THE CONSTITUTION OF THE SOIL 67 



substances of the soil. The difficulty of working with insoluble, un- 

 stable bodies mingled with twenty times or more their weight of sand, 

 silt and clay has hitherto proved almost insuperable. The ideas current 

 in the text-books go back to the time before organic chemistry arose, 

 and have come down direct from C. Sprengel (269), Mulder (204), and 

 Detmer (85). 



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 formation, 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 sub- 

 soil 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 contain more 

 than -I per cent. The percentage of carbon fluctuates, but is usually 

 five to ten times that of nitrogen (199). 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 10 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 animal constituents not decomposable in the soil. 



The undecomposed material is important as the reserve supply for 

 the entire chain of reactions to be considered later. It also has a 

 certain mechanical effect in opening up the soil and facilitating aeration 

 and drainage, an effect useful on clays but often harmful on sands 

 where these processes already tend to go too far. 



The partially decomposed material forms a particularly vague and 

 indefinite group containing all the non-volatile products of bacterial, 

 fungal, enzymic and other actions on the plant residues. It shades off 

 in one direction into the simple soluble decomposition products, and in 

 the other into undecomposed plant fragments, so that it cannot be 

 sharply defined or accurately estimated. A detailed study of the 

 group being thus out of the question, we must ascertain in the first 

 instance what part it plays in determining those relationships between 

 the soil and the livjng plant that it is our business to study, and then, 

 when we know what to look for, try to discover what constituents are 



