GENERAL PRINCIPLES. 47 



ually becomes richer in ava'ilable fertility, except in 

 so far as the product of such growth is removed from 

 the land. This explains why the producing power of 

 grass-producing lands grazed by sheep is increased. It 

 also explains at least in part why poor soils become more 

 productive when they have become covered with ^ 

 growth of grass. 



Grasses, however, improve the mechanical condition 

 of the soil. They put humus into it not only in the 

 decay of their rootlets, but also in the decay of the sod 

 which they have formed when it is buried with the 

 plough. The vegetable matter thus furnished yields up 

 plant food in a readily available form, binds light soils 

 so that they lift less with the winds, and renders stiff 

 soils more friable and consequently easier of access to 

 the roots of plants. And they greatly increase the power 

 of all soils to retain moisture in the cultivable area for 

 the benefit of succeeding crops. 



The extent of the benefit thus conferred will increase 

 with the increase of the supply of vegetable mat- 

 ter thus furnished and with the slowness of the decay in 

 the same. When grasses have been grovra upon a 

 piece of land for a time the maximum production of hu- 

 mus is practically reached. If, therefore, when this 

 limit has been reached, the pasture is broken up, and the 

 same land is again laid down to pasture it is very evi- 

 dent that much more humus will be put into the soil thus 

 rotating grasses than by growing them for long periods. 

 In tillable areas, therefore, it is greatly important that 

 grasses shall be rotated with other crops. In climates 

 lacking in humidity and warmth and also in moisture, 



