62 
IOWA ACADEMY OP SCIENCES. 
the roots of plants to the moisture contained in them — which 
is here called sap-acidity — probably generally falls within, and 
not very far within, one per cent, calculated as crystalized citric 
acid. Citric acid is chosen to express the acidity, partly on 
account of its being an organic acid, and in that sense kindred 
to other root- sap acids; partly because it is the acid generally 
used by those who have attempted to determine the available 
phosphoric acid in manures by means of weak acid, in particu- 
lar by Tollens, Stutzer, A. Thompson, and L. Wagner; and 
partly because it is at hand in every agricultural laboratory in a 
state of purity, and therefore a convenient acid. On the whole 
these sap- acidity determinations, however desultory and imper- 
fect in a scientific sense, seemed to confirm the wisdom of 
Stutzer in adopting (Tollens had suggested various strengths) 
a one per cent solution of citric acid as a standard test of the 
availability of phosphates in manures, though he appeared to 
have lighted on that strength by experimental enquiry based on 
quite other grounds.” 
The several plots of soils which Dyer examined by the method 
indicated, had each received the same fertilization continuously 
for thirty-eight years — some phosphatic, some potassic and 
some mixed; all had been cropped continuously with barley. 
The soils were sampled to a depth of nine inches. Two hundred 
grammes of the air dried soil were treated with 2,000 c. c. of 
the one per cent solution of citric acid, and left in contact there- 
with for seven days, with frequent shaking. For comparison 
with the citric acid results, determinations of potash and phos- 
phoric acid soluble in strong hydrochloric acid were also made. 
The averages for phosphoric acid were as follows: 
Percentage of total phosphoric acid-— soluble in strong HCl. 
— in the eight plots receiving no phosphates, was .1U6; in the 
eight plots receiving phosphates, .178. These numbers are 
nearly in the ratio 1 : 1.7. Percentage of phosphhoric acid dis- 
solved by citric acid from the eight plots receiving no phos- 
phates, was .00078; from the eight plots receiving phosphates 
. 0463, These figures are in the ratio of nearly 1 :6. ‘ ‘The difference 
in the percentages of the phosphoric acid soluble in dilute citric 
acid is thus comparatively overwhelming.” 
Striking as these results are, those obtained in the potash 
determinations are even more so. The average percentage of 
HCl- soluble potash in the eight plots receiving no potash, was 
.195; in the eight plots receiving potash, .266. These figures 
