14 BULLETIN 83. 
The two most direct services rendered by the drains are, first, 
the removal of surplus water; second, the elimination of sodic 
carbonate from the soil. The scope of this bulletin will not per¬ 
mit any further discussion of these subjects, besides we are con¬ 
vinced that the facts are more important than any attempt to 
explain them would be. 
Repeated examinations have failed to show the presence of 
more than traces of phosphoric acid in the drain and ground 
waters. This is in marked contrast with the aqueous extracts of 
some of the soils. The importance of this is that this very valu¬ 
able, and for our soils particularly desirable substance, is held 
pretty firmly within the soil, and though the other salts are in¬ 
volved in probably many changes of solution, this substance re¬ 
mains held by the soil particles and is given up under the influence 
of the plant whose needs it is to supply. How it is held, I do not 
pretend to say, but we know that it must be retained in some way, 
for we know that carbonated waters will extract it from the rocks 
in which it occurs. Phosphoric acid occurs in the soil in which 
there is both water and carbonic acid, and yet the water within 
the soil and that which drains out of it carry no more than traces 
of it. 
The exhaustion of the fertility of our soils by the drain waters 
proceeds then very slowly, so far as the potash and the phosphoric 
acid is concerned. The former is removed by these means more 
rapidly than the latter, both in absolute and relative quantities. 
An acre of good soil taken to a depth of one foot contains about 
78,750 pounds of potash and about 9,000 pounds of phosphoric 
acid. The drain waters contain easily determinable quantities of 
potash and only traces of phosphoric acid, for the detection of 
which we have to use large quantities of water or the residues rep¬ 
resenting it. If we should take a larger quantity of felspar which 
occurs in these soils, grind and treat it with water and carbonic 
acid, we could find upon examining the water, after it had been in 
contact with the felspar for a few days, that it contained easily 
determinable quantities of phosphoric acid. Why then do the 
ground and drain waters contain none or only a trace of it? We 
answer this question by appealing to the observed property of the 
soil particles in mass to retain certain salts, which we have 
seen illustrated in a very marked degree in the case of sodic sul¬ 
fate which we found present in the ground water of the soil in 
large quantities, but as good as absent or wholly so in the drain 
waters. 
The claim often presented, that we add a significant quantity 
of fertilizing ingredients with our irrigation waters, cannot be se¬ 
riously urged for them. Almost the only good they do is in sup¬ 
plying moisture to the plants. Even such waters as have been 
