2T4 HOW CROPS FEED. 



not having been absorbed entirely by the plants. Observe, 

 however, what a remarkable coincidence exists between 

 the ratios of supply of nitrogen in form of a nitrate and 

 those of growth of tlie 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 i*ecently made by the process of water- 

 culture {H. 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 ajjpreciable 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 as 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 aii-- 

 dry and in some cases highly impure jieat, was 1.4 ]ier 

 tent. This nitrogen belongs to the (>rL,'a-)ir ni;ittcrs in 



