'a' lands. Copper combines strongly with most 

 soils as evidenced by the many soils that have been 

 treated for decades with copper sulfate as a fungi- 

 cide without displaying copper toxicities. Only 

 water irrigating sands very low in organic matter 

 would need to remain classified for use on type 'a' 

 lands. Limits for water to be used on type 'b' lands 

 could safely vary up to at least 5 mg/1. 



Iron: Iron is not likely to be a problem with 

 irrigation waters. In those instances where im- 

 balances due to excess iron develop, they can be 

 controlled with management practices. 



Fluoride: The most serious effect of fluoride is 

 not its effect on plant growth, but the ultimate 

 effect on the consuming animal including man. 

 The uptake of fluoride by plants is restricted 

 both by a combination of the element with soils, 

 favored by low pH, and a discrimination against 

 fluoride by plant roots (20). Some plant species 

 do accumulate large amounts of fluoride, but for 

 the most part they are not consumed by man or 

 livestock. The principal pathway for fluoride poi- 

 soning then is through direct imbibition of toxic 

 waters or plant accumulation of fluoride from the 

 air. No limits are proposed at this time, but it 

 is recommended that it be placed in a warning 

 category to be considered as specific cases arise. 



Lead: Results from adding lead to nutrient so- 

 lutions are somewhat contradictory (35, 105). 

 Toxicities have been reported from additions of 

 as little as 1 mg/1. But considerably higher levels 

 have been used in some cases without injury. Since 

 even sandy soils can be expected to adsorb lead, 

 the tolerance limit of 5 mg/1 is proposed. 



Lithium: Crops sensitive to sodium are also 

 sensitive to Hthium. Most crops can tolerate 5 

 mg/1 and this limit is proposed for water to be 

 used on type 'a' lands {18, 20). The same limit 

 is proposed for water to be used on type 'b' lands, 

 since it might be expected that a steady state 

 will be approached within a period of years on 

 most soils. 



Manganese: Manganese toxicities have been ob- 

 served down to 0.5 mg/1, but a great deal af varia- 

 tion occurs among species and conditions of nu- 

 trient imbalance. With suitable management prac- 

 tices, it should be possible to tolerate up to 2 mg/1 

 for nearly all species of plants. 



Molybdenum: Molybdenum presents a particu- 

 larly unique problem in that ground waters fre- 

 quently carry levels of the element that give rise 

 to plant concentrations toxic to cattle. In nutrient 

 solution and soil solution measurements, 0.01 



mg/1 molybdenum in solution will produce leg- 

 umes containing in order of 5 mg/kg molyb- 

 denum or more in the tissue (82). This level is 

 commonly accepted as the upper limit for safe 

 feeding to cattle and is, therefore, proposed as the 

 tolerance limit, even though levels of 0.001 to 

 0.002 mg/1 molybdenum in river waters are not 

 uncommon; and the Colorado River at Yuma, 

 Ariz., is reported at 0.0069 (44). 



An upper limit of 0.05 mg/1 is proposed when 

 the irrigation water is added to acid soils with 

 a large capacity to combine with the element. 

 The reason for this action is to protect against 

 the possibility of inducing molybdenum toxicity 

 at a later date as a result of overtiming in humid 

 and subhumid areas. 



Nickel: Nickel toxicities occur in nature in con- 

 junction with high levels of chromium in soils 

 developed from serpentine rock. These soils may 

 contain 400 to 5,000 mg/kg, compared with 

 about 5 to 100 for most soils (35). Surprisingly, 

 when the occurrence of serpentine-derived soils 

 is considered, few results are available relating 

 nickel toxicity to solution concentrations. Growth 

 of flax is depressed by the presence of 0.5 mg/1 

 nickel and this value is suggested here for a tenta- 

 tive tolerance limit. Examination of more sensitive 

 crops may suggest a lower value. 



Selenium: Tolerance limits for selenium should 

 be based on animal toxicities, rather than those 

 of plants. Plants containing 4 to 5 mg/kg sele- 

 nium are commonly considered to induce toxic 

 symptoms in animals. From results of Broyer 

 (28), this level of selenium could result in many 

 species from a level of 0.05 mg/1 selenium in 

 solution. Tolerance limits will, therefore, be placed 

 at this value. The assumption is made that there 

 will be sufficient management of irrigated lands 

 so that selenium-accumulating plants will not be 

 a factor. Fertilizer trials in greenhouse experi- 

 ments indicate that the same limit might best be 

 applied to water used on type 'b' lands as well. 



Tin, Tungsten, and Titanium: Tin, tungsten, 

 and titanium are effectively excluded by plants. 

 The first two can undoubtedly be introduced to 

 plants under conditions that wiU produce specific 

 toxicities, but not enough is known about any of 

 the three to prescribe tolerance limits at this time. 

 Titanium is too insoluble to be of great concern. 

 Tungsten has been observed to interfere with 

 ascorbic acid metabolism in animals (162). 



Vanadium: Vanadium toxicities have been in- 

 duced in several plant species in concentrations in 

 the neighborhood of 10 mg/1 of the vanadate 



154 



