196 POWER AND THE PLOW 



reaching distance. It checks the cooling of the soil by sur- 

 face evaporation, and thus favors the germination of seeds. 

 It retards the loss of moisture in the heat of summer. It 

 promotes bacterial action in the soil, the fixation of atmos- 

 pheric nitrogen by bacteria, and the change of plant food 

 from the insoluble to the available form. It enables the soil 

 to recover in the shape of dew a part of the moisture lost by 

 evaporation. By more than a thousandfold increase in the 

 area of the soil grains or kernels and the volume of the air 

 spaces between, it enables the soil to fix a larger amount of 

 nitrogen from the air without bacterial aid. 



Plowing with properly designed moldboards accomplishes 

 by far the greatest amount of pulverization. In addition, it 

 checks the growth of weeds which steal food and moisture, 

 burying them beneath the surface, where they decompose to 

 improve the physical condition of the soil, and to yield up 

 supplies of humus and plant food for the benefit of plants of 

 economic importance. An analysis of the action of the mold- 

 board upon the furrow shows how the primary objects of 

 plowing are accomplished. Professor King has likened the 

 action of the share and the moldboard to what takes place 

 when all the pages of a book are grasped between the thumb 

 and fingers and bent abruptly. The furrow slice is divided 

 into thin layers which slide over each other like the leaves of 

 the book, dividing the soil into horizontal flakes. It will be 

 remembered that it was the old Berkshire plow's separation 

 of the furrow into layers that aroused and held Jethro Tull's 

 admiration. 



The inner wall of the furrow slice, next the land, must 

 travel faster than the outer edge as it comes up and over. In 

 the low moldboard of the prairie breaker this additional travel 

 is not great enough, nor the turns abrupt enough, to break the 

 tension of the elastic sod, which is usually cut in very shallow 

 strips. With a deep furrow, the horizontal shearing of the 

 soil layers over each other takes place, even though the elas- 



