126 NATURE TEACHING 



which remain in the ground, together with the leaves, 

 stems, etc., may be returned to the soil to increase its 

 nitrogenous store. The result is that the soil 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. 



Leguminous plants, accordingly, are frequently made 

 use of to increase the fertility 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 opera- 

 tion is usually referred to as "green dressing" from the 

 fact that in this method of working, the crop is buried 

 while it is in a green and fresh condition, instead of 

 dressing the soil with dead or decaying material of 

 the nature of farmyard manure, or with chemical 

 substances. 



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. 



The nitrogen question is of the first importance to 

 the practical cultivator, a large part of his efforts being 

 directed towards securing a sufficient supply of this 

 important plant food. This question also demands 



