THE BIOLOGICAL CONDITIONS IN THE SOIL 243 



The general mixed flora developing on nutrient gelatine 

 plates is also much less numerous on sour than on a neutral 

 soil: Fischer (98) and Bear (11) have both shown marked 

 increases in bacterial numbers after applying lime to soils. ^ 

 The rate of ammonia production does not appear to be so 

 much affected as the numbers of bacteria, but this is explained 

 by assuming that fungi play an increasingly important part 

 in ammonia production as the acidity of the soil increases.^ 

 Nevertheless there is on the whole a marked increase in the 

 rates of decomposition of organic matter and of production of 

 nitrate^ when lime is added to soil. 



Organisms causing plant diseases also vary in their degrees 

 of tolerance of acidity : in some cases the true acidity was 

 determined. Pythium, which produces damping off in seedlings, 

 is somewhat intolerant ; consequently no good, and sometimes 

 harm, is done by adding lime to affected soils. Also the 

 organism producing potato scab {Spongospora subterraned) is 

 somewhat intolerant, a fact which permits the control of the 

 disease since the potato itself tolerates a higher degree of 

 acidity. Actual measurements by Gillespie and Hurst (105^) 

 show that scab is common on the Washburn loams of North 

 Maine where the Ph value is 59, but rare on the Caribou 

 loam where the Ph value is 5*2, indicating a higher acidity. 



On the other hand, some organisms are more tolerant of 

 acid conditions or even grow better in them. Plasmodiophora 

 brassica, producing finger and toe, does so ; it is common on 

 acid soils, but is usually kept in check by addition of lime. 

 Further, it is commonly stated that fungi predominate in acid 

 soils, and this may be the case,^ but the evidence is rather 

 a priori in character, since no satisfactory method of enumera- 

 tion is known (p. 257). 



^ See also data in Christensen's and Hutchinson's papers. 



"^ Lyon and Bizzell, Cornell Memoir, i, 1913 ; N. Kopeloff, Soil Sci., 1916, 



& 571- 



' For quantitative measurements see Blair and McLean, Soil Sci., 1916, i^ 

 489-504. 



*It has been noticed at sewage works, e.g. at Huddersfield, that fungi 

 develop when the sewage becomes acid through trade effluents 



16* 



