148 PRINCIPLES OF PLANT CULTURE 



cation by favoring warming of the soil. Cultivation in 

 dry weather further promotes plant nutrition by pre- 

 venting the accumulation of soluble plant-food in the 

 dry surface soil, where it is deposited above the reach of 

 roots through evaporation. 



258. Partially-decomposed organic manures act more 

 promptly than fresh ones, because nitrification has already 

 begun in these materials. 



259. Value of legumes. Leguminous plants enrich 

 the soil with nitric acid (255), which is formed from atmos- 

 pheric nitrogen 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 certain 

 species of Lathyrus. Highly valuable as are these crops 

 for the nitrogen they leave in the soil, it should be remem- 

 bered 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. Value of rain and snow. 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 



