222 



Physics of the Soil. 



Here is a mean difference of 3.1 F. at 1.5 inches, and 

 2.9 F. at 3 inches deep in favor of the rolled surface. 



270. Influence of Thorough Preparation of the Seed-bed on 

 Soil Temperature. It follows, from what has been said in 

 previous paragraphs, that the practice of thoroughly pre- 

 paring the seed-bed before sowing or planting must have 

 the effect of decreasing the capillary rise of cold water 

 from below and its loss by evaporation from the soil. This 

 then would tend to concentrate the sun's heat in the seed- 

 bed itself, first by lessening its rate of conduction down- 

 ward, and second by diminishing its loss, by lessening the 

 evaporation. In -the spring, then, early and thorough 

 preparation of the seed-bed tends to make the seed-bed 

 warmer ; it diminishes the loss of soil moisture ; it increases 

 the formation of nitrates, thus making the soil richer; it 

 hastens and makes stronger /the germination and it enables 

 one or more crops of weeds to be destroyed before the crop 

 is up in the way of cultivation. Hence there is much to 

 gain and little to lose in the thorough preparation of the 

 seed-bed before planting. 



271. Controlling Soil Temperature by Underdraining. 

 When land naturally too wet for tillage early in the spring 

 has been thoroughly underdrained, the soil is brought into 

 fit condition for seeding much earlier than would be pos- 

 sible without this improvement, and one of the great points 

 gained is the warming of the soil to a greater depth, on 

 account of the removal of the water and tke lessening of 

 the loss of heat by evaporation. 



