58 PRODUCTIVE FARMING 



When it dries it becomes very hard and unworkable. A good 

 farm soil should be between these two conditions, open 

 enough to permit the proper entrance and movement of air 

 and water, and yet compact enough to prevent the washing 

 out of plant-food. 



Farm Soil. The best farm soil does not belong strictly 

 to any of these classes already described, but is one which is 

 a mixture of sand, clay, and humus in certain proportions. 

 With reference to the proportions of sand and clay present, 

 the useful farm and garden soils may be divided into three 

 general classes, sandy loam, medium loam, and clay loam 



(Fig. 31). A perfect soil is 

 one whose physical nature 

 and chemical composition 

 make it best adapted for 

 the purpose or crop in- 

 tended. It contains just 

 enough sand to enable it to 



FIG. o2. A subsoil plow has a long . 



shank, it is used in the bottom of furrows absorb air and moisture in 



behind a turning plow where the subsoil 



is too compact, it stirs the subsoil but proper amounts and to 



does not bring it to the surface. 



make it warm and easily 



worked. It contains enough clay to keep it from getting too 

 warm and to prevent too rapid loss of water. Lime must 

 be present to perform its several duties. Humus is there to 

 control the amounts of moisture and air, to furnish nitrogen, 

 and to help produce valuable chemical changes in the soil. 

 There are a few other conditions necessary to provide suffi- 

 cient plant-food. 



Perfect soils are hard to find. They are also hard to make 

 in any of the known ways of soil-improvement. We usually 

 can make slight improvements in the soils found in nature. 

 Since this is true we must select crops for the farm which are 

 best adapted to the soil found there. 



The subsoil lies under the soil, which generally occupies 

 the surface six to twelve inches. There are several differ- 



