18 SUCCESSFUL FARMING 



is dissolved small quantities of various soil solids. The gases consist 

 chiefly of the air intermingled with various quantities of other compounds, 

 such as carbon dioxide, marsh gas, etc. 



The soil and subsoil include all material to the depth to which plant 

 roots distribute themselves. It, therefore, constitutes a wide range of 

 material, both in depth and character. It may be deep or shallow, loose 

 or compact, wet or dry, coarse or fine in texture, having all degrees of 

 variation in its physical, chemical and biological properties. 



The Soil Solids. The solid part of the soil consists of the minerals 

 and organic matter. In practically all soils the minerals form ninety-five 

 per cent or more of the solids. The exception to this would be the peat 

 and muck soils, which may contain as much as eighty per cent or more 

 of organic matter. The mineral matter of the soil consists chiefly of the 

 minute particles or fragments of the mother rock from which the soil has 

 been derived. In case of residual soils this will correspond in a large 

 degree to the rock formation generally found beneath the soil and subsoil 

 at varying depths. In transported soils the mineral particles, having been 

 transported either by water, glaciers, or wind, may have come from dif- 

 ferent sources, and will generally show a greater diversity in character. 

 It is significant, however, that the minerals of all soils contain all the 

 essential mineral elements for plant growth, although these may vary 

 widely in their relative proportions. 



The minerals of the soil are sparingly soluble in the soil water and the 

 solubility is influenced by a number of factors that will be discussed in a 

 subsequent chapter. It is fortunate that this solubility takes place very 

 slowly, otherwise soils would be dissolved and disappear in the drainage 

 waters too rapidly, and the waters of the earth would become too saline 

 to be used by plant and animal life. Loss of the mineral constituents 

 takes place by leaching. The drainage waters from land always contain 

 a very small quantity of many of the elements of which the soil is com- 

 posed. Nitrogen, the most valuable decomposition product of the organic 

 matter of the soil, is most rapidly leached away in the form of nitrates. 

 Likewise, lime slowly disappears from the body of the soil. Limestone 

 soils, formed from the disintegration and decomposition of limestone 

 rocks, sometimes ninety per cent or more carbonate of lime, generally 

 contain not more than one-half of one per cent of carbonate of lime. The 

 rate of leaching corresponds in a large measure to the rainfall of the region. 

 In regions of sparse rainfall very little leaching takes p'ace, and the soil 

 solution frequently becomes so concentrated that the soils are known as 

 alkali soils. Such soils are either bare of vegetation or produce only crops 

 that are tolerant of alkali. The soils of arid regions are as a rule very 

 productive when placed under irrigation. 



The Soil Fluid. This consists of water in which is dissolved minute 

 quantities of the different minerals of the soil together with organic prod- 

 ucts and gases. The soil solution moves through the soil by virtue of 



