May 16, 1921 
271 
Quality of Irrigation Water 
It will be noted from Table I that the alkalinity and turbidity of the 
percolate continued through many leachings after the use of distilled 
water was resumed, while the chlorids quickly disappeared. The rate 
of percolation also declined sharply when the leaching with distilled 
water was resumed. 
It is probable that the course of events indicated in part in the experi¬ 
ment just described may be explained as a partial change of the bases 
in the silicates of the clay in the soil. It is assumed that some of the 
sodium displaces some of the calcium in these silicates and that the 
calcium passes out as calcium chlorid. The resultant sodium silicates 
remain insoluble in the presence of such strong acid radicles as the 
sulphates and chlorids, but as these are leached away the silicates become 
soluble and by their high viscosity retard the rate of percolation. 
A similar substitution of bases is well known and widely used in the 
so-called zeolite process of water softening. In this process an artificial 
zeolite, rich in sodium, is used. The “hard” water to be treated is 
leached through the zeolite with the result that the lime is absorbed and 
some of the sodium is given up. From time to time the zeolite is re¬ 
stocked with sodium and the absorbed calcium is replaced by leaching 
it with a strong salt solution. 
It has been shown by Kelley and Cummins 1 that such a substitution 
of bases takes place in the soil, presumably in the silicates, when soils 
are digested with dilute salt solutions. They found that calcium is ex¬ 
tensively replaced by sodium and that sodium is replaced by calcium, 
though in the dilutions and with the soils with which they worked the 
latter reaction was much less marked than the former. The final para¬ 
graph of their conclusions is sufficiently significant to warrant quotation. 
It is suggested that the continued addition of soluble salts in the open field where 
the products of the reactions are removed by either the growth of crops or intermittent 
leaching must ultimately result in building up a chemical system different from that 
originally present. As will be shown later, the physical properties of the system 
also may be materially altered. 
EFFECT OF CALCIUM SALTS 
It was remarked above that the salts of calcium and of aluminum 
react differently in relation to subsequent leachings with distilled water 
than the salts of sodium. The use of gypsum (calcium sulphate) has 
been widely recommended as a corrective for difficulties with black 
alkali. These recommendations are usually based on the assumption 
that calcium sulphate reacts with the sodium carbonate in the soil solu¬ 
tion to form calcium carbonate, which is very slightly soluble, and 
sodium sulphate, which is one of the least toxic of the sodium salts. 
It is well established by numerous field experiments that gypsum 
exerts a beneficial effect when used on hard land. It is not very soluble, 
1 Kelley, W. P., and Cummins, A. B. chemical effect of salts on soils. In Soil Sci., v. n, no. a, 
p.139-159. 7 fig- 1921. References, p. 158-159- 
