IOWA ACADEMY OF SCIENCES. 
65 
condition or constitution, rather than chemical, seems to be the 
controlling factor in the case of the ‘ ‘poorest” of the first two soils 
of the list — marked A. This soil contains decidedly more citric 
acid- soluble Pg 0^, Kg O, and N, than does its mate marked 
“best”; also more of these elements soluble in HCl, except PgO^ 
— and respecting this the difference is only slight. 
In order to find the explanation in this case — the only excep- 
tional one in the list — I made inquiry of the sender (Mr. J. O. 
Overholt, of Havelock, Pocahontas county) to which he replied; 
‘ ‘The sample marked ‘poorest, ’ is soil that needs tile drainage, 
and on wet years is difficult to farm. The one marked ‘best’ 
is high land, and is productive every year, wet or dry.” These 
facts explain the apparent anomoly, and forcibly illustrate the 
necessity of considering the physical as well as the chemical 
constitution of soils, in studying the conditions affecting fer- 
tility. 
The tentative conclusions to which Dyer was led by his work 
on the Rothamsted soils are as follows : “It would perhaps not 
be unreasonable to suggest that, when a soil is found to contain 
as little as about .01 per cent of phosphoric acid soluble in a one 
per cent solution of citric acid, it would be justifiable to assume 
that it stands in immediate need of phosphatic manure.” Con- 
cerning potash he says it is difficult ‘ ‘ to draw from the figures 
any fairly plausible suggestion as to what percentage limit of 
citric- acid- soluble potash should be regarded as marking the 
non-necessity of special potash applications. Probably this 
limit lies below .005 per cent.” How far below he does not 
venture an opinion. Nor can we draw any more definite con- 
clusion from the results on the Iowa soils. So far as they bear 
witness, however, they seem to justify Dyer’s tentative conclu- 
sions. Evidently it will require much careful work — chemical 
work correlated with field observations — to fix the exact limits 
below which the several elements may, with certainty, be 
declared deficient for the porposes of immediate crop production; 
and doubtless any such limits will have to be qualified by con- 
ditions as to the physical constitution of the soil. 
Although much remains to be done in the future along this or 
some similar line of research, before the desired end will be 
attained, sufficient has already been learned to justify the 
opinion that the citric acid method of soil analysis, as pro- 
posed by Dyer, is a distinct advance upon the method commonly 
in use for ascertaining deficiencies in available phosphoric acid 
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