A BIOLOGICAL FACTOR 293 



which could be extracted by water, the amount of which 

 was greatest in the spring soon after the application of the 

 manures and diminished as the year progressed, there was 

 present a much larger quantity of comparatively insoluble 

 free humic acid. Clearly this had arisen by the action of 

 the mineral acids, set free year by year from the ammonium- 

 salts, upon the neutral humus or calcium humate originally 

 present in the soil, and the humic acid had been able to 

 accumulate because it is but slightly soluble in water. 

 Without discussing the other details bearing upon the 

 question (see Hall, Miller, and Gimingham, Proc. Roy. Soc., B. 

 80 (1908), 196), it became pretty clear that the acidity of the 

 Eothamsted grass soils has arisen from the action of various 

 micro-fungi upon the ammonium-salts that had been annually 

 applied to these plots ; such fungi have become very abundant 

 in the soil, and are able to attack ammonium-salts and set free 

 the acid, taking the ammonia to themselves to supply the 

 nitrogen they require for nutrition. At Bothamsted the acid 

 soils have not been rendered absolutely sterile ; the ground 

 is still covered by herbage, but it has a very unhealthy appear- 

 ance, and resembles in the most interesting manner the vegeta- 

 tion of naturally acid soils. The grasses have a characteristic 

 dark ugly colour and grow in tufts with bare spaces between, 

 the surface of the ground in these bare spaces being occupied 

 by a layer of peaty vegetable matter, as though the dead grass 

 had been unable to decay in the normal manner. Half of 

 each of the plots has been limed 2000 Ib. per acre of ground 

 lime having been applied in January 1903, and again in 

 January 1907. Table XCVIII. shows the great increase of 

 crop which has followed the liming, the effect of which is 

 also seen in the restoration of the herbage to a normal appear- 

 ance and a close sward, accompanied by the disappearance of 

 the peaty layer. 



The cause of the comparative infertility of the acid soils 

 must be set down to the fact that they are permeated by the 

 micro-fungi which can grow in an acid medium, whereas the 



