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 rolhng 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 soilmulch, by preventing evaporation, conserves much 

 heat for the growth of crops. (4) By drainage. In weU-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 weU-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 (Rei. 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 



