SOILS AND FERTILIZERS 313 



this constituent diminishing with depth. The color of the sub- 

 surface soil is often very similar to that of the surface. It varies 

 with the type, but depends primarily upon the amount of organic 

 matter and iron present. There are some places where the top soil 

 is only 6 or 8 inches deep, and rests directly upon the subsoil. In 

 such cases there is no subsurface stratum, as the plow line and the 

 subsoil line coincide. 



The subsoil extends usually from 18 or 20 inches to unknown 

 depths. The color is usually yellow, and the subsoil is frequently 

 spoken of as clay, although it commonly contains much more silt 

 than clay. The physical composition of the subsoil is frequently 

 different from the top soil in that it contains more of the finer soil 

 particles that have been carried downwards from the soil above by 

 percolating water. It generally contains only a small amount of 

 organic matter. The character of the subsoil is of very great im- 

 portance, for upon that, to a large degree, depends the drought- 

 resisting power of the soil. If the subsoil is coarse sand or gravel 

 the water will pass through it and cannot be drawn back by capil- 

 larity and so is lost to the crop, which will suffer from drought in 

 consequence. If, however, the subsoil is silty or clayey, a large 

 amount of water will be retained in it and much of what passes to 

 lower depths will be brought up within reach of the roots by cap- 

 illarity. Subsoils sometimes contain a layer of impervious material 

 such as compact clay, carbonate of lime, or oxide of iron, which is 

 commonly called hardpan. This is very objectionable if within 

 three feet of the surface, since it prevents the upward and downward 

 movement of water and the deep penetration of plant roots. The 

 crops on level hardpan land suffer from much rain because the 

 water cannot get down through this to drain away, and they suffer 

 from drought because the water below this layer cannot be brought 

 up through it by capillarity. (Bu. of Soils, B. 55; Fla. Ex. Sta. 

 B. 93; Colo. Col. B. 65; Y. B. 1897; Cornell R. C. Soil Series 1; 111. 

 Col. B. 82, 124; Bu. of Soils B. 22.) 



Organic Matter in Soils. Less is known about the nature of 

 the organic constituents than about the mineral constituents of the 

 soil. All kinds of animal and plant remains and their decomposi- 

 tion products are liable to occur in the soil. The great problems of 

 chemistry of the soil will probably be found in a study of its organic 

 constituents. 



The color of a soil is not a sure indication of the amount of 

 organic matter which it contains. Some characteristically black 

 soils have less organic matter than some of the red or yellow soils. 

 This appears to be a question of the condition of the organic matter, 

 resulting from different processes of decay. In some soils it breaks 

 down into a black product, the kind of material which we recognize 

 by the undefined term humus. In other soils the material is of a 

 much more neutral tint or even colorless. Such soils usually yield, 

 however, an intensely black solution when extracted with a dilute 

 alkali, especially on standing in contact with the air. 



