88 KANSAS UNIVERSITY SCIENCE BULLETIN. 



the bacterial flora. Certain plant species will favor certain 

 types of bacteria and inhibit others and thus disturb the bac- 

 terial equilibrium of the soil. The new flora thus established 

 then produces specific changes in the composition of the soil 

 which affect subsequent plant growth, favoring some plant 

 species and retarding others. 



The problem as stated above is complex and presents many 

 difficulties. With the three variable factors, crop, soil and 

 bacteria, it was realized that little more than substantiation 

 of the principles above set forth, in a few specific instances 

 could possibly be hoped for. The establishment of the fact of 

 such an interrelationship between soil crop and soil bacteria, 

 was deemed worthy of an attempt. 



On the basis of Schreiner's work, it was assumed that ex- 

 tracts made from sterilized soils in which different types of 

 bacteria had been grown, would show differences when em- 

 ployed as nutrient solutions for the growth of various types of 

 plant seedlings. 



"The good or bad properties of a soil as shown by the growth 

 of plants upon it, are largely transmitted to the aqueous ex- 

 tract of that soil." 15 



In the same manner a soil continuously cropped with one 



plant must show differences when employed in extract form 

 for the growth of soil organisms. 



"We must regard the excreta of the growing plant roots 

 as one of the main causes of the low yields obtained in improper 

 rotation of crops. By alternating the crops one invariably 

 secures larger yields than where one crops continuously with 

 a given crop." 10 



These two principles were employed as the basis for the 

 work herein reported : First, to ascertain in individual cases 

 what influence the growth of different pure cultures of soil 

 organisms in sterile soil will have upon certain plant seedlings 

 grown in the extracts of such soils ; second, to ascertain what 

 influence the growth of different plants in the same soil may 

 have upon pure cultures of bacteria grown in the extracts of 

 such cropped soils. 



Considerable difficulty was experienced in the use of tho 

 ordinary perforated cork for the growth of the seedlings. After 



15. Bureau of Soils, Bulletin 28, p. 21. 



10. Bureau of Soils, Bulletin 40, i>. 40. 



