66 SOIL CONDITIONS AND PLANT GROWTH 



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 living plant that it is our business to study, and then, 

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

 important from our point of view and fix attention on them. For 

 the preliminary inquiries recourse is had to the indirect method of 

 correlation already used in ascertaining the properties of the mineral 

 fractions of the soil. Numerous studies on these lines have proved 

 that this group possesses at least six properties not shown by the un- 

 decomposed plant residues : — 



1. It gives a dark brown or black colour to the soil. 



2. It can withdraw various ions — NH^, K, PO4 — from their so- 

 lutions. The experiments of van Bemmelen (19, 21) indicate a 

 complete parallelism with clay in this respect. 



3. It causes the soil to puff up, or in the expressive phrase of the 

 German farmer, to " ferment " {Bodengdrung), and so leads to an 

 increase in the pore space (see p. 104). From this results a marked 

 improvement in the tilth and general mechanical condition. The 

 Rothamsted mangold plots receiving no organic manure, and therefore 

 poor in this group, get into so sticky and " unkindly " a state that the 

 young plants have some difficulty in surviving however much food is 

 supplied, and may fail altogether if bad weather intervenes in the 

 spring (as in 1908 and 191 1); the dunged plots which are rich in this 

 group are much more favourable to the plant and never fail to give a 

 crop. But the puffing up or " lightening " may go too far, and some- 

 times causes much trouble in old gardens that have long been heavily 

 dunged. 



4. It increases the water-holding capacity of the soil. The 

 amounts of moisture present in adjacent plots at Rothamsted 

 were : — 



