68 



SOIL CONDITIONS AND PLANT GROWTH 



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 4 , K, PO 4 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. 105). 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 1911); 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 : 



The variations in water content follow very closely the variation in 

 the amount of organic matter present. So marked are these physical 

 effects that if 1 5 or 20 per cent, of organic matter is present in a soil 

 the operation of other factors ceases to count for much, and the dis- 

 tinctions between sands, loams, and clays are obliterated. Thus, much 



