MICROBIOLOGICAL ANALYSIS OF SOIL 717 



basis of the fact that all normal cultivated soils contain active nitrifying 

 organisms, which transform ammonia into nitrate. 



There are other cases on record where the nitrate and ammonia 

 formation in the soil and bacterial numbers are not correlated. When 

 fresh organic matter, particularly of non-nitrogenous nature, is added 

 to the soil, there is a rapid increase in the number of microorganisms. 

 This is not accompanied by an immediate increase in the amount of 

 ammonia or nitrate in the soil, but rather by a decrease, due to the 

 fact that the microorganisms use up the available nitrogen compounds 

 in the process of growth and multiplication. Russell and Appleyard 

 also observed that the curve for nitrate always lags behind that of 

 bacterial numbers. Also as a result of partial sterilization of soil, the 

 bacterial numbers greatly increase without any corresponding increase 

 in nitrates; the ammonia increases but not necessarily in proportion to 

 the numbers. The lack of correlation between certain bacterial proc- 

 esses, such as nitrogen changes, and soil fertility may be due to the 

 fact that, in these cases, the latter is limited by some factor other than 

 the nitrogen supply, such as moisture, temperature, aeration. 



Whenever plant growth is limited by the supply of compounds pro- 

 duced by bacterial activities, the relationship between bacterial numbers 

 and activities and plant growth is definite. Otherwise it may be 

 accidental or it may not exist at all. 



CARBON DIOXIDE EVOLUTION 



The carbon dioxide produced by microorganisms from the decomposi- 

 tion of organic matter has both a chemical and physical action upon 

 the soil. It renders certain insoluble soil minerals soluble and it imparts 

 to the soil, after spring plowing and cultivation, a condition of ripeness 

 ("Gare" in German). An increased carbon dioxide production also 

 stimulates plant growth. Since all heterotrophic aerobic microbiologi- 

 cal processes are accompanied by the production of carbon dioxide, 

 this can be readily taken as an index of the microbiological activities 

 in the soil. After harvesting a crop of rye, oats, clover or alfalfa, 

 considerable amounts of organic matter are left in the soil, so that the 

 quantities of carbon dioxide formed, as a result of the decomposition 

 of these residues in the soil, are quite appreciable. The larger part 

 of the organic matter is decomposed in the first few days, the rate of 

 decomposition soon falling off. 



In 1905 Russell 19 pointed out that soil oxidation, when measured 



19 Russell, 1905 (p. 683). 



