64 FERTILISERS CONTAINING NITROGEN [chaP. 



there is no calcium carbonate present to neutralise the 

 acids, they combine with the calcium of the humus and 

 set free humic acid, which accumulates from year to year, 

 until, with the small amount of mineral acid from the 

 salts that is also left free, the acidity becomes consider- 

 able enough to be detected. But the injury to the 

 crop seems to be less due to the direct effect of the 



Table XV.— Woburn. Yield of Barley, 1904. 



acids upon the plant than to the way the acidity tends 

 to suspend the normal bacterial activities of the soil, as, 

 for example, the process of nitrification, and to replace 

 them by the growth of moulds and fungi. In the acid 

 grass soils at Rothamsted, for example, nitrification 

 is almost at a standstill, the organisms are very few 

 in number, and the plant is chiefly feeding on the 

 unchanged ammonia of the manure. 



Although under ordinary farming conditions an 

 actually acid reaction is not likely to arise through the 

 use of sulphate of ammonia, the experiments at Woburn 

 and Rothamsted clearly indicate that it is not a desirable 

 source of nitrogen for soils which are deficient in 

 calcium carbonate. The reaction of ammonium salts 

 with the soil, resulting in the withdrawing of the 

 ammonia from solution, gives a clue to the difference 

 in both the yield and the character of the crop when 



