36 SOILS 



drawn from a near-by swamp. The chief 

 reason for adding humus to a soil is to improve 

 its texture, but another benefit, and one that 

 is often quite important, is to improve its 

 colour. 



The buff yellow and yellowish-brown colours of 

 soils are usually due to the presence of iron oxides. 

 These soils are most common south of the glaciated 

 part of the United States, particularly in the south- 

 ern Appalachian states. 



The Influence of Tillage on Soil Temperature. 

 The way in which a soil is handled has much to do 

 with its warmth. Uneven, ridged soil, like that 

 left by fall plowing, loses more neat than smooth, 

 level soil. However, ridging may warm the soil 

 by drying it, and this usually more than counter- 

 balances the loss of heat because of the greater 

 surface exposed. Rolling land in fair and warm 

 weather makes it warmer, but rolling it in cloudy 

 and cold weather, especially if it is wet, makes it 

 colder. Deep plowing makes the soil cooler, 

 because loose soil is a poor conductor of heat. The 

 decay and fermentation of farm manure plowed 

 into a soil may raise its temperature several de- 

 grees; it produces as much heat in the soil as it 

 would if burned in the open air. Manured soil 

 is usually about 2 warmer in spring than unma- 

 nured soil. Thorough tillage, especially in the 

 preparation of a seed bed, has a marked influ- 

 ence on soil temperature; it prevents the evap- 

 oration of soil moisture and hence keeps in the soil 

 the large amount of heat that it takes to evaporate 

 water. Good tillage saves heat, then, as well as 

 water, especially in early spring. This means that 

 the soil for early crops should be plowed early and 

 tilled often. 



