144 LIVING ORGANISMS OF THE SOIL [chap. 



particularly where the natural herbage is allowed to fall 

 and decay, there is abundant material for the purposes 

 of the organism, and fixation in consequence can become 

 a marked feature in the history of the soil. Two small 

 portions of land at Rothamsted have been allowed to 

 run down from arable into a mass of rough grass and 

 weeds, nothing having been done to the land for more 

 than a quarter of a century. The coarse derelict herbage 

 that has sprung up — natural prairie — falls back to the 

 soil every year, and an analysis of these soils before and 

 after the growth of the wild vegetation show a notable 

 fixation of nitrogen, over 90 lb. per acre per annum for 

 the twenty-five years during which the land has been 

 left to itself This experiment also helps us to under- 

 stand how forest soils get enriched with nitrogen ; the 

 autumnal fall of leaf adds a large amount of carbonaceous 

 matter to the soil and thus enables the Azotobacter to 

 go to work. The enormous accumulations of nitrogen 

 in the deep black soils of the prairies, the steppes, and 

 similar areas, probably also owe their origin to Azoto- 

 bacter ; for in these soils the requisite conditions are 

 fulfilled — carbonate of lime exists in the soil, and there 

 is a steady addition of carbonaceous matter as each 

 year's growth dies down in the autumn. 



We are now in a position to get some general con- 

 ception of the work of the bacteria in the soil ; the 

 actual numbers are very great, as many as forty millions 

 have been estimated in a gramme of a garden soil 

 fairly rich in organic matter, and the number of species 

 that can be distinguished is considerable. It is, how- 

 ever, better to consider them by groups according to 

 their function, than by species, and the main features of 

 the chief groups have been set out above. All these 

 different groups are at work together in the soil, but 

 which will predominate and what degree of activity it 



