16 THE CHEMISTRY OF THE FARM. 



tion. It is the principal nitrogenous ingredient of soils. 

 A black soil, rich in humus, is sure to be also rich in 

 nitrogen ; a soil destitute of humus will contain scarcely 

 any nitrogen. The fertility of virgin soils is largely due 

 to the nitrogenous humus which they contain. 



Of all soil ingredients sand has the least, and humus 

 the greatest capacity for retaining water. Light sandy 

 soils thus suffer most from drought, while applications of 

 farmyard manure, or the ploughing in of green crops, 

 increase the water-holding power of a soil by increasing 

 the proportion of humus. The capillary power of soil, 

 by wdiich water is raised from the subsoil to the surface 

 in dry weather, is least in open sandy soils composed of 

 coarse particles, and greatest in the case of loam or clay. 



Dark-coloured soils absorb the greatest amount of heat 

 from the sun's rays, and light-coloured soils least. The 

 presence of himius is thus favourable to soil warmth. 

 Quartz sand is an excellent conductor of heat ; chalk is a 

 bad conductor. A soil rich in sand will thus be warmed 

 or cooled more rapidly, and to a greater depth than a soil 

 containing but little sand. Water has a very considerable 

 effect in cooling a soil, partly from its high specific heat, 

 and partly from the immense consumption of heat during 

 its evaporation. A wet soil is always colder than a dry 

 one. The drainage of wet land will thus result in a 

 gi*eater warmth of the surface soil, and consequently an 

 earlier growth in spring. 



The proportion of plant food present in soils is veiy 

 small, even when the soil is extremely fertile. The surface 

 soil (first 9 inches) of a pasture may contain when dry 0.25 

 of nitrogen per cent, while soil of the same depth from a 

 good arable field may yield 0.15 per cent., and a clay 



