136 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



Water Capacity. The water capacity of a soil is the amount 

 of water held by the partly drained soil. Fifty grams of soil 

 are placed in a tube i% inches in diameter, allowed to drain, and 

 weighed. The gain in weight, expressed in per cent., is the water 

 capacity. Other methods which vary in detail are used. The 

 percentage of water held will vary with the height of the column 

 of soil, and to some extent, with the time of draining, but the 

 results are comparable if the same method is used on different 

 soils. 



The following figures show the water capacity of some types 

 of soils, determined by the method given above : 



Water capacity 



of some soils 1 



Per cent. 



Tarboro sand 25. i 



Norfolk sand 29 .6 



Cecil clay 45.0 



Cecil sandy loam 36.8 



Durham sandy loam 28.9 



Amount of Water at Disposal of Plant. The amount of water 

 at the disposal of the plant varies from time to time, according to 

 a number of factors. The principal groups of factors are as 

 follows: (i) The available water in the soil; (2) the root area of 

 the plant; (3) the losses and gains of water by the soil; (4) the 

 depth of the water-table. 



Available Water in the Soil. The amount of available water 

 in the soil depends upon the nature of the soil, and the nature of 

 the plant. 



Nature of Soil. The forces which cause water to enter a plant 

 are opposed by the osmotic pressure of the soil solutions and by the 

 hygroscopic attraction of the soil particles for water. The soil 

 attraction increases as its moisture content diminishes, con- 

 sequently decreasing the rate of entrance into the plant, and 

 diminishing the production of organic substance if the amount 

 supplied is below the optimum. When the outgo of water 

 becomes greater than the income, the plant wilts. The point at 

 which wilting takes place varies with the nature of the plant, but 

 1 Rep. N. C. Exp. Sta., 1902-3, p. 39. 



