KINDS OF SOIL 99 



glaciers is called till or glacial drift, and that the soils af- 

 fected by glaciers are usually quite fertile. 



Soil as to Nature. The classification of soils as to 

 their chemical nature is much too complicated a subject 

 to take up in a book of this kind. But we should note 

 that this is a very important matter in scientific farming. 

 By means of chemical analysis it is possible to discover 

 in what mineral and organic substances a given soil is 

 rich or poor, as the case may be. The substances which 

 crops need being known, it is evident that such analyses 

 give the farmer a scientific basis upon which to determine 

 what kinds and amounts of fertilizers should be applied 

 to his fields. Without such analyses, the use of fertilizers 

 is largely a matter of guesswork. We mean by fertilizer 

 anything used to improve the fertility of soil. Lime, 

 rock-phosphate, bone-dust, potash, and manure are exam- 

 ples of fertilizers; all of these contain substances which are 

 very necessary to the growth of plants. 



The classification of soil as to its physical nature de- 

 pends chiefly upon the size of the soil grains. Thus, sand 

 has much larger grains than clay, while loam, as you 

 have noted, is a mixture of the two. You have also 

 learned (see page 83) how important a part the size of 

 soil grains plays in the matter of water retention by the 

 soil. Generally speaking, for agricultural purposes, sand 

 is too porous; it lets the water escape too readily. Clay 

 is too dense; it holds water too tenaciously and is too 

 difficult to work. Loam is a "happy medium" between 

 the two, especially if its richness be increased by a quan- 

 tity of humus. 



Of course soils vary exceedingly as to the relative pro- 



