96 



CONSERVATION AND IMPROVEMENT OF SOIL 



is plowed; gradually deepen the soil by increasing the depth of 

 tillage each time. By this method a little of the subsoil is very 

 gradually incorporated with the surface, and the depth of tillage 

 is increased without detriment. Too much of the subsoil turned 

 up at one time might be injurious, particularly if it be sour or 

 contain matter poisonous to plants. It has been said that a 

 man with a forty-acre field which was usually plowed to a 

 depth of six inches can double the yields by gradually doubling 

 the depth of tillage. Thus, when he is able to plow the field regu- 

 larly to a depth of twelve inches he will have in effect an eighty- 

 acre field. In fact the management of the forty-acre field, taken 

 as a whole, will be much less expensive than if he really had eighty 



FIG. 70. This tractor has wheels of the "caterpillar" type and may be used on rather 

 rough land without difficulty. Tractors give enough power to allow very deep tillage of 

 soils. Farming thus becomes both more intensive and more extensive. 



acres plowed to the original depth of six inches. If organic matter 

 be added from time to time during this process of deepening the 

 soil, the increased depth may be attained much more rapidly. 



Disk plows and disk harrows (Fig. 71), have greatly improved 

 the method of tillage. Heavy soils turned with the original mold 

 board plows are usually not sufficiently crumbled and mixed 

 during the process. If the plowed field be thoroughly disked the 

 tenacious particles are thoroughly crumbled and mixed. 



The effects of tillage may be either physical or chemical. 

 Proper tillage also includes the conditions for soil bacteria. The 

 moisture-holding effects of tillage have already been considered. 

 Dynamiting the soil is for moisture effects and is seldom practiced 

 except for the planting of trees (Fig. 72). 



