INFLUENCE OF ROOT EXCRETIONS 



193 



to the sand. A non-legume (oats) was planted in the small pots 

 and a legume (peas) in the outer pots. As growth progressed, the 

 oats in the unglazed pot developed far better than the oats in the 

 glazed pot, showing effects of nitrogen fertiUzation. It is appa- 

 rent that some nitrogenous substances became available to the oat 

 plants as a result of development of the peas. 



Unfavorable conditions for root development enhance the 



Fig. 74. — Influence of a legume (peas) on growth of a non-legume (oats). 



Oats in inner pot, peas in the outer. Porous inner pot on left; glazed inner 



pot on right (from Lipman). 



activity of microorganisms about the roots, on account of death 

 of the root parts and attack by soil saprophytes. Under condi- 

 tions of deficient aeration, certain substances may accumulate 

 which become toxic to plants and microorganisms; they generally 

 disappear rapidly under conditions favoring thorough aeration 

 of soils. The conditions brought about by continued cultivation 

 of cereals and the introduction of appreciable amounts of organic 



