122 NATURE TEACHING. 



is richer in nitrogen after the crop has been 

 removed than before. In this case it is assumed 

 that a reasonable proportion of the growth, 

 that is of roots, leaves and stems, is left upon 

 the land. 



2. Leguminous plants, accordingly, are 

 frequently made use of to increase the fer- 

 tility of soils. Crops of these plants are grown, 

 and when the crop is well developed, the whole 

 of it is buried in the soil. This method increases 

 the store of nitrogen in the soil, that in the 

 crop being largely derived from the air. At 

 the same time it adds greatly to the store of 

 humus. This operation is usually referred to 

 as ' green dressing, ' from the fact that it is 

 best to bury the crop while it is in a green, 

 vigorous condition, instead of dressing the soil 

 with dead or decaying material of the nature 

 of pen manure, or with chemical substances. 



3. It will be understood why it is more 

 profitable to use leguminous plants for green 

 dressings, than plants belonging to other orders. 

 The latter will, it is true, increase the store 

 of humus, yet the nitrogen which they contain 

 is nitrogen which was already present in the 

 soil. With leguminous plants there is a gain 

 of nitrogen a constituent which it is costly to 

 purchase: 



