316 HOW CROPS FEED. 
er nutritive elements, viz., chlorine and sulphuric acid, in 
the Table, p.311, leads to the question, May not the aqueous 
solution of the soil be altogether lacking in some es- 
sential kinds of mineral plant-food in certain instances? 
May it not happen in case of a rather poor soil that it will 
support a moderate crop, and yet refuse to give up to 
water all the ingredients of that crop that are derived 
from the soil ? 
The weight of evidence supports the conclusion that 
water is capable of dissolving from the soil all the sub- 
stances that it contains which serve as the food of plants. 
The absence of one or several substances in the analytical 
statement would seem to be no proof of their actual ab- 
sence in the solution, but indicates simply that the sub- 
stance was overlooked or was too small for estimation by 
the common methods of analysis in the quantity of solu- 
tion which the experimenter had in hand, It would ap- 
pear probable that by employing enough of the soil and 
enough water in extracting it, solutions would be easily 
obtained admitting of the detection and estimation of ev- 
ery ingredient. Knop, however, asserts (Chem. Central- 
blatt, 1864, 168) that he has repeatedly tested aqueous 
solutions of fruitful soils for phosphoric acid, employing 
the soils in quantities ranging from 2 to 22 Ibs., and water 
in similar amounts, without in any case finding any traces 
of it. On the other hand Schulze mentions having inva- 
riably detected it in numerous trials; and Von Babo, in 
the examination of seven soils, found phosphoric acid in 
every instance but one, which, singularly enough, was 
that of a recently manured clay soil, Inno case did he 
fail to detect lime, potash, soda, sulphuric acid, chlorine, 
and nitric acid; magnesia he d:d not look for. (Hof? 
mann’s Jahresbericht der Ag. Chem., I. 17.) 
So Heiden, in answer to Knop’s statement, found and 
estimated phosphoric acid in four instances in proportions 
