CHAPTER IV. 
> 
NITROGEN. DECOMPOSITION OF NITROGENOUS 
COMPOUNDS. 
The nitrogenous foods of plants are next in importance to carbon 
dioxid and water. Plants cannot grow without nitrogen, and they 
need it in larger quantity than any other mineral foods. The ni- 
trogenous fertilizers have commonly a more noticeable effect in 
stimulating crops than other minerals. The amount of material 
in the world that can serve directly as nitrogen food for plants is 
decidedly limited, and therefore it is expensive. For these as well 
as other reasons, the problem of continued soil fertility is more 
closely bound up with the matter of nitrogen than any other chemical 
element. 
SOURCES OF NIROGENOUS FOOD. 
Plants take their nitrogen from the soil, chiefly in the form 
of nitrates. While it is true that they can utilize ammonium 
salts also, under ordinary conditions ammonia furnishes little 
food directly to the plant, the far larger part being furnished 
by soil nitrates. The amount of nitrate in any soil is however, 
very limited, there being only from o.i per cent, to 0.2 per cent, 
in ordinary soils. As crop after crop is grown, the small amount in 
the soil is gradually used up and must be replaced if the soil is to 
continue yielding crops. The farmer buys nitrates in the form of 
commercial fertilizers to replace the amount taken from his soils by 
his crops. These commercial nitrates, however, are also limited 
in amount. They are confined to a few deposits of nitrates, chiefly 
in warm dry regions. The best known come from Chili, whose 
nitrate mines to-day furnish the greater part of the nitrates for the 
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