690 
Journal of Agricultural Research voi. xxvn, No. 9 
These two experiments show very definitely that the addition of 
sulphuric acid to the soil solution in the soil does not give the results 
that would be expected if the soil were not present. They tend to 
confirm the view that the soil is not to be regarded as an inert mass 
in which a solution is suspended by capillarity, as water is held in a 
sponge, but rather that reactions are going on continually between 
substances in the solution and substances combined with the soil. 
The present hypothesis is that the sodium that causes soil defloccu¬ 
lation and impermeability is the sodium that is combined with the soil 
and not the sodium that is in the soil solution. If this is correct, the 
treatment designed to flocculate the soil and improve its permeability 
should be aimed to replace the combined sodium from the soil by getting 
it into the solution and then to remove the solution. The difficulty 
can not be remedied by merely removing the solution. 
A soil may be rich in combined or replacable sodium and consequently 
impermeable and at the same time contain a large proportion of calcium 
as calcium carbonate. With such a soil it is to be expected that any 
treatment that would bring into solution the calcium of the calcium 
carbonate would favor the reaction by which the combined sodium 
would be replaced from the soil and thus permit its removal by leaching. 
If a soil is found to be deficient in combined calcium, it may be necessary 
to treat it with calcium or some other base, preferably in the form of a 
soluble salt. In any case, the removal of combined sodium from the 
soil appears to require the substitution for it of another base, and if the 
soil is to be flocculated and its condition improved the substitution 
must be made by an earthy base. 
The removal of sodium from the soil solution under field conditions 
is not a difficult matter if the soil has not absorbed so much sodium 
as to become impermeable. In other words, it is possible and not very 
difficult to pre^nt the accumulation of harmful quantities of sodium 
in irrigated soils. It is quite another matter to remove combined 
sodium from the soil in the field when as a result of such combination 
the soil has become impermeable to water. In dealing with sodium in 
irrigated soils an old adage may be used even with added emphasis. 
It would not seem extravagant to say that one ounce of prevention 
is worth a thousand pounds of cure. 
SUMMARY 
The effective supply of water that may be stored in the soil to last 
from one irrigation to the next is limited on the one hand by the field- 
carrying capacity and on the other by the inability of the plants to take 
water from the soil beyond what is known as the wilting point. „ 
The soil may hold as much as 6 inches of water per foot of depth, but 
ordinarily its net effective storage capacity is not much above 2 inches 
per foot of depth. 
The subsoil, which is usually more compact than the surface soil, may 
become saturated when it contains 5 inches or less per foot, and the 
addition of 1 inch of water to a saturated subsoil may raise the level of 
the underground water as much as 1 foot. 
When irrigation water is applied to the soil it penetrates in part by 
flowing into cracks and also by way of the small spaces between the 
particles of soil. 
