266 
Journal of Agricultural Research 
Vol. XXI, No. 4 
Where this problem has been investigated on desert or irrigated land 
it has been the consensus of opinion that the condition is due to, or is 
at least associated with, the occurrence of sodium carbonate or “black 
alkali” in the soil solution. As a result of the present investigation we 
have reached the conclusion that the trouble is not due to sodium car¬ 
bonate alone but that it may be induced by the so-called neutral salts 
of sodium as well. 
It is well known that if a soil containing clay is leached or irrigated 
with water containing sodium carbonate, even in very dilute solution, 
the rate of percolation will become greatly retarded. Sharp 1 observed 
-a similar retardation in the rate of percolation as a result of leaching 
with pure water soils to which sodium chlorid and sodium sulphate had 
been added. We have found that when a soil treated with sodium 
chlorid or sodium sulphate is leached with pure water or with solutions 
of these salts much more dilute than the soil solution, the rate of perco¬ 
lation is retarded and the percolate shows a strongly alkaline reaction. 
In other words, in leaching from the soil the neutral salts of sodium the 
permeability of the soil is reduced and the soil solution is made alkaline. 
These are the characteristic features of land that becomes “hard” 
after a period of irrigation and of salty land that has been reclaimed by 
drainage. 
We have also observed in these investigations that this injurious 
effect of the salts of sodium is much reduced if the salty soil is first 
leached with a solution of some salt of calcium or aluminum. A soil so 
treated may subsequently be leached with pure water without devel¬ 
oping the symptoms of hard land, namely, reduced permeability and an 
alkaline percolate. Similarly we have found that if the irrigation 
water contains calcium salts equal to or exceeding in quantity the salts 
of sodium, the soil rendered salty by such water is less likely to become 
impermeable when subsequently leached with pure water or with water 
containing these salts in the same proportion but in lower concentration. 
These observations lead us to the conclusion that the quality of irri¬ 
gation waters should be judged not only by considering the total quan¬ 
tity of the salts in solution or the proportions of the acid radicles, but 
also the proportion of the sodium to the calcium and magnesium. It 
is suggested that water, to be safe for long-continued irrigation, should 
be relatively rich in calcium and magnesium. 
In colloquial language, waters containing relatively large quantities 
of calcium and magnesium are known as “hard” waters. The results 
of the present investigation may be summarized in the simple state¬ 
ment: Hard water makes soft land and soft water makes hard land. 
1 Sharp, I,. T. fundamental interrelationships between certain soluble salts and colloids. 
In Univ. Calif. Pub. Agr. Sci., v. i, no. io, p. 291-339, 3 fig. 1916. Bibliographical footnotes. 
