BULLETIN 318 TH E EFFECTS OF ALKALI ox CITRUS TREES 325 



Chlorides are undoubtedly the most injurious constituents that 

 occur in the irrigation waters of southern California, but it must not 

 be concluded that other alkali salts can be ignored. In certain locali- 

 ties some of the waters also contain injurious amounts of sulfates and 

 carbonates. Hilgard says, "Unfortunately it is not easy to give abso- 

 lute rules in regard to the exact figures that constitute an excess of 

 salts for irrigation purposes, since not only the composition of the salts, 

 but the nature of the land to be irrigated, and the frequency of irriga- 

 tion required, must be taken into consideration." He concluded that 

 40 grains per gallon (684 parts per million) is the maximum amount 

 of the total mineral matter a water can safely contain, unless the 

 mineral constituents be composed largely of gypsum. But he also 

 said: "When a large proportion of the solids consists of carbonate of 

 soda or common salt, even a smaller proportion of salts than 40 grains 

 might preclude its regular use." We regret to state that a consider- 

 able number of irrigation waters contain salts in excess of this limit, 

 some of them greatly so. 



It has also been shown that the continued application of nitrate 

 of soda may bring about an increase in the amounts of alkali in the 

 soil. It is true, the amount of nitrate that has been applied to the 

 experimental plot we have studied is excessive and the results might 

 have been different on other soil or even on the same soil, had it been 

 treated differently in other respects. If nitrate of soda be applied 

 in moderate quantities to a porous soil, especially where the rainfall 

 is heavy, or if used in conjunction witli manure and cover crops, it 

 is highly probable that the sodium will be leached more deeply than 

 it has been on the experimental plot at Riverside. 



However, it is especially important for farmers to understand 

 clearly that citrus trees, as well as many other crops, do not absorb 

 more than very limited amounts of sodium, and that wherever nitrate 

 of soda be applied, either in combination with other fertilizer con- 

 stituents or alone, the sodium will, for the most part, be rejected by the 

 crop and left in the soil. This sodium remains mostly in soluble form 

 and contributes directly to the alkali content of the soil. 



All sodium salts are really alkali salts. As has been repeatedly 

 stated, small amounts of none of them art' injurious, and in the case 

 of the nitrate, limited amounts undoubtedly stimulate the growth of 

 practically all crops; but unless drainage effectively leach the salts 

 from the soil, the continued application of sodium nitrate, or of saline 

 irrigation water, will ultimately produce a harmful concentration of 

 alkali in anv soil. 



