EXTENSION COUESE IN SOILS. 33 
of soil can be much improved by rolling, especially when the surface 
is loose. By this means an additional amount of heat can be carried 
into the subsurface soil. At a depth of 3 inches rolling commonly ' 
warms the soil as much as 3°. If the surface soil is moist, however, 
the rolling should be followed at once by cultivation to prevent evap- 
oration of moisture. (3) By use of a soil mulch. As has been stated 
above, the evaporation of moisture takes a great deal of heat from 
the soil. The soil mulch, by preventing evaporation, conserves much 
heat for the growth of crops. (4) By drainage. In well-drained 
soils the gravitational water is drawn off from beneath instead of 
evaporating from the surface. Soil that is tile drained is 5° to 10° 
warmer in the spring than it was before it was drained. The tem- 
perature of the soil in turn affects the temperature of the air in im- 
mediate contact with it, and frost often occurs on poorly drained 
soil at night where it does not form on well-drained soil. 
DRAINAGE. 
An excess of water prevents the entrance of the necessary air into 
the soil; it hinders the normal development of soil microorganisms; 
it leads to the puddling of clay soils and consequently produces poor 
tilth; it keeps the soil cold, especially in the spring; and, finally, it 
causes a leaching of plant-food substances from the soil. 
Conditions where drainage is necessary (Ref. No. 8, pp. 14-16). — It 
is usually not difficult to detect the need of drainage. There are 
cases, however, when late in summer it is difficult to determine 
whether partial crop failure was caused by poor drainage earlier in 
the season or from the lack of necessary elements of plant food. 
Water should not stand on the surface of cultivated soils any longer 
than can be helped. Especially in the Northern States, where the 
growing season is short, it is desirable to have drainage in the spring 
as thorough as possible. Soil should not be saturated within 3 feet 
of the surface for most crops, though many grasses will make a very 
good growth on land which is saturated within 18 inches of the 
surface, or even nearer, for a portion of the growing season. Drain- 
age is especially desirable in irregular fields where the drainage of 
wet portions will permit the laying out of a field of proper dimen- 
sions and also make it possible for the whole field to be tilled at one 
time. This not only increases the acreage of available land but 
greatly increases the efficiency with which operations of tillage and 
harvesting can be performed. Drainage in any case simply removes 
the gravitational water, and it is a mistake to think that good drain- 
age is detrimental to crops, even in dry seasons. 
It is customary to speak of surface and subsurface drainage, 
referring to the removal of surface or flood water in one case and to 
the withdrawal of the excess of water from the subsoil in the other. 
21862°— Bull. 355—16 3 
