142 Principles of Plant Culture. 
mosphere in rain and snow, so that it is impossible to ex- 
haust the soil of plant food, even with the most improvident 
culture. But the cultivator should aim at the largest returns 
from his soil, and these are impossible without restoring cer- 
tain materials that continued crop removal invariably re- 
duces below the limit of profitable yields. 
254. The Food Elements Most Likely to be Defi- 
cient, when plants are properly supplied with water, are 
nitrogen, phosphorus and potassium. These are all liberated, 
in greater or less quantities, when vegetable and animal 
material (organic matter) decays in the soil; hence all such 
material has more or less value as fertilizers. But we need 
not wholly depend upon refuse organic matter for fertilizers, 
since the leguminous plants add nitrogen to the soil (260), 
and compounds of nitrogen, phosphorus and potassium can 
often be purchased in artificial fertilizers at prices that place 
them within the reach of the cultivator. 
255. Nitrogen is the Most Important Fertilizing 
Element to the cultivator, because it is liberated in small- 
est amount by rock decay, and is most expensive in the 
market. Nitrogen is chiefly used by plants in the form of 
nitrates, i. e., in combination with certain other substances 
as potash, lime, soda, magnesia and iron. Ammonia, which 
is a gaseous compound of nitrogen and hydrogen, is also 
used to some extent by plants. Free nitrogen, the most 
abundant constituent of the air, plays no direct part in 
plant nutrition. 
256. The Sources of Nitrates in the Soil are 
a — Nitrification (nit-ri-fi-ca’-tion), by which the nitrogen 
contained by organic matter in the soil is changed to nitric 
acid, through the agency of microscopic plants (bacteria). 
