CHAPTER IV 



CONDITIONS AFFECTING SOIL FERTILITY 



Structure. While humus, water, and air are necessary 

 constituents, mineral matter is the basis of all fertile soils, 

 forming from 60 to 90 per cent of their weight. The prevail- 

 ing mineral constituent is nearly always silica, with varying 

 amounts of alumina or clay and the oxides of iron, calcium, 

 magnesium, and others. Even in the poorest soils there are 

 enough of the elements needed by plants for at least a hundred 

 ordinary crops, and the subsoil contains immense additional sup- 

 plies ; but these are often so solidly bound in the rocks as to be 

 only slowly available to plants. The texture of the soil, then, 

 determines in great measure not only what crops can grow 

 upon it, but the rapidity with which weathering can make the 

 needed elements available. A block of stone will support only 

 a few mosses and lichens ; grind it to sand and many more 

 highly specialized plants will grow upon it ; reduce it to pow- 

 der and it will grow our cultivated crops. A gram of good 

 soil contains from two billions to twenty billions of soil par- 

 ticles. In a cubic foot of the finest clay the total exposed 

 surface of the particles is not far from 175,000 square feet. In 

 a sandy soil the area falls to about 10,000 square feet. The 

 particles in a cubic foot of light loam have a total surface area 

 of about one acre. Since water containing the dissolved food 

 materials is held on the surface of these particles, one easily 

 understands how a fine soil and a fertile soil are nearly 

 synonymous terms. In the soil the finest particles are not 

 separate, but are flocculated, or bound together in small groups 

 called soil crunibs. When for any reason the soil crumbs are 



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