58 AGRICULTURE 



a soluble condition, a succession of heavy rains would dis- 

 solve and wash all of the plant-food out of the soil and 

 .carry it to the ocean. But since only a small part of the 

 phosphoric acid and potash of the soil are in a soluble 

 condition, no soil can be permanently or completely ex- 

 hausted. It is possible to restore the fertility of any soil 

 that has a fair proportion of clay in it. 



Clay and sand. Examine a little sample of clay soil 

 and another of sandy soil. When you rub them between 

 your fingers the clay soil feels smooth, while the sandy 

 soil feels coarse and gritty. Sand grains are hundreds of 

 times larger than the tiny grains of clay. They are so 

 large that they do not settle closely together, and the 

 spaces between them allow water to run very rapidly 

 through. Sandy soil, therefore, will not hold water well. 



We can scarcely understand how small the separate 

 particles of clay are. It would require more than fifty 

 thousand fine particles of clay side by side to cover a 

 line one inch long. Since the grains or particles of clay 

 are so small, they can be packed tightly together, leav- 

 ing very little space between. It is difficult, therefore, 

 for air and water to penetrate a clay soil. 



Granulation. Fortunately, in well-cultivated, well- 

 drained, clay soils, supplied with vegetable matter, a 

 number of the tiny particles cling together in one group 

 or granule. Each of these groups acts like a single sand 

 grain, leaving spaces between granules open enough for 

 water to drain through and for air to enter. This granu 

 lation, or grouping into granules, is the condition the farmer 

 wishes his clay soil to assume. If, however, he plows when 



