92 KANSAS UNIVERSITY SCIENCE BULLETIN. 



contrast to that with Proteus vulgaris. The yield of clover 

 with the yellow ammonifier on the manured soil is four times 

 as great as that in the sterile manured soil. These are varia- 

 tions which are well beyond the factor of error. It is appar- 

 ent that individual organisms when permitted to grow in pure 

 culture in soils, influence the composition of the soil solution 

 sufficiently to reveal an effect upon the crops grown in such 

 solutions. On the supposition that continued one-cropping will 

 modify the bacterial flora of the soil by favoring some forms 

 and hindering others, it seems reasonable to expect that such 

 a change of the flora will exert an influence upon subsequent 

 crops. 



THE EFFECT OF CROPS ON THE GROWTH OF ORGANISMS IN SOIL 



EXTRACTS. 



Experiments were next performed in an endeavor to show 

 that certain crops can so modify the soils' composition as to 

 favor certain types of organisms and perhaps retard others. 

 These were carried on in the following manner : 



Three distinct types of soils, sand, loam and marsh, were 

 employed. For each soil eight pots were filed with 3000 grams 

 of soil. These were then planted to corn, oats and clover, using 

 two pots to each soil for each crop ; the fourth set being left 

 unsown to serve as controls on the respective soils. 



After luxuriant growth had occurred with penetration of 

 the root systems throughout the soil mass (about six weeks' 

 growth), the soil was shaken free from the roots, run through 

 a Vi-inch sieve and then air-dried at 37° C. Of the dry soil so 

 obtained, 1000-gram portions were vigorously shaken with 1000 

 cc. of distilled water and then immediately filtered through 

 paper as previously described. In this manner between 600 

 and 800 cc. of clear extract were secured from each cropped 

 soil. In the case of the marsh soil 1200 cc. of water were em- 

 ployed, owing to its large water-holding capacity. 



The extracts thus secured were then measured with a burette 

 into test-tubes in 10 cc. quantities. After plugging, these tubes 

 were sterilized and subsequently used as culture media for the 

 growth of pure cultures of soil bacteria. The rate of multipli- 

 cation of the organisms in these extracts was used as an index 

 of the influence, whether beneficial or detrimental, which the 

 respective extracts exerted. 



