THE COMPOSITION OF THE SOIL 129 



priate for plant nutrition. We may look upon its constituents 

 as taking part in a perpetual cycle : in one stage nourishing 

 the growing plant and storing up the energy of sunlight, in 

 the other stage nourishing micro-organisms and liberating 

 energy. In addition, it has important physical effects on the 

 soil. Unfortunately, not much is known of the highly com- 

 plex components of the plant and even less is known about 

 the important organic substances of the soil. The difficulty 

 of working with insoluble, unstable bodies mingled with 

 twenty times or more their weight of sand, silt, and clay has 

 hitherto proved almost insuperable. The ideas current in the 

 textbooks go back' to the time before organic chemistry arose, 

 and have come down direct from C. Sprengel (270^), Mulder 

 (205) and Detmer (84). 



We can thus only speak in the most general terms about 

 what is admittedly the characteristic component of soil. Two 

 great groups are to be carefully distinguished : one furnished 

 by recent generations of plants ; the other deposited with the 

 soil during its forrricition, and therefore as old as the soil itself. 

 Unfortunately, no actual method of separation is known, but 

 some idea of the amount and properties of the original organic 

 matter can be obtained from a study of the subsoil at depths 

 below the root-range of plants. Ten feet or more below the 

 surface, sandy subsoils usually contain less than -oi per cent 

 of nitrogen and clays less than -05 per cent., but shales con- 

 tain more than O'l per cent. The percentage of carbon 

 fluctuates, but is usually five to ten times that of nitrogen 

 (2000). Now these values are about one-tenth to one-fifth of 

 those obtained in the surface soil, so that at the very outside, 

 and assuming there has been no decomposition, not more than 

 I o to 20 per cent, of the surface organic matter is original. 



The organic matter furnished by recent vegetation may 

 roughly be classified as: (i) material that has not yet had 

 time to decompose and still retains its definite cell structure ; 

 (2) partially decomposed and still decomposing material ; (3) 

 simple soluble decomposition products ; (4) plant or animali 

 constituents not decomposable in the soil. 



9 



