150 Principles of Plant Culture. 



ing warming of the soil. Cultivation in dry weather 

 further promotes plant nutrition by preventing the accu- 

 mulation of soluble plant food in the dry surface soil, 

 where it is deposited above the reach of roots through 

 evaporation. 



259. Partially-Decomposed Organic Manures Act More 

 Promptly than fresh ones, because nitrification has already 

 commenced in these materials. 



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 micro- 

 scopic 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 is richer in nitro- 

 gen 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 remembered 

 that they do not contribute phosphoric acid or potash, 

 and hence must not be wholly depended upon for soil 

 fertility (263, 264). 



Leguminous plants are supplied with nitrogen by the 

 micro-organisms in their roots (113), and hence do not 

 require this element in fertilizers. 



261. 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. 



262. Nitrogen may be Purchased for fertilizing pur- 

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



