THE CONSTITUTION OF THE SOIL 65 



Organic Matter. 



The distinguishing characteristic of soil is that it contains part of 

 the complex material synthesised by plants. This material affords 

 energy to numerous micro-organisms, and is gradually converted by 

 them into simple substances appropriate 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 complex components 

 of the plant and even less is known about the important organic 

 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 -01 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 



