SOIL WATER 91 



Drainage 



95. Best Amount of Water. Water is the most im- 

 portant plant-food, the one that most frequently limits 

 the crops of the world. It is also the plant-food that 

 frequently causes injury by appearing in too large quan- 

 tities. 



For the best growth of -crops, the water content of 

 the soil should be maintained at about 50 to 60 per cent 

 of the water capacity of the soil. (Laboratory Exercise, 

 page 107.) If there is either much more or much less 

 water, the growth of the plant is injured. Commonly the 

 soil is saturated with water during the early part of the 

 season, and later becomes too dry, so that the crop is 

 injured by both extremes. 



96. Harmful Effects of Too Much Water. The most 

 serious result of having too much water in the soil is the 

 exclusion of air, which is essential for plant growth and 

 for the activities of the soil organisms (page 97). It also 

 prevents plant roots from growing deeply into the soil, 

 makes the soil cold and delays farm work. Since farm 

 work cannot be done at the proper time, weeds are more 

 likely to obtain a foothold. Wet land is nearly always 

 weedy land. 



97. All Soils Require Drainage. If a soil is not drained, 

 the excess of water will prevent the growth of crops. 

 Or, if there is no excess of water, salts and acids will ac- 

 cumulate to such an extent as to kill crops, as in the case 

 of alkali and marsh soils. Fortunately a very large pro- 

 portion of the farm land is underlain by porous subsoil, 

 so that drainage takes place naturally. Whenever the 



