Life in the Soil 129 



rains in a section where there is far more rain than freez- 

 ing in winter. A winter cover-crop becomes of special 

 importance in the South, and is also important anywhere. 

 This cover-crop must be a green winter-growing crop, 

 capable of taking up and holding in the form of organic 

 matter, the nitrates that would otherwise be washed from 

 the soil and lost. We have no figures to show how great 

 this loss is, but it has evidently been one of the most 

 serious causes of the loss of nitrogen in Southern soils, 

 where the oxidation of the organic matter in the soil has 

 been promoted by constant exposure. In the case of the 

 phosphates already in the soil, or appHed to it in a soluble 

 form, there is far less danger of loss, since the phosphoric 

 acid, unused at once by plants, reverts in the soil to a less 

 soluble form; and the absorptive character of the soil is 

 such that it will retain its hold on the phosphoric acid and 

 potash, till called for by plant roots; and the loss of phos- 

 phates and potash is mainly through the crops carried 

 off, and the young animals raised on the farm and sold; 

 for the making of the bony frame of a growing animal is 

 one of the chief sources of loss of phosphates from the 

 soil. 



The fact that nitrogen, as a nitrate, is rapidly washed 

 from the soil shows, too, the importance of having a supply 

 in the form of organic decay; for when we apply nitrogen 

 that is already a nitrate, as in the nitrate of soda, it must 

 be used at once by plant roots; while the organic nitrogen 

 is more slowly coming into use as the plants need it. 

 Hence, the very soluble form of nitrogen in nitrate of soda 

 should never be used except when the plants are in rapid 

 growth, and then, not in excessive amoimts. 



