30 AGRICULTURAL TEXT-BOOK. 



ficial layer, which is commonly called the mould or vegetable 

 earth; this is a layer generally impregnated with humus, and 

 is looser and more friable than the sub-soil upon which it rests. 

 The thickness of this superficial layer is extremely variable; 

 varying from two inches to two feet. Depth of mould or vege 

 table soil is always, practically, advantageous; it is one of the 

 best conditions to successful agriculture. If we have depth of 

 soil, and the roots of our plants do not penetrate sufficiently to 

 derive benefit from the fertility that lies below, we can always, 

 by working a little deeper, bring up the inferior layers to the 

 surface, and so make them concur in fertilizing the soil. Farther, 

 a deep soil suffers less from excess or deficiency of moisture \ 

 the rain that falls has more to moisten, and is therefore absorbed 

 in greater quantity than by thin soils; and, once imbibed, it 

 remains in store against drought. 



104. Soils also vary much in color, as (a,) black; (b^) white; 

 (cj blue ; (d,) red ; (e,) brown. The colors of soil have a 

 considerable influence in regulating the quantities of heat ab 

 sorbed from the sun s rays; the darker colored absorb more 

 heat than the lighter colored ; and the dark colored reflect the 

 least. 



105. According to Shubler, while the thermometer was 77 in the 

 shade in August, sand of a natural color indicated a temperature of 

 112} c ; black eand, 123&amp;gt; ; and white sand, 110 c , exhibiting a dif 

 ference of 13 in favor of the black color. 



106. Color also influences the power of soil in retaining the 

 heat of the sun. Dark colored soils, in the absence of the sun s 

 rays, radiate or part with the heat more quickly than light col 

 ored. Thus sand cools more slowly than clay, and clay than a 

 soil containing much vegetable matter. This principle has a 

 strong practical influence on the deposit of dew, and injury to 

 vegetation by night frosts. 



107. Soils also vary in their power of absorbing and retaining 

 moisture. The absorption is greatest in clay soils, especially 

 when they contain humus, (vegetable matter.) 



