SALINITY FACTOR 283 
TABLE 1 
Permissible Limits of Boron for Several Classes of Irrigation Waters (5) 
Boron Class Sensitive Crops Semitolerant Crops Tolerant Crops 
(ppm) (ppm) (ppm) 
Excellent <0. 33 <0.67 <1.00 
Good 0.33 to 0.67 0.67 to 1.33 I.00 to 2.00 
Permissible 0.67 to 1.00 TG GEEOLZEOO 2.00 to 3.00 
Doubtful I.00 to 1.25 2.00 to 2.50 3.00 to 3.75 
Unsuitable SiO 22.59) = B.915 
Boron is a minor constituent of practically all natural waters, 
and irrigation waters should be analyzed for this element if there 
is any reason to suspect its presence at toxic levels. Although 
boron is an essential microelement for plant growth, it may be 
toxic at concentrations only slightly in excess of those needed for 
optimum growth. Toxicity may develop with boron-sensitive 
crops when the concentration is as low as 1 ppm but, for most 
crops, water containing I to 2 ppm is satisfactory, and waters up 
to 3 ppm may be used with the more boron-tolerant crops. Water 
containing in excess of that amount of boron is, in most cases, 
unsuitable for irrigation purposes. The permissible limits of boron 
for several classes of irrigation waters considered on the basis of 
the relative sensitivity of the crop to boron are given in Table 1. 
Characteristics of Return-Flow Water 
The pollution of streams by irrigation residues and the charac- 
teristics of return-flow water have been discussed by Scofield 
(3), Howard (1) Wilcox (6), and others. 
In some cases, the water available for irrigation is unfit for 
irrigation use in its natural state, owing to unsatisfactory quality, 
but there are other instances where the quality of the irrigation 
supplies has been impaired by drainage and return flow down- 
stream. The major effects of use and reuse of irrigation waters as 
related to quality are (1) an increase in the total amount of 
dissolved solids, (2) the loss of calcium, magnesium, bicarbonate, 
and sulfate by precipitation, and (3) an increase in the quantity 
and proportion of sodium and chloride in the water. 
