FERTILITY AND HOW TO MAINTAIN 49 



accompanying this we have the unsuccessful farmer. Plowing is an expen- 

 sive operation. It is estimated that the power required annually to plow 

 the farm land of the United States exceeds that used in the operation of 

 all the mills and factories in the country. 



There is a limit to the profitable depth of plowing, and numerous 

 experiments indicate that it is seldom profitable to plow deeper than 

 eight to ten inches. There doubtless are some exceptions to this found in 

 case of the production of intensive crops or the occasional deep plowing 

 for the preparation of a deep-rooted crop like trees or alfalfa. Deep plow- 

 ing increases fertility by increasing the area of pulverized soil in which 

 the roots of the plants find pasturage. Such plowing increases the aera- 

 tion of the soil, encourages the multiplication of bacteria to a greater depth 

 in the soil, and results in increased availability of plant food. Deep plow- 

 ing also incorporates the organic matter applied as manure or as the stubble 

 of the preceding crop in a deeper stratum of soil, thus increasing its water- 

 holding capacity. Deep plowing also increases the penetration of rainfall 

 and provides for greater storage of it. This provides a larger water supply 

 for the growing crops in periods of drought. 



Tillage is Manure. Cultivation of the soil, and especially the inter- 

 tillage of crops, such as corn, potatoes and truck crops, aids in maintaining 

 fertility: first, by conserving soil moisture; second, by more thorough 

 aeration of the soil; third, by a fuller incorporation and distribution of 

 the organic matter with the mineral matter; and fourth, by the destruc- 

 tion of weeds which consume plant food and water to the detriment of 

 the crop grown. 



Rotations are Helpful. Crop rotations also help to maintain fertility. 

 By means of rotating crops the soil may be occupied for longer periods of 

 time than when one crop is planted year after year on the same soil. The 

 roots of different crops, having very different habits, occupy somewhat 

 different zones in the soil. A shallow-rooted crop may be advantageously 

 followed by a deep-rooted one. One takes the major portion of its plant 

 food from near the surface and the other from a somewhat lower stratum. 

 All crops do not use mineral constituents in the same proportion. One 

 which demands large amounts of nitrogen may appropriately follow one 

 which has the power of gathering nitrogen from the air. For example, 

 corn appropriately follows clover, the corn benefiting by the nitrogen left 

 in the soil by the roots and stubble of the clover crop. 



Rotations Reduce Diseases. Rotations also make for fertility by 

 checking the epidemics of plant diseases and the depredations of insects. 

 As a rule, a plant disease is common only to one crop and where that 

 one crop is grown year after year on the same soil the disease increases 

 until finally the crop must be abandoned. Many of the insect pests of crops 

 either live permanently in the soil or have but little power of migration. 

 These likewise prey upon certain crops and do not bother others, and the 

 rotation of crops prevents serious injury by them. While these do not 



