262 SOILS: PROPERTIES AND MANAGEMENT 



the capillary water is called the maximum water content 

 for plant growth. The bad effects of free water on the 

 plant arise largely from the poor aeration that results 

 from its presence. Not only are the roots deprived of 

 their oxygen, but toxic materials tend to accumulate. 

 Favorable bacterial activities, such as nitrification and 

 ammonification, are much retarded also. 



The various forms of water in the soil and their avail- 

 ability to the plant are illustrated diagrammatically in 

 the following figure. 



HYGROSCOPIC W P0INT LENTOC/iPILLARtTY MAX/MUM WATER 



COEEE/C/ENTk I POINT CONTENT 



mW05C0P/(>[\ ^OPTIMUM WATER CONTENT \ \EREE«JUPEfm/VIBLE 



AVAILABLE MOISTURE 



Fig. 41. — Diagram showing the forms of water in the soil and their 

 relationship to the plant. 



184. Optimum moisture for plant growth.— It is 

 very evident that there must be some moisture condition 

 of a soil which is best for plant development. This is 

 usually designated as the optimum content. It is not 

 to be assumed, however, that the total range of the 

 available soil water represents this condition for optimum 

 plant growth.  Nor is this optimum water content in 

 any particular soil to be designated by a definite per- 

 centage. In reality the moisture in a soil may undergo 

 considerable fluctuation and yet allow the plant to develop 

 normally. This is because the physical condition of the 

 soil changes with varying water content and the plant is 

 able to accommodate itself to such a fluctuation without 

 a disturbance in its normal development occurring. 

 Wollny 1 has shown that the optimum moisture for com- 



1 Wollny, E. Untersuchung iiber den Einfluss der Wachs- 

 thumsfaktoren auf des Produktionsvermogen der Kultur- 



