to extrapolate from these sand and solution cul- 

 tures to soil conditions. Toxic limits determined in 

 solution cultures might apply to irrigation water 

 if it were not for the fact that soil conditions could 

 influence the element's availability to the plant. 



Comprehensive reviews of literature dealing with 

 trace element effects on plants have recently been 

 published {20, 35, 105). Another reference {68) 

 deals with reactions of trace elements in soils. 

 Additional research is needed to predict reactions 

 between ions in irrigation water and various soil 

 types, and the resultant effect on various plant 

 species. 



In developing a workable program of acceptable 

 limits for trace element pollution of irrigation 

 waters, three considerations should be recognized : ^ 



( 1 ) The inherent difficulty of establishing gen- 

 eralizations. Many factors affect the uptake 

 of and tolerance to trace elements. The 

 most important of these being genetic 

 variability of plants and animals, reactions 

 within the soil, and nutrient interactions, 

 particularly in the plant. 



(2) A system of tolerance limits must, to the 

 greatest extent possible, provide sufficient 

 flexibility to cope with the more serious 

 factors above. 



(3) At the same time, restrictions must be 

 defined, as precisely as possible. 



To translate these considerations into workable 

 recommendations, two types of soil groupings that 

 may be irrigated are defined: 



(a) Lands having a significant fraction of well- 

 drained soils classified as sands, loamy 

 sands, or sandy loams. 



(b) Lands made up principally of finer textured 

 soils and generally more slowly drained. 



Individual minor element limits for water to 

 be used on type 'a' lands are calculated assuming 

 that steady state may be approached in a relatively 

 short period of time and, therefore, that the con- 

 centration in irrigation waters approximates that 

 of the soil solution. In areas where irrigation 

 water accounts for most of the water applied to a 

 field, the values may have to be adjusted down- 

 ward to allow for concentration in the soil. 



Upper limits that may be set for minor element 

 tolerances in water for type 'b' lands are somewhat 

 more arbitrary. They are drawn largely from maxi- 

 mum safe fertilizer additions that might be applied 

 to soils under the most favorable conditions for 

 fixing the element in the soil. The term "short 

 time" used in table IV-15 means a period of 

 time as long as two decades. 



It is beyond the scope of this report to present 

 a critical literature review on phytotoxic ions. 

 Some references are cited to illustrate both the 

 importance and the complexity of the problem. 

 Emphasis must be placed, however, on research 

 needs. Due to the vast scope of the problem, it 

 is recommended that research be initiated as prob- 

 lems arise to derive specific recommendations. 

 The following list of trace element effects indicate 

 in part the potential problem and suggested trace 

 element tolerances for irrigation waters are shown 

 in table IV-15. 



Aluminum: Aluminum toxicities to plants have 

 been reported for both acid and alkaline condi- 

 tions, but are probably of little consequence at 

 near-neutral pH values. One milligram per liter 

 is taken as the tolerance limit, even though sev- 

 eral reports of toxic effects have been observed 

 at 0.5 mg/1 {35). The reason for this is that even 

 sandy soils could be expected to reduce aluminum 

 toxicities somewhat and management practices 

 could be used to avoid marginal toxicities. 



Arsenic: Arsenic may be present in fairly high 

 concentrations without inducing injury to some 

 plants such as lemons and sudan grass (55), but 

 toxic effects on other species have been observed 

 down to 1 mg/1 {105). This value is selected as 

 the tolerance level here, but a better understand- 

 ing of the effects of management practices on 

 the uptake of arsenic might indicate that a higher 

 value could be used. 



TABLE IV-15. Trace Element Tolerances for 

 Irrigation Waters 



For short-term use 



on fine textured 



soils only 



^ Basic information on trace elements was supplied by 

 J. F. Hodgson of the U.S. Soil, Plant, and Nutrition 

 Laboratory, Ithaca, N.Y. 



mg/1 



Aluminum LO 



Arsenic 1.0 



Beryllium 0.5 



Boron 0.75 



Cadmium 0.005 



Chromium 5.0 



Cobalt 0.2 



Copper 0.2 



Fluorine Q) 



Iron O 



Lead 5.0 



Lithium 5.0 



Manganese 2.0 



Molybdenum 0.005 



Nickel 0.5 



Selenium 0.05 



Tin O 



Tungsten Q) 



Vanadium 10.0 



Zinc 5.0 



mg/l 



20.0 



10.0 



1.0 



2.0 



0.05 



20.0 



10.0 



5.0 



O 



Q) 



20.0 



5.0 



20.0 



0.05 



2.0 



0.05 



(') 



10.0 

 10.0 



' See text. 



152 



