322 Manures, Composts, Green Manures 



19. Waksman, S. A., Humus; Origin, Chemical Composition and Importance in 

 Nature, Williams & Wilkins Co., Baltimore, 2nd Ed., 1938. 



20. Waksman, S. A., Tenncy, F. G., and Diehm, R. A., Chemical and micro- 

 biological principles underlying the transformation of organic matter in the 

 preparation of artificial manures, /. Am. Soc. Agron., 21:533-546, 1929. 



