SOILS 385 



The chief causes of the wearing of rocks are exposure to 

 air, heat, cold, rain, and frost, extending over immense 

 periods of time. We may even see the effects oi these 

 disintegrating forces in the stone employed for building 

 purposes. 



The soil, therefore, as a rule partakes of the nature of the 

 rock it covers, but there are exceptions as in the alluvial 

 soils brought down by big rivers like the Nile, Ganges, and 

 Amazon, which overlay the natural rock of the part, and 

 bear no relation to it. Such are spoken of as soils of 

 transport. 



Ordinary soil is a mixture of sand, clay and lime, with a 

 variable proportion of saline and organic matter. The 

 difference in soils depends upon the relative proportion of 

 these constituents, while its fertility, within certain limits, 

 is affected by the proportion of humus and inorganic 

 matter present. 



Sand by itself is of no agricultural value, it would neither 

 afford support for the roots of plants nor sufficient moisture, 

 but it confers a certain property on soils when mixed with 

 them, viz., lightness, and it opens them for the admission 

 of both air and moisture. 



Clay in a pure condition is of no agricultural value, as a 

 matter of fact it is seldom pure, but the point to be noticed 

 here is not its fertility but its physical function in the soil. 

 It is plastic, retentive of moisture, and consequently cold ; 

 it is the exact antithesis of sand in these respects. The 

 admixture of clay with sand causes the latter to retain its 

 moisture and gives it tenacity, while the sand raises the 

 temperature of the clay, opens it out, and renders the soil 

 more porous. 



When clay and sand exist together in a soil in moderately 

 definite proportions they form what is known as a loa^n ; if 

 the percentage of sand exceeds that of the clay it is known 

 as a sandy loam ; if the clay is in excess it is called a clay 

 loam ; when neither one nor the other are greatly in excess 

 it is termed a loam. 



The function of these two important soil constituents is 



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