INVESTIGATIONS ON THE PRODUCTION OF PLANT FOOD. 185 



are valueless and may even be indirectly harmful to the plant : after 

 decomposition they begin to be valuable. The speed and completeness 

 of the decomposition are therefore of fundamental importance in soil 

 fertility. 



The agents bringing about the decomposition are numerous. Large 

 organisms, earth-worms, <xc, play an important part in the dissemina- 

 tion of the material, but otherwise do not seem to be essential ; there 

 is a sufficient variety of micro-organisms to complete the change 

 without them. 



The speed at which the change takes place obviously depends on 

 the activity of the organisms and on the composition of the material. 

 When for any reason it is slow there is a considerable accumulation of 

 undecomposed substance which has characteristic and usually harmful 

 effects on the soil. Certain of the grass-plots at Rothamsted have 

 received large annual dressings of sulphate of ammonia for so many 

 years that the soil has become distinctly acid. The speed of the 

 decomposition processes has been reduced so much that the dead 

 vegetation lies on the surface in thick mats, through which nothing 

 can push its way except an occasional plant of sorrel or a runner of 

 Yorkshire fog or Alopecurus pratensis. Thus the surface tends to 

 become covered with peaty patches bare of vegetation. As soon as 

 lime is put on, the decomposition becomes more rapid, the dead 

 residues become decomposed and are cleared out of the way, so that 

 the ground once more becomes covered with vegetation. 



Persistent dryness produces the same effect with certain differences. 

 In the first place the vegetation tends to be xerophytic, and the hard, 

 narrow-leaved, waxy plants do not easily decompose, especially in the 

 dry soil. The soil of a sandy common or woodland, for instance, often 

 contains a considerable amount of nitrogenous organic matter, which, 

 however, is mainly undecomposed pieces of bracken frond, &c. 



Two distinct cases arise when the soil is too wet. In presence of 

 calcium carbonate the general conditions favour a grassy type of 

 vegetation which decomposes fairly well, and can at any time be made 

 to go through the remainder of its stages in a normal way by making 

 the conditions a little more favourable. These soils are therefore 

 eminently suited for reclamation ; they constitute the fen land. 



Where, however, calcium carbonate is absent a much tougher 

 type of vegetation arises — heathers, mosses, cotton grass, &c. — which 

 even under the best conditions would not easily decompose, and under 

 the conditions here obtaining only breaks down very slowly. Thick 

 layers therefore accumulate, forming the great deposits of Sphagnum, 

 &c, on the moors. As in the earlier cases, however, decomposition will 

 proceed as soon as the conditions become favourable. Draining the 

 moor removes the limiting factor and sets going the chain of processes : 

 the peat may decompose right down to the solid rock or soil surface, 

 or the processes may be stopped at some convenient stage, as in 

 Bottomley's bacterized peat. 



So far as evidence is available, it shows that the general decom- 



