Beryllium: Beryllium is toxic to both animals 

 and plants. Growth of beans has been inhibited at 

 0.5 mg/1 (20), and this value is selected as the 

 tolerance limit. Beryllium toxicity will be moder- 

 ated by reaction with soils, but because it repre- 

 sents a relatively serious problem, its limits for 

 water use even for type 'b' land should be restricted 

 to 1 mg/1 until better information is available on 

 its uptake from soils. 



Boron: Boron is an essential plant micronu- 

 trient almost up to concentrations of 0.5 mg/1 

 in irrigation water. However, boron in irrigation 

 water has caused destruction of, or damage to, 

 sensitive crops when concentrations in the irri- 

 gation water are somewhere between 0.5 and 1.0 

 mg/1. Most of the work on boron, done under the 

 leadership of Eaton (49), found a range of toler- 

 ance of crops, as shown in table IV-16. Water 

 containing more than 4 mg/1 of boron is generally 

 unsatisfactory for all crops. In general, sensitive 

 crops will show slight to moderate injury at boron 

 levels of 0.5 to 1.0 mg/1; semitolerant, 1.0 to 2.0 

 mg/1; and tolerant crops, 2.0 to 4.0 mg/1. In terms 

 of content in the soil saturation extract, a limit 

 of 0.7 mg/1 of boron is considered safe. Probably 

 the effect varies inversely with the percentage of 

 applied water that is passed through the root 

 zone, but this has not been evaluated. Most prob- 

 lems of excess boron have been encountered in 

 waters derived from the coast range mountains 

 of California and from the Hot Creek area in the 

 Owens Valley of California on the eastern slope 

 of the Sierra Nevada Mountains. 



Cadmium: Cadmium toxicities have been impli- 

 cated in hypertensive diseases of man. Its contri- 

 bution to pollution, therefore, bears very close 

 scrutiny. Present understanding of the problem is 

 sufficiently limited so that cadmium will definitely 

 require reappraisal as evidence accumulates for 

 or against its toxicity and as understanding of its 

 behavior in the soil-plant-animal chain is im- 

 proved. The tolerance Hmit of 0.005 mg/1 is sug- 

 gested assuming: (1) reported toxicities are valid 

 (88, 149, 150); (2) cadmium behaves similarly 

 to zinc in its uptake by plants and reactions with 

 soil. It is very likely that higher levels of cadmium 

 could be regulated by appropriate management 

 practices; but in the absence of yield depression, 

 farmers have little inducement to employ such 

 practices. 



Chromium: Both the chromic and chromate 

 ions display toxicities. Use of large amounts of 

 chromium for processes, such as tanning, increase 

 the importance of controlling this element. 



Tolerance to the two ions varies with plant 



species, but more sensitive plants are adversely 

 affected at about 5 mg/1 for each ion (35). 



The chromic ion could be expected to combine 

 fairly strongly with neutral soils so that class 'b' 

 soils could likely tolerate considerably more 

 chromic ion than the above value. There is not 

 sufficient information available on chromate ion 

 to recognize the presence of type 'b' situations for 

 this ion. Furthermore, the possibility of oxidizing 

 chromic to chromate ions is too great to include 

 water of a higher chromium content for irrigating 

 alkaline soils of type 'b' lands even when the chro- 

 mic ion is present in the water, until further in- 

 formation is available. 



Cobalt: Cobalt toxicities have been observed on 

 several species grown in sand culture. On the other 

 hand, field occurrence, of cobalt toxicity is rare. 

 The tolerance limit suggested here is somewhat 

 higher than the 0.1 mg/1 cobalt which has been 

 observed to be toxic to tomato plants. It is felt 

 that management practices should be capable of 

 relieving marginal toxicities of this element (105). 



Copper: Copper toxicities have been observed 

 at copper concentrations as low as 0.1 mg/1 in 

 nutrient solution (35, 105). A value of twice this 

 is taken for the tolerance limit for water for type 



TABLE IV-16. Relative Tolerance of Plants to 

 Boron (181) 



[In each group, the plants first named are considered as being 

 more tolerant and the last named more sensitive.] 



153 



