306 HOW CROPS FEED. 
that 99 per cent and more of the soil, exclusive of water, 
does not in the slightest degree contribute directly to the 
support of the present vegetation of our ordinary field 
products. 
The hay crop is one that takes up and removes from 
the soil the largest quantity of mineral matters (ash- 
ingredients), but even a cutting of 2} tons of hay car- 
rics off no more than 400 Ibs. per acre. From the 
data given on page 158, we may assume the weight of 
the soil upon an acre, taken to the depth of one foot, 
to be 4,000,000 Ibs. The ash-ingredients of a heavy 
hay crop amount therefore to but one ten-thousandth of 
the soil, admitting the crop to be fed exclusively by the 
12 inches next the surface. Accordingly no less than 100 
full crops of hay would require to be taken off to consume 
one per cent of the weight of the soil to this depth, We 
confine our calculation to the ash-ingredients because we 
have learned that the atmosphere furnishes the main sup- 
ply of the food from which the combustible part of the 
crop is organized. Should we spread out over the surface 
of an acre of rock 4,000,000 Ibs. of the purest quartz 
sand, and sow the usual amount of seed upon it, maintain- 
ing it in the proper state of moisture, etc., we could not 
produce a crop; we could not even recover the seed. Such 
a soil would be sterile in the most emphatic sense. But 
should we incorporate with such a soil a few thousand 
Ibs. of the mineral ingredients of agricultural plants, to- 
gether with some nitrates in the appropriate combinations 
and proportions, we should bestow fertility upon it by this 
addition and be able to realize a crop. Should we add to, 
our acre of pure quartz the ashes of a hay crop, 400 lIbs., 
and a proper quantity of nitrate of potash, we might also 
realize a good crop, could we but ensure contact of the 
roots of the plants with all the added matters, But in 
this case the soil would be fertile for one crop only, and 
after the removal of the latter it would be as sterile as 
