336 



W. C. VISSER 



The logarithm of the vertical co-ordinate of curve A in Fig. 8, with the 

 horizontal asymptote as zero line, also approaches a straight line and shows 

 that there exists a close parallel between the growth factor governing the 



Yield depression 

 O 



15 20 



40 60 80 lOO I40 200 



Ground- woler level in cm-surtace 



Fig. 9. The influence of the factor water, derived by the subtraction of the logarithms 

 of the yields, plotted against the numerical value of the yield. 



Yleld^ depression in % 



50- 

 70- 



20 



40 60 80 120 



Ground-water level in cm-surface 



Fig. 10. The deficit in yield of Fig. 9, plotted logarithmically against the logarithm of 

 the water depth, shows parallel hnes pointing to the applicabihty of formula 1.5. 

 The curves may be shifted to coincidence, giving curve B for the influence of the 

 groundwater depth. 



left-hand side of the curves and the air content in the soil profile, as is 

 found by comparmg with pF-curve. 



The relation between the yield and the groundwater depth, as derived 

 from the available field observations, is not in contradiction to the yield 

 formulae nor to experience with respect to soil physics and plant physiology. 



