294 



SCIENCE PROGRESS 



The Part Played by the Soh. 



It has already been stated that the water, temperature, and 

 air conditions of the soil are intimately bound up together, 

 and that the effects of water supply and temperature depend 

 on the food present. An ideal set of curves expressing this rela- 

 tionship in the case of water and food supply would resemble 

 those in the figure : the temperature curves would probably be 



CT. CdA 



Curves showing the nature of the relationship between plant growth, 

 food supply and water supply. 



similar. For a particular amount A of food-stuff present in the 

 soil, the crop might be anything between a and c according to 

 the water supply or the temperature ; and an increase in food 

 supply from A to B would lead in the first case to a small 

 increase aa' and in the latter to a larger one cc\ corresponding 

 to the vertical distances between the points a and a' and c and c' 

 respectively. These varying increases in crop for the same 

 dressing of manure may be taken as an indication of the 

 " efficiency " of the various soils — a factor of great importance 



