Water in the Soil 



69 



of soils, gives a good idea of the water-storing power of 

 the different soils: 



The second column shows the per cent of water 

 that evaporated in four hours, when spread over a given 

 surface. It is seen that soils having capacity for large 

 amounts of capillary water part with it very slowly. 



102. What amount of water is most favorable to 

 the growth of plants? This has been experimentally 

 studied by Hellriegel, who found that oats, wheat, 

 and rye growing in sand able to hold twenty-five per 

 cent capillary water made maximum yield with fifteen 

 to twenty per cent water. He observed that the plants 

 would grow with no less vigor when the soil contained 

 even only 2.5 per cent water. Below this the plants 

 would wilt. It is not generally true that the most 

 favorable amount of moisture for the growth of a plant 

 is the full capillary power of the soil, as might be inferred 

 from the above results. The results of some investi- 

 gations of the United States Department of Agriculture 

 show that plants might suffer for lack of water (drought 

 limit) when the soil contained 15 per cent moisture, 

 while in other soils the plants were well supplied 



