262 SOILS: FROPEBTIES 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- 

 abilit}" to the plant are illustrated diagrammatically in 

 the following figure. 



I4/71T/A/G 

 HYGmSCOPlC f^oiNT LENTOCmLLABITY MAXIMUM IV/fTiff 



COCmOENTs. I POINT CONTINT 



HYGHOXOPIC^n ^OPTIMUM WATER CONTEhIT \ xFREE^'^WERmimLt 



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 ^ has shown that the optimum moisture for com- 



^WoUny, E. Untersuchung liber den Einfluss der Waehs- 

 thumsfaktoren auf des Produktionsvermogen der Kultur- 



