THE FACTORS OF DISTRIBUTION 49 



cold; but, on the other hand, the water conducts and 

 retains the heat, so that moist soils are warmer in 

 autumn than dry ones. A covering of grass or other 

 vegetation screens the soil from the sun and checks- 

 radiation, so that the mean temperature of soil thus- 

 covered is lowered, and it experiences less extremes. 



Coarse-grained, well-drained soils, retaining little 

 water and thus with a low specific heat, are readily 

 warmed in spring, and are termed early, as opposed to- 

 fine-grained, retentive ones. 



THE CHEMISTRY OF THE SOIL. The principal con- 

 stituents of soils are sand, clay, carbonate of lime, and 

 humus. Sand consists mainly of coarse, incoherent, 

 insoluble grains of quartz, generally coloured by iron- 

 oxides. By itself it forms a light soil, i.e. one that offers 

 but little resistance to the plough, and is too unretentive 

 to be fertile. 



Clay is hydrated aluminium-silicate, generally coloured 

 by iron; green, blue, or black being due to a silicate, 

 carbonate, or sulphide; yellow or brown to hydroxides. 

 It consists of particles not exceeding -g-l^ of a millimetre 

 r-g-^i-- in.) in diameter, and has great power of retaining 

 water, and if " puddled," or kneaded, when wet, is 

 impermeable and excessively " heavy ," i.e. tenacious 

 and resistant either to the plough or to root-action. 

 Minute proportions of acids or of certain salts, such as 

 bicarbonate of lime, have the power of coagulating or 

 clotting the clay-particles; and in this condition, which 

 is known as flocculation, it can be readily crumbled. 

 This crumbling takes place when clays are alternately 

 frozen and thawed, as on fallow lands. Mixtures of 

 clay and sand are known as loams. Neither sand nor 

 clay themselves form material of plant food. 



Carbonate of lime occurs commonly in rocks as lime- 

 stone, or, when earthy, chalk; and, in smaller proportions, 

 in other rocks. Mechanically it renders soils lighter in 

 colour, more permeable and friable; but its most 

 important action is probably the neutralising of acids in 

 soils, such as those resulting from the decay of vegetable 

 matter. Clays or sands containing from 5 to 20 per 

 cent, of lime are known as marls. 



Hu:nus is the black or dark -brown carbonaceous and 

 nitrogenous substance resulting from the decay of 



getable and animal matter in the soil. Its formation 



