicate that other pesticide 

 contamination of irrigation water would be detri- 

 mental to plant growth or accumulate in or on 

 edible plants in toxic concentrations under normal 

 use (pp. 156-157). 



Biochemical Oxygen Demand (BOD) and 

 Dissolved Oxygen 



Insufficient information is available to suggest 

 guidelines or to indicate that low BOD values or 

 dissolved oxygen content of an irrigation water as 

 such will have a deleterious effect on plant growth 

 or well-drained soils. 



TABLE IV-4. Levels of Herbicides in Irrigation 



Water at Which Crop Injury Has Been 



Observed ' 



Crop injury threshold in 

 irrigation water mg/l 



Acrolein Flood or furrow: beans-60, 



corn-60, cotton-80, soy- 

 beans-20, sugar beets-60. 

 Sprinkler: corn-60, soybeans- 

 15, sugar beets-15. 



Aromatic solvents .__ Alfalfa-> 1,600, beans-1,200, 

 (xylene). carrots-1,600, corn-3,000, 



cotton-1,600, grain sor- 

 ghum->800, oats-2,400, 

 potatoes-!, 300, wheat- 

 1,200. 



Copper sulfate Apparently, above concentra- 

 tions used for weed control. 



Amitrole-T Beets (rutabaga)->3.5, corn- 



>3.5. 



Dalapon Beets->7.0, corn-<0.35. 



Diquat Beans-5.0, corn-125.0. 



Endothall Na and K Corn-25, field beans-<1.0, 

 salts. alfalfa->10.0. 



Dimethylamines Corn->25, soybeans->25, 



sugar beets-25. 



2,4-D Field beans->3.5<10, grapes- 



0.7-1.5, sugar beets-3.5. 



Dichlobenil Alfalfa-10, corn->10, soy- 

 beans-!. 0, sugar beets- 

 1.0-10. 



Fenac Alfalfa-1.0, corn-10, soybeans- 



0.1, sugar beets-0.1-10. 



Picloram Corn->10, field beans-0.1, 



sugar beets-<1.0. 



1 Data submitted by crops research division, ARS, USDA 

 (unpublished). 



NOTE. — Where the symbol ">" is used, the concentrations in 

 water cause no injury. Data are for furrow irrigation unless 

 otherwise specified. 



118 



