CHAPTER VI 



THE CONTROL OF SOIL FERTILITY 



We now turn to the final part of our subject: the 

 study of the methods by which the fertihty of the soil 

 may be increased, or, in other words, the soil may be 

 made more favourable for the growth of plants. The 

 plant requires from the soil six conditions, viz. : 



1. Proper water supply. 



2. Proper air supply. 



3. Suitable temperature. 



4. Nutrient salts. 



5. Ample root room. 



6. Absence of injurious substances or pests. 

 These six are all quite distinct: it is no good satisfjdng 



five of them if the sixth is not attended to : any single 

 one left unsatisfied may operate as a limiting factor and 

 render the soil infertile. 



Thus the problem of increasing the fertility of the soil 

 reduces itself to the discovery, first of the factor or 

 factors limiting the growth of the crop, and then of the 

 best methods of overcoming the limiting factors. 



Sometimes the fault lies with the soil, sometimes with 

 its surroundings: in the first case the defects may be 

 caUed intrinsic, in the second extrinsic. It is necessary 

 carefull}^ to distinguish between these : there is obviously 

 no point in spending time and money in doing some- 

 thing to the soil when the surroundings are unsuitable, 

 or in elaborately trying to improve the surroundings 

 when the soil is not worth it. 



