182 HOW CROPS FEED 
that held 150 per cent of water. So the humus of Schiib- 
ler imbibed 181 per cent; the peat of Zenger, 377 per cent ; 
while Wilhelm examined a very porous peat that took up 
519 per cent. These differences are dependent mainly on 
the mechanical texture or porosity of the material. 
The want of capillary retentive power for water in the 
ease of coarse sand is undeniably one of the chief reasons 
of its unfruitfulness. The best soils possess a medium re- 
tentive power. In them, therefore, are best united the 
conditions for the regular distribution of the soil-water 
under all circumstances. In them this process is not hin- 
dered too much either by wet or dry weather. The re- 
taining power of humus is seen to be more than double 
that of clay. This result might appear at first sight to 
be in contradiction to ordinary observations, for we are 
accustomed to see water standing on the surface of clay 
but not on humus. It must be borne in mind that clay, 
from its imperviousness, holds water like a vessel, the wa- 
ter remaining apparent; but humus retains it ivisibly, 
its action being nearly like that of a sponge. 
One chief cause of the value of a layer of humus on 
the surface of the soil doubtless consists in this great re- 
taining power for water, and the success that has attended 
the practice of green manuring, as a means of renovating 
almost worthless shifting sands, is in a great degree to be 
attributed to this cause. The advantages of mulching are 
explained in the same way. 
Soils which are over-rich in humus, especially those of 
reclaimed peat-bogs, have some detrimental peculiarities 
deserving notice. Stoeckhardt ( Wilda’s Centralblatt, 
1858, 2, 22) examined the soil of a cultivated moor in 
Saxony, which, when moist, had an imbibing power of 
60-69" |... After being thoroughly dried, however, it lost 
its adhesiveness, and the imbibing power fell to 26-30" |,. 
It is observed in accordance with these data that such 
yoils retain water late in spring; and when they become 
