54 Plant Physiology 



types with respect to the water-holding capacity in order 

 properly to understand the relation of the plant to the 

 soil. The water-holding capacity of the soil as ordinarily 

 measured is the ratio of the weight of water, when the 

 soil is at the point of saturation, to water-free soil, or dry 

 weight of soil. 



The following table is an indication of what may be 

 expected in this regard in certain general soil types : 



Ordinary sand 20- 30 per cent 



Rich sandy loam 30- 40 per cent 



Rich clay loam . . 40- 60 per cent 



Very heavy clay 60- 70 per cent 



Garden soil, rich in humus 70- 90 per cent 



Humus 100-300 per cent 



It is considered to be a general rule that a* large number 

 of cultivated plants find most favorable conditions for 

 rapid growth when the soil contains from 40 to 50 per cent 

 of its maximum water capacity. In the case of a sand, 

 therefore, with a maximum capacity of 25 per cent the 

 optimum for plant growth would be 10 to 12| per cent. 

 This will depend somewhat, however, upon the other con- 

 ditions under which grown. 



Under the conditions encountered in the laboratory and 

 greenhouse, where in general the soil employed has been 

 well stirred, or is constantly well aerated, the optimum 

 moisture content may be much higher. In some cases 

 this optimum may run as high as 70 to 75 per cent. Under 

 field conditions it is apparent that the amount of organic 

 matter and the aeration of the soil are most important in 

 determining the optimum. When organic matter is abun- 

 dant in the soil, especially when the soil is compact, a 



