IOWA ACADEMY OF SCIENCES. 
59 
sometimes dilute, but more frequently quite strong (twenty- 
three to twenty-five per cent), sometimes hot, sometimes cold, 
acting for periods ranging from one to forty -eight hours'^'. 
Now hydrochloric acid of this strength, or anything 
approaching it, is a much more powerful solvent than is the 
root- sap of plants. It is not surprising, therefore, that the 
results obtained by chemisal analysis have not been found relia- 
ble measures of the relative amounts of the different plant-foods 
existing in readily assimilable form in the soils analyzed, except 
possibly in the case of new (virgin) or nearly new soils, where 
a large “total” of any element maybe assumed to indicate a 
large supply of that element in available form, and vice versa. 
(Hilgard.) 
Since the sap of plant roots is mildly acid with vegetable 
acids it seems reasonable, not to say evident, that in our attempts 
to discriminate between the available and the unavailable con- 
stituents of the soil we should imitate nature by employing 
weak solutions of some kind of vegetable or organic acid. But 
what acid, and how weak the solution? These are questions 
that at present need answering. Some work has, however, been 
done on these lines. 
In 1872 H. von Liebigf reported some work on the soils of 
the Rothamsted wheat plots, in which he used “dilute” nitric 
and acetic acids as solvents; but the extent of dilution was in 
neither case stated. Each plot had received annually the same 
treatment as to manuring — the treatment recorded below — for 
nearly thirty years. All had been cropped continuously with 
wheat. The soil samples examined represented the first and 
second depths of nine inches each for each plot. Results were 
as follows: 
*Of sp. gr. 1.115 (=23.4 per cent strength) for thirty-six hours at the boiiing point 
of water. (Ass’n of Offlciai Agricuiturai Chemists, 1893), 
Of 25 per cent strength, at ordinary temperature fpr forty-eight hours; or of 10 
per cent strength at temperature of boiling water for three hours. (Ass’n of Agr. 
Expt. Stations of Germany, 1890). 
Of sp. gr 115 (=30.3 per cent strength), at boiling temperature for one hour. 
Wahnschalfe, Brannt’s translation, 1891). 
Of about 15 per cent strength, on water bath for five hours. (Fresenius.) 
t Quoted in Jour. Oh. Society, London, 1891, p. IIT. 
