ix.] FERTILITY 273 



effect much improvement by the use of manures ; in fact, 

 manuring will not turn bad into good land, the con- 

 ditions limiting the amount of crop being other than 

 the food supply. Of course, by the continued incorpora- 

 tion of.humus into a light soil, its physical texture may 

 be improved at the same time as its richness, until it 

 becomes sufficiently retentive of water for the needs of 

 an ordinary crop, just as a heavy soil may be lightened 

 by similar additions of humus. It has already been 

 mentioned that many subsoils, especially of the heavier 

 loams and clays, are extremely infertile when brought 

 to the surface, even though they may possess a fair 

 proportion of phosphoric acid and potash and be arti- 

 ficially supplied with nitric nitrogen. Some of this 

 effect is due to texture, part to the very scanty 

 bacterial flora they possess ; but it is to be noted that 

 in arid climates the subsoils, which are not more fine- 

 grained than the surface soils, do not show the same 

 infertility when brought to the surface. 



The soils which show the greatest fertility are, as 

 a rule, soils of transport, uniform and fine-grained in 

 texture, but with particles of a coarser order than clay 

 predominating, so that, while moving water easily by 

 capillarity, they are freely traversed by air and per- 

 colating water. As a rule, they also contain an appreci- 

 able amount of organic matter at all depths ; in Britain 

 they have been deposited from running water, and 

 represent the silt from which both the coarsest sand 

 and the finest clay particles have been sifted, together 

 with a certain amount of vegetable debris. We have 

 nothing comparable with the typical " black soils " of 

 the North American prairies or the Russian steppes, 

 which contain very large proportions of organic matter 

 to considerable depths in the subsoil : as, for example, 

 a soil from Winnipeg that contained 0-428, 0-327, 0-158, 



T 



