310 GENERAL SCIENCE 



where surface earth is removed by running water, the soil 

 will be impoverished. 



Soil Water. - - The productiveness of soil is limited by 

 the amount of plant food that it contains. Water is the 

 most important plant food. Since plants are able to take 

 their food only in water solutions, the productiveness of 

 any soil is limited by the amount of water that it can hold 

 and by the facility with which it yields up this water to 

 growing plants. Soil water differs from rain water in 

 that it contains the plant foods in solution, while rain 

 water is chemically pure. , 



Water exists in soil as ground water which drains away, 

 and capillary water which varies with the kind of soil. 

 The finer the soil particles the greater the amount of capil- 

 lary water or film water. 



Regulation of the Amount of Soil Water. Soil water 

 may limit the crop by being present in either too great or 

 too small amounts. Dry farming and irrigation are prac- 

 tical ways of obtaining crops in regions where there is too 

 small an amount of soil water. Commonly there is too 

 much water in the soil in the early part of the season and 

 too little as the season advances, so that the crop is injured 

 by both extremes. 



When there is too much water in the soil, air, which is 

 essential for plant growth, is excluded, and the soil 

 remains cold. This retards the growth of the plant and 

 delays farm work. 



Artificial Drainage. Underground drainage of some 

 sort is essential for the growth of ordinary crops. Where 

 the soil is not drained, the excess of water prevents 

 the growth of crops, and where there is no excess of 

 water to drain away, the soil becomes either acid or 

 alkaline due to the accumulation of salts and acids, 



