BACTERIOLOGICAL STUDIES OF A SOIL SUBJECTED TO 

 DIFFERENT SYSTEMS OF CROPPING FOR TWENTY- 

 FIVE YEARS 



By P. L. Gainey and W. M. Gibbs, 



Department of Botany, Missouri Agricultural Experiment Station 



INTRODUCTION 



During the past few years a number of soil biologists have reported 

 their findings regarding the effects of different agricultural practices upon 

 the bacteria of the soil. The majority of these investigations have 

 been concerned with determining the gross effects of a particular treat- 

 ment upon the physiological activities of the flora as a whole. Such 

 factors as continuous cropping, rotational systems, cultural methods, 

 application of chemical fertilizers, and manures have been studied. 

 Among the more recent workers reporting such investigations in the 

 United States may be mentioned King and Doryland (io), 1 Stevens and 

 Withers (18), Stewart and Greaves (19), Lyon and Bizzell (12), Temple 

 (20), Jensen (9), Given and Willis (6, 7), Brown (2, 3, 4), Hill (8), 

 Allen and Bonazzi (1), Wright (22), and McBeth and Smith (13). 



It is not necessary to give any extensive review of the work that has 

 been done as the papers referred to above contain full summaries of the 

 results thus far obtained. Special attention may be called to the review 

 given by Temple (20) as to the effect of stable manure, by Hill (8) as to 

 the effect of other organic materials, and by Lyon and Bizzell (12) as 

 to the effect of different growing crops. The available data leave little 

 doubt that certain of the above-mentioned factors do exert a marked 

 effect upon soil organisms. In very few instances, however, has any 

 serious effort been made to ascertain just how such factors exert their 

 influence. In most instances the treatment in question has been in 

 operation a comparatively short time. Brown (3), for example, studied 

 the effects of a 4-year rotation while the fourth crop was still on the 

 soil, or before the cycle was completed the first time. It is true that 

 the quantities of nitrate nitrogen have been determined in situ, following 

 long-continued cropping systems. With our present very limited knowl- 

 edge as to the demands any particular crop makes upon soil nitrates, 

 such information gives us little insight into nitrate formation. Allen and 

 Bonazzi (1) carried out a few laboratory experiments, following a long- 

 continued treatment, but obtained such irregular results that they 

 regarded them of little value. Given and Willis (6, 7) have also re- 



1 Reference is made by number to "Literature cited," p. 974-975. 



Journal of Agricultural Research, Vol. VI, No. 24 



Department of Agriculture, Washington, D. C Sept. 11, 1916 



fj Mo.— 3 



(953) 



