644 
Journal of Agricultural Research 
Vol. XXVII, No. 9> 
bicarbonate. On the other hand, it has been observed that certain sam¬ 
ples of underground water when first taken show no phenolphthalein 
reaction, but if these are allowed to stand for a few hours exposed to the 
air they give a strong reaction for normal carbonate. 
One of the practical difficulties in titrating soil solutions is encoun¬ 
tered when the solution is strongly colored or turbid, due to the presence 
of dissolved or suspended organic matter. With such solutions the end 
point of the indicator reaction is not easy to see. It has been found 
that this difficulty may be overcome by placing the solution to be 
titrated in a bottle which can be corked and shaken after each addition 
of acid. The shaking causes the formation of foam which persists at 
least momentarily after the shaking stops. The indicator colors can be 
seen in this foam quite as sharply as in a clear solution. 8 
The discussion of this subject of the determination of the carbonate- 
bicarbonate constituents of the soil solution may be summarized by 
pointing out that these determinations are at least difficult to interpret. 
It is regrettable that this is so because these constituents are known to 
be closely associated with some of the most important reactions that 
take place in irrigated soils. They are also regarded as highly toxic to 
crop plants. 
NITRATE 
The nitrate content of the solution is determined by the use of phenol- 
disulphonic acid with which the nitrate reacts to produce a yellow color 
when the solution is made alkaline, as with ammonia. A comparison 
of the unknown solution with solutions of known strength affords a 
basis for estimating the nitrate content. 
SILICA 
Silica in solution may be precipitated by evaporating to dryness, 
acidulating with hydrochloric acid, evaporating again, and finally taking 
up the soluble residue with dilute acid. The silica may be separated 
from the other constituents of the solution because it does not redissolve 
in an acid solution after being dried. 
In making a silica determination with soil solutions it is essential to 
free the solution of all suspended matter. In the water extracts of 
irrigated soils and in some turbid irrigation waters silica is an important 
constituent of the suspended matter. In this condition it is not, how¬ 
ever, a part of the true solution. 
STATING THE RESULTS OF ANALYSES 
From what has been said on the preceding pages it is obvious that in 
reporting the results of analyses of samples of the soil solution or of 
irrigation or drainage waters one can not with certainty report the salts 
as such. The analysis permits the identification of certain elements or 
ions, but gives no clue as to how these are combined if at all. Indeed 
such evidence as we have would indicate that in the dilutions with 
which we commonly have to deal, these constituents exist largely in a 
dissociated condition. 
8 The writer is indebted to J. F. Breazeale for this detail of technique. 
