I30 SOIL CONDITIONS AND PLANT GROWTH 



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 relation- 

 ships 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 (or some component) possesses at least 

 six properties not usually shown by the undecomposed 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 solutions. The experiments of van Bemmelen (19, 21) 

 indicate a complete parallelism with clay in this respect. 



Baumann and Gully (10) and Od6n (21 8(5) show that un- 

 decomposed sphagnum can absorb ions from solutions, but the 

 phenomena differ in detail from those shown by humus. 



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. 220). From 

 this results a marked improvement in the tilth and general 

 mechanical condition. The Rothamsted mangold plots re- 



