THE SOIL AND WATER 

 EFFECT OF MOISTURE ON CROP. 



127 



The most favorable quantity of water in this case was 40 per 

 cent, of the water capacity of the sand. Necessarily in this 

 respect there will be a difference for different kinds of plants. 



According to this experiment, there is an optimum condition 

 of the soil at which the moisture content is most favorable to 

 plant growth. Below or above this optimum, there is a decrease 

 in yield, independent of the wilting of the plant. Wilting is a 

 sign of distress, signifying that moisture is lacking to an extent 

 that endangers the life of the plant. The growth may suffer 

 from lack of water long before wilting takes place. The experi- 

 ments of Hellriegel were carried on in pots, in which there was a 

 limited amount of soil at the disposal of the plants. In the open 

 field, in which a greater range of root development is permitted, a 

 smaller percentage of available water may suffice. 



Forms of Water in the Soil. Water is present in the soil as 

 water of hydration, hygroscopic water, capillary water, and flow- 

 ing water. 



Water of hydration is water in chemical combination with cer- 

 tain soil constituents, such as hydrated silicates (zeolites) and 

 hydrated oxide of iron. Most of it is retained when the soil is 

 dried at 100, and is driven off on heating the soil to a high tem- 

 perature. As water of hydration cannot be taken up by plants, it 

 cannot be considered to be of value to the plant. 



Hygroscopic Water is water which is absorbed by the soil from 

 the atmosphere. Every body in a moist atmosphere has a layer of 

 water upon it, the thickness of which depends upon the tempera- 



