358 W. C. VISSER 



The constant Xq indicates at what level of the production parameter the 

 optimum frequency is found. This optimum is the result of the combined 

 effect of all the competing crops together within the farm economy. The 

 constant ^ indicates with what frequency the crop is planted under optimal 

 conditions and depends therefore on the importance of the crop within the 

 farm organisation and the farm economy. 



The constant a points out how the farmers react to decrease in profits 

 and increase in costs under sub-optimal conditions and may be considered 

 as a more sociological parameter. Can this parameter be independent of 

 the soil and the crop ? 



Considering that according to the yield equation for the law of diminish- 

 ing returns the yield depression D may be described with 



D = ^^-^(^-c) (3) 



and considering that b is claimed to be constant, it may be assumed that the 

 yield decrease for different soils and crops will not differ much by variation 

 of the growth factor and it seems not too bold a step to assume that a 

 same size of yield decrease will induce the farmer to react in the same way. 

 If t in formula 3 is near constant for the same growth parameter it makes 

 sense also to take a in formula 2 as constant for the same production 

 parameter. This assumption is the more acceptable, because constancy or 

 inconstancy cannot be proved statistically, due to the restricted accuracy of 

 the data commonly available. 



The Clay Content and the Cropping Pattern 



The clay content is a useful parameter to indicate the moisture available in 

 the soil profde. It governs rather strongly which crops will be grown. 

 Potatoes and rye, for instance, are typical crops for dry profdes with low 

 clay content. 



To study the relation between cropping pattern and clay content the 

 crops over five years of 2000 fields were collected. For successive groups 

 of clay content the frequency of occurrence of the different crops was 

 estabhshed. The logarithm of the frequency percentage, plotted against 

 the clay content, yielded curves as given in Fig. i. 



These curves now have to be fitted around a paraboHc curve. This 

 parabola is only partly covered by the longest curve which is possible in 

 view of the variation in clay content. The relative unimportance of the 

 value of the constant a clearly results from the inaccurate determination of 

 the degree of curvature, due to the relative shortness of these partial curves. 

 The probabihty that the reaction curve is a parabola must to a greater 



