126 



SOIL CONDITIONS AND PLANT GROWTH 



produce ammonia and nitrate, for relatively large amounts of these sub- 

 stances are left at the end of the experiment. 



In similar manner the growth both of bacteria and of plants may 

 be helped by the same cause. Speaking generally it is found that the 

 bacterial numbers increase as the intensity of the farming increases. 

 Thus Stoklasa and Ernest (273) found only 1-2 million organisms per 

 gram in their barley land, 3-5 millions on the better treated sugar- 

 beet land, and 7-8 millions on the clover land. Again, the addition of 

 plant residues to the soil increases the bacterial numbers by furnishing 

 the organisms with additional food ; it also commonly increases the 

 crop. Moorland soils contain only few bacteria and are very unsuited 

 to the growth of most plants. But after cultivation and treatment 

 with lime and manures they become much better media both for 

 plants and bacteria. Fabricius and von Feilitzen (960) found o - i 

 millions of bacteria per gram in the raw moorland soil, but 7 millions 

 in similar soil that had been cultivated and manured. 



It may often be difficult in practice to determine whether the 

 relationship between the bacterial numbers and plant growth is causal 

 or accidental, but the principle is perfectly clear ; the relationship is 

 causal only when the plant growth is limited by the supply of com- 

 pounds produced by bacterial activity. The recognition of this central 

 principle greatly facilitates investigation, for it shows the futility of 

 haphazard attempts to correlate bacterial activity and plant growth 

 over a set of soils that are not strictly comparable. The better course 

 is to narrow down the problem and confine it to the elucidation of the 

 connection between bacterial activity and nitrate production. 



Bacterial Activity and Nitrate Production. 



Effect of Temperature. Bacteria being living organisms it is 

 natural to expect that their activity increases with the temperature up 

 to a certain point. The amount of nitrate does show this expected 

 increase but the bacterial numbers do not, there being no steady rise 

 as the temperature of storage increases (Table LI.). 



TABLE LI. EFFECT OF TEMPERATURE OF STORAGE ON BACTERIAL NUMBERS AND 

 NITRATE PRODUCTION. RUSSELL AND HUTCHINSON (24001). 



