THE ORIGIN OF SOIL. 57 



ganic ingredients with which the fertility of the soil is most 

 distinctly associated. The mineral constituents of soil are 

 abundant enough almost everywhere, but the organic constitu- 

 ents are limited. Clear sand, even though it contain large 

 amounts of the necessary mineral matter for plant food, will 

 not sustain vegetation, and for the proper life of plants the 

 organic materials, or their equivalents, are necessary and fun- 

 damental. Indeed, we generally look upon the fertility of 

 any soil as strictly parallel with the amount of organic matter, 

 or humus, contained in it. It will be the most convenient way 

 of considering our subject to divide it into two divisions, one 

 relating to the origin of the mineral and the other to the origin 

 of the organic factors, although no sharp line can be drawn 

 between them. 



MINERAL MATTERS. 



The mineral compounds in the soil, upon which plant life 

 depends, are numerous but they are chiefly salts of potassium, 

 magnesium, calcium, phosphorus, iron and sulphur. These 

 salts exist. in the soil in varying amounts, and each contributes 

 more or less to the substance that goes to make up plant 

 foods. It is true that plants need only small amounts of these 

 minerals, but these small amounts are none the less necessary. 

 The mineral matters in our soils are derived chiefly from the 

 decomposition of the rocks, for the basis of most soil on the 

 earth appears to be decomposed rock. The decomposition of 

 the rocks is brought about chiefly by purely physical agencies. 

 The effect of "weathering" upon the rocks, as is well known, 

 results in slowly crumbling them into fine fragments. The 

 weathering has been, in general, attributed to physical and 

 chemical phenomena. The direct mechanical action of the 

 rains, the effect of the freezing and thawing of water, the sol- 

 vent action of various waters, especially if they contain car- 

 bonic acid in solution, the direct oxidation of the rocks by the 

 oxygen of the atmosphere, all of these are the important fac- 

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