156 Principles of Plant Culture. 



259. Leguminous Plants Enrich the Soil with nitric 

 acid (255), which is formed from atmospheric nitro- 

 gen in the tubercles on their roots through the agency 

 of microscopic plants (112). 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 is 

 richer in nitrogen than before the crop was planted. 

 The principal leguminous crops are the clovers, peas, 

 beans, lentils, sanfoin, vetches, alfalfa, lupine and cer- 

 tain 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 phosphoric 

 acid or potash, and hence must not be wholly depended 

 upon for soil fertility (262, 263). 



Leguminous plants are supplied with nitrogen by the 

 micro-organisms in their roots (112), and hence do not 

 require this element in fertilizers. 



260. Rain and Snow Add Nitrogen to the Soil in 

 small quantities, both as nitric acid and ammonia, which 

 they have taken from the air, but the amounts thus 

 added, while useful to plants, are not under our control. 



261. Nitrogen may be Purchased for fertilizing pur- 

 poses as sodium nitrate (nitrate of soda, Chili-saltpeter), 

 ammonium sulfate (sulfate of ammonia), and in organic 

 materials. The former is available as plant food when 

 dissolved in the soil water. It is best applied imme- 

 diately before the planting of a crop or in small quan- 

 tities 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 



