124 ELEMENTS OF AGRICULTURE 



118. Effect of Cultivation on the Water Supply of 

 Soils. — Besides- preparing the soil for the development 

 of plant roots, cultivation has a marked effect on the 

 water supply of soils by increasing the number and 

 reducing the size of the soil particles. The effect of the 

 number and size of the soil particles on film moisture 

 has already been described in Chapter XVII, and the 

 effect of drainage on the free water in Chapter XXII. 



119. Aeration. — Surface cultivation also admits a 

 freer circulation of air in the soil, by making the sur- 

 face more open or porous, thus supplying oxygen for 

 the roots of plants and for the soil bacteria. 



120. Cultivation Destroys Insects and Weeds. — Of 

 the many kinds of insects that feed on cultivated crops 

 there are a number that build their nests in the 

 ground, where their eggs are deposited to be hatched 

 out by the spring sunshine. Cultivation dectroys these 

 nests, bringing many insects and their eggs to the sur-* 

 face, where they are eagerly devoured by birds or killed 

 by exposure to the weather. 



Many soils are covered by a growth of weeds which, 

 when plowed under, rot and add to the soil a supply 

 of humus and nitrogen. If left to grow these weeds 

 seriously interfere with the growing crops. 



121. When to Cultivate.— There is great difference 

 of opinion as to whether soils to be planted in the 

 spring should be plowed in the fall and allowed to lie 

 bare all winter, or else plowed a short time before 

 planting the crop. As a matter of fact, the time of 

 plowing should be regulated by the character of the 

 soil. Soils that have a tendency to wash should never 



