ion (105). Since this represents additions of 

 about 100 pounds per acre of vanadium per year 

 for 40 inches of irrigation, no increased level of 

 vanadium for water use on type 'b' lands is pro- 

 posed. 



Zinc: Zinc has produced toxic symptoms in 

 various plants in concentrations from 3 to 10 mg/1 

 (35, 105). A tolerance limit of 5 mg/1 is pro- 

 posed here, since zinc is bound strongly to even 

 coarse-textured soils. A limit of 10 mg/1 is sug- 

 gested even for water used on type 'b' land until 

 evidence is presented to indicate that larger addi- 

 tions are acceptable. At irrigation additions of 

 40 inches per year, this would introduce about 

 100 pounds of zinc per acre per year. 



Other Considerations 



Acidity and alkalinity and common elements, 

 including chlorides and bicarbonates, are discussed 

 here only as they relate to irrigation water in gen- 

 eral. Specific quality characteristics relating to 

 arid region irrigation agriculture, or to water for 

 supplemental irrigation in humid regions, will be 

 discussed later. No attempt will be made to assign 

 criteria to each, but appropriate guidelines and 

 extent of importance will be described. 



Acidity and Alkalinity in normal irrigation 

 water, as measured by pH, have little direct sig- 

 nificance. Since water itself is unbuffered, and the 

 soil is a buffered system (except for extremely 

 sandy soils low in organic matter), the pH of the 

 soil will not be significantly affected by application 

 of irrigation water. There are, however, some ex- 

 tremes and indirect effects. 



Water having pH values below 4.8 applied to 

 acid soils over a period of time may possibly render 

 soluble iron, aluminum, or manganese in concen- 

 trations large enough to be toxic to plant growth. 

 Similarly, addition of a neutral or acid irrigation 

 water high in salts to an acid soil could result in a 

 decrease in soil pH, thereby rendering these ele- 

 ments soluble. In some areas where acid mine 

 drainage contaminates water sources, pH values 

 as low as 1.8 have been reported. Waters having 

 unusually low pH values such as this would be 

 strongly suspect of containing toxic quantities of 

 certain heavy metals or other elements. 



Water having pH values in excess of 8.3 are 

 highly alkaline and may contain high concentra- 

 tions of sodium, carbonates, and bicarbonates. 

 These constituents affect soils and plant growth 

 directly or indirectly and these effects will be dis- 

 cussed later under specific ions. 



Since most of the effects of acidity and alkalinity 

 in irrigation waters are indirect as they relate to 

 soils and plant growth, it is not practical to set 



narrow limits. Water having pH values in the 

 range of 4.5 to 9.0 should not present any insur- 

 mountable problems assuming that no indirect 

 limitations develop resulting from its use. 



An imbalance of common nutrient elements can 

 create an unfavorable environment for plant 

 growth. Among the common ions which are essen- 

 tial for plant growth in relatively large quantities, 

 there is a wide variation in their effect upon spe- 

 cific crops according to their total and relative 

 concentrations. Essential ions such as calcium, 

 magnesium, potassium, and sulfate may deter 

 growth if the total or relative concentrations are 

 out of balance. Plants vary in their tolerance of 

 high concentrations of calcium in the soil solution. 

 Masaewa (103) found that both calcium chloride 

 and calcium nitrate were more toxic to soil cul- 

 tures of flax than added sodium chloride. Wad- 

 leigh and Gauch (184), however, found some spe- 

 cies such as guayule to be more tolerant of added 

 calcium salts than of other neutral salts. Although 

 harmful concentrations of calcium are rare, this 

 illustrates a potentially unfavorable effect of one 

 of the most beneficial ions. Magnesium is fre- 

 quently more toxic than other elements at the same 

 osmotic concentration and potassium may have 

 effects similar to those of magnesium which may 

 be alleviated by the presence of high calcium con- 

 centrations in the substrate. Sulfate has specific 

 deleterious effects on many crops and has been 

 found to limit calcium uptake. Sodium, which is 

 very common in saline waters, affects irrigated 

 crops in many ways. In addition to its effect on 

 soil structure and permeability, sodium has been 

 found by Lilleland, et al. (90) and Ayers (8) to 

 be absorbed by plants and cause leaf bum in 

 almonds, avocados, and in stone fruits grown in 

 culture solutions. Bernstein (16) has indicated 

 that water having SAR ^ values of 4 to 8 may 

 injure sodium-sensitive plants. It is difficult to 

 separate the specific toxic effects of sodium from 

 the effect of absorbed sodium on soil structure. 

 This latter factor will be discussed later. The 

 complex interactions of the total and relative con- 

 centrations of these common ions upon various 

 crops preclude their consideration as individual 

 components for general irrigation use, except for 

 sodium and possibly chlorides in areas where fruit 

 would be important. 



Chlorides are not generally phytotoxic to most 

 crops. For this reason, no limits should be estab- 

 lished because detrimental effects from salinity 

 per se ordinarily deter crop growth first. 



' SAR: Sodium Adsorption Ratio =^ 



pressed as me//. 



Na+ 



|/s^ 



Mg" 



155 



