THE SOIL AND ITS IMPROVEMENT. 99 



This substance does not furnish any food to the roots of plants, 

 but it has a wonderful power of retaining moisture, and the 

 elements of fertility. In any ordinary soil it is always under- 

 going decay, and by this process the elements of plant food 

 contained in the humus itself, and also those contained in 

 the sand and clay, of which the remainder of the soil is com- 

 posed, are set free in forms which the plant can use. When 

 in excess, humus renders the soil light and chaffy, and it is 

 commonly noticed that in reclaimed bogs and swamps, the soil, 

 which is usually largely composed of this substance, though 

 very productive at first, soon loses its fertility. This is due to 

 the lack of sufficient mineral elements. 



Humus is not a constant element in the soil. As we have 

 already seen, it is always undergoing decomposition; the min- 

 eral matters it contains return to their original form, while the 

 carbon, combining with oxygen from the air, is converted 

 into carbonic dioxide, and escapes into the air. The more thor- 

 oughly the land is cultivated, the more rapidly does this pro- 

 cess take place, and a soil that contains but a, moderate per 

 cent of humus may lose nearly all of it after a few years' 

 persistent cropping and cultivating. Such a soil is said to be 

 " run down," and though it may still contain in abundance the 

 materials needed for the sustenance of plant life, it becomes in- 

 fertile, because the chemical processes by which this material is 

 made available for the plant proceed but slowly, unless there is 

 a reasonable percentage of humus in the soil. 



Changing the Character of Soil. The gardener, with 

 a small amount of ground yielding crops of great propor- 

 tionate value, may find it profitable to change the character of 

 his soil by hauling on clay if too sandy, or sand if it is a heavy 

 clay, but in the operations of the farm, this is impracticable. 

 Humus alone, of the three great constituents of the soil, can 

 practically be controlled by the farmer. If it is in excess, he 

 can diminish it by cultivation, and the use of lime and mineral 

 manures. If it is deficient, it can be increased by growing 

 clover, rye, buckwheat, or other similar crops, and plowing 

 them under. The reader will also notice that it corrects the 



