144 Principles of Plant Culture. 
260. Leguminous Plants Enrich the Soil with nitric 
acid (256), which is formed from atmospheric nitrogen in the 
tubercles on their roots through the agency of microscopic 
plants (113). Even when a part of these crops is removed 
from the land, as when clover is harvested for hay, or peas 
for their seed, the land remains richer in nitrogen than be- 
fore the crop was planted. The principal leguminous crops 
are the clovers, peas, beans, lentils, sanfoin, vetches, alfalfa, 
lupine and certain species of Lathyrus. Highly valuable as 
are these crops for the nitrogen they leave in the soil, it 
should be remembered that they do not contribute phos- 
phoric acid or potash, and hence must not be wholly de- 
peuded upon for soil fertility. 
Leguminous plants are supplied with nitrogen by the 
bacteroids in their roots, and hence do not require this ele- 
ment in fertilizers. 
261. Rain and Snow Add Nitrogen to the Soil in 
small quantities, both as nitric acid and ammonia, which 
have been taken up from the air, but the amounts thus ad- 
ded, while useful to plants, are not under our control. 
262. Nitrogen may be Purchased for fertilizing pur- 
poses as sodium nitrate (nitrate of soda, Chili-saltpeter), and 
ammonium sulfate (sulfate of ammonia). The former is 
available as plant food as soon as it is dissolved in the soil 
water. It is best applied immediately before the planting 
of a crop, or in small quantities at intervals during 
growth; since it is in danger of being washed out of the soil 
in drainage water. Sodium nitrate is especially useful for 
garden crops started early in spring, when the soil is too 
cool for active nitrification (256). The surface soil is apt to be 
poor in nitrates in spring, because they are often washed 
down by the autumn and winter rains. 
