234 HOW CROPS FEED. 
and 100 grams of the solution left on evaporation 132 
megrms. of residue. The same amount of humus extract, 
that had been kept in a contiguous vessel containing no 
plant, left a residue of 186 mgrms. The disappearance of 
humus from the solution is thus mostly accounted for by 
‘its oxidation. 
De Saussure considers that his experiments demonstrate 
that humic acid and (in his third exp.) the matters ex- 
tracted from peat by water (crenic and apocrenic acids) 
are absorbed by plants. Wiegmann and Polstorf attrib- 
ute any apparent absorption in their trials to the una- 
voidable errors of experiment. The quantities that may 
have been absorbed were indeed small, but in our judg- 
ment not smaller than ought to be estimated with certainty. 
Other experiments by Soubeiran, Malaguti, and Mulder, 
are on record, mostly agreeing in this, viz., that agricul- 
tural plants (beans, oats, cresses, peas, barley) grow well 
when their roots are immersed in, or watered by, solutions 
of humates, ulmates, crenates, and apocrenates of ammo- 
nia and potash. These experiments are, however, all un- 
adapted to demonstrate that humus is absorbed by plants, 
and the trials of De Saussure and of Wiegmann, and Pol- 
storf, are the only ones that have been made under condi- 
tions at all satisfactory to a just criticism. These do not, 
perhaps, conclusively demonstrate the nutritive function 
of humus. It is to be observed, however, that what evi- 
dence they do furnish is in its favor. They prove effec- 
tually that humus is not injurious to plants, though Liebig 
and Wolff have strenuously insisted that it is poisonous. 
Let us now turn to the probabilities bearing on the 
question. 
In the first place there are plants—those living in bogs 
and flourishing in dung-heap liquor—which throughout 
the whole period of their growth must tolerate, if not ab- 
sorb, somewhat strong solutions of humus. 
Again, the cultivated soil invariably yields some humus 
