SUBSOILING AND DEEP TILLING 



147 



and plant-food elements. On deep, rich soils deep plowing is not 

 absolutely necessary. On the better soils it is not necessary to 

 plow deep for small grams — three to f om- mches is usually better 

 than six inches. On the poorer soils fall plowing to a depth of six 

 inches for grain is often more desirable than shallow spring plowing. 

 For corn, and especially for the sugar beet, a deep seed bed is to 

 be preferred. 



Deep hillside plowing is frequently recommended for fields 

 that wash easily (Fig. 84). This causes much of the rainfall to 

 soak into the ground, thus checking or lessening the run-off. All 

 hillside plowing should be done at right angles to or across the slope. 



Fig 84 — Hillside plow (Walking type ) 



New or virgin lands seem to produce best when initial plowing 

 is comparatively shallow. Since in such soils the helpful soil 

 organisms necessarily inhabit a shallow surface zone, too deep 

 plowing buries them so deep that they cannot properly perform 

 their function in making plant-food elements available. 



It is good farm practice to vary the depth of plowing from year 

 to year. If a soil is always plowed at the same depth, the tramping 

 of the horses and the weight of the plow on the furrow bottom 

 tend to compact and harden it, causing the formation of a so-called 

 *' plow-sole." This danger is greatest in wet clay soils. A plow- 

 isole is detrimental because it retards or prevents percolation, 

 entrance of air and root penetration. 



Subsoiling and Deep Tilling. — ^Plows have been built to stir the 

 soil at greater depths than can be accomplished by the' common 

 plow. These implements are called subsoilers and deep-tillers 

 (Figs. 81, 82 and 83). A subsoil plow is used when the subsoil is 

 so compact that water and roots cannot penetrate it. This imple- 



