10 An Account of the Soil [pt. i 



of view but they are much more difficult to investigate 

 than mere changes in weight. 



From these and similar experiments we may deduce 

 three general principles of the highest importance in the 

 study of soil fertility : 



(1) Six separate soil factors are necessary for the 

 successful growth of the plant: there must be an ade- 

 quate supply of food, water, air, a suitable temperature, 

 sufficient root room and an absence of harmful sub- 

 stances. If any of these conditions is not complied with 

 the plant fails to grow well: the lacking condition is 

 called the limiting factor and it must be supplied or in- 

 creased before further growth takes place. 



(2) By increasing the supply of any of the factors 

 necessary for the plant (food, water, temperature, etc.) 

 an increase in growth is obtained. But a limit is sooner 

 or later reached beyond which further growth will not 

 take place. Additional increases in the food, water 

 supply, temperature, etc. may do positive harm. 



(3) When a crop has been increased by improving 

 one of the soil conditions {e.g., the food supply, water 

 supply, etc.) it is always possible that some other factor 

 which sufficed for the original crop is no longer sufficient 

 for the new and larger crop. Thus a hmiting factor 

 comes into play and prevents the farmer from getting 

 as large a return as he should from his outlay. It is 

 therefore necessary in all cases where land has been 

 improved to see that the screwing up of efficiency has 

 extended to all the six soil conditions, and finally to see 

 if some new variety of crop with larger power of growth 

 cannot be obtained that will do even better than the 

 best of the old varieties. 



