124 THE FERTILITY OF THE SOIL [ch. 



CHAPTER IX 



CONCLUSION 



We can now sum up the general conclusions to 

 which the previous chapters lead. 



The problem of making a soil fertile consists in 

 finding out first what conditions the plant requires 

 for its proper development, and then altering the soil 

 as far as possible to make it meet these requirements. 

 If the discrepancy between the actual and the ideal 

 is too great the plant may be altered by the breeder 

 in such a way that its new requirements shall be 

 nearer the actual possibilities of the case. 



It is always most economical to select crops 

 naturally adapted to the climatic and soil conditions, 

 so that the gap to be bridged shall be small. Our 

 problem is to alter the soil : in the first instance it is 

 necessary to ascertain what the actual soil conditions 

 are, and then to find which constitutes the limiting 

 factor and to change that one. Probably another will 

 now be found to set the limit : this must in turn be 

 changed and so on until the limit is set by the 

 incapacity of the plant to make further growth, and 

 not by any soil factor. The responsibility of the soil 

 investigator is now at an end, and the problem becomes 

 one for the plant breeder. 



