Jan. 13, 1919 Nitrates, Nitrification, and Bacteria of Acid Soils 39 



materially in the nitrogen-fixation processes of acid soils. Small applications of 

 calcium carbonate are, as a rule, relatively more effective than large applications as 

 a means of increasing the bacterial activities in acid soils. 



The problem of maintaining soil fertility resolves itself into maintain- 

 ing and increasing the available supply of organic matter and nitrogen 

 in the soil and the replenishing of the mineral elements. One system now 

 generally recommended and used is to apply lime and phosphates, then 

 to grow legumes, and to plow them under. This system of soil mainte- 

 nance and impro^"ement is in accordance with the important role of soil 

 bacteria in plant nutrition, and the results obtained in the controlled 

 investigations reported here illustrate some good reasons for such a 

 method of soil management. 



When the soil was limed, the aerobic bacteria concerned with oxida- 

 tion reactions increased in numbers. This is illustrated by the increased 

 bacterial numbers and nitrification wherever the soils were limed. 



Plenty of organic matter is necessary for high bacterial numbers, a 

 condition which is well illustrated by the low bacterial content and 

 nitrate results with the limed yellow silty clay (low in organic matter) 

 compared with the high bacterial contents and nitrates on the limed 

 brown silt loam and dark-brown peat (high in organic matter) . 



Mineral fertilizers serv'e as food for larger crops and larger crops in 

 turn leave more residues in roots and stubble for bacterial food. 



The number of bacteria in an arable soil can be correlated with crop 

 yield to about the same degree that soil moisture can be. Soil moisture 

 is conceded to be the most vital single factor influencing crop yields ; yet 

 because of so many other variable conditions it is not always possible to 

 correlate soil moisture and crops any more than it is possible to always 

 correlate bacterial numbers and crops. Below a certain minimum in 

 moisture or bacterial numbers field soils will not produce crops; above 

 that minimum, everything else being equal, crops may be in general 

 correlated with bacterial numbers as well as with moisture. 



Changes in bacterial numbers, especially differences in the propor- 

 tions of aerobes to anaerobes, are of prime importance in soil-biology 

 studies. The results here reported under controlled conditions make 

 it evident that soil-fertility investigations should include both chemical 

 and biological examinations of the soil. 



SUMMARY 



(i) Controlled greenhouse investigations were conducted on five 

 typical acid soils. In part of the experiments the soils were fertilized 

 with calcium carbonate, acid phosphate, and complete fertilizer, cropped 

 to wheat and clover, and kept at optimum moisture content, while in 

 another series the soils were unfertilized, uncropped, and kept one- 

 fourth, one-half, and fully saturated with water. 



(2) The results reported include crop yields, soil-acidity determina- 

 tions, nitrates in the soil when sampled and after incubation with ammo- 



