274 HOW CROPS FEED. 
not having been absorbed entirely by the plants. Observe, 
bowever, what a remarkable coincidence exists between 
the ratios of supply of nitrogen in form of a nitrate and 
those of growth of the several crops, as exhibited in the 
last two columns of the Table. Nothing could demon- 
strate more strikingly the nutritive function of nitric acid 
than these admirable investigations. 
Of the multitude of experiments on vegetable nutrition 
which have been recently made by the process of water- 
culture (ZZ C. G., p. 167), nearly all have depended upon 
nitric acid as the exclusive source of nitrogen, and it has 
proved in all cases not only adequate to this purpose, but 
far more certain in its effects than ammonia or any other 
nitrogenous compound. 
§ 6. 
NITROGENOUS ORGANIC MATTERS OF THE SOIL. 
AVAILABLE NITROGEN.—QUANTITY OF NITROGEN 
REQUIRED FOR CROPS. 
In the minerals and rocks of the earth’s surface nitrogen 
is a very small, scarcely appreciable ingredient. So far as 
we now know, ammonia-salts and nitrates (nitrites) are 
the only mineral compounds of nitrogen found in soils. 
When, however, organic matters are altered to humus, 
and become a part of the soil, its content of nitrogen ac- 
quires significance. In peat, which is humus compara- 
tively free from earthy matters, the proportion of nitrogen 
is often very considerable. In 32 specimens of peat ex- 
amined by the author (Peat and its Uses us Fertilizer and 
Fuel, p. 90), the nitrogen, calculated on the organic mat- 
ters, ranged from 1.12 to 4.31 per cent, the average being 
2.6 per cent. The average amount of nitrogen in the air- 
dry and in some cases highly impure peat, was 1.4 per 
cent. This nitrogen belongs to the organic matters in 
