292 ACTIONS OF ARTIFICIAL MANURES 



interchange of bases between the salt and the zeolites took 

 place as already described, but the resulting liquid remained 

 perfectly neutral; with humus a similar interchange took 

 place, also giving rise to no free acid. The humus of normal 

 soils consists of calcium salts of the indefinite acids grouped 

 together as humic acid ; when attacked by a solution of 

 ammonium sulphate or chloride, calcium comes out into the 

 solution, while an equivalent amount of ammonium goes 

 into combination with the humic acid. Even when the 

 mixtures of humus and ammonium-salts were repeatedly 

 evaporated to dryness in a current of air or carbon dioxide 

 to represent in ah exaggerated way conditions which might 

 occur in the soil, no production of free acid took place. 

 In consequence of these failures, search was then made for 

 some living agency in the soil which would set free acid 

 from ammonium-salts, and small quantities of the acid soils 

 from the Rothamsted grass plots were introduced into nutrient 

 solutions containing ammonium - salts and organic matter. 

 A clue was at once obtained to the actions going on in 

 the field, for the soils were found to induce in the nutrient 

 medium a very rapid and abundant growth of moulds and 

 other minute fungi, the development of which was accompanied 

 by an increasing acidity in the culture liquid. The moulds 

 require nitrogen for their nutrition, and in order to obtain 

 it they split up the ammonium-salts and set free the acid. It 

 was shown that the degree of acidity thus generated corre- 

 sponded approximately to that prevailing in the water in the 

 soil of the acid plots at Rothamsted soon after the application 

 of the ammonium - salts in the spring, being such as could 

 be produced by the liberation of the acids contained in 

 the manurial salts. The surface soil of these plots was found 

 to be swarming with microscopic fungi, and several species 

 were separated and identified, all of which would attack 

 ammonium - salts and liberate acids, though to a different 

 degree. Further examination of the Rothamsted acid grass 

 soils showed that in addition to the small quantity of acid 



