SOIL FERTILITY. 1 5 



humus supply. This is necessary only on poor soils. With a 

 field badly run down and lacking in humus, it may well repay the 

 outlay. If a grain crop is used and allowed to ripen before 

 turning under, the grain may at once spread and form a second 

 cutting, to still further add to the supply. If heavy crops of 

 clover or grain are turned under while green, it sometimes has 

 the eflFect of souring the soil to such an extent as to injure the 

 succeeding crops. 



The- most important means of maintaining the humus supply 

 in ordinary farm operations is by means of a systematic rotation, 

 in which a heavy sod is frequently turned under. A mistake 

 all too common is to allow grass land to remain until it is badly 

 run out and the sod largely lost before plowing it up. In that 

 case there is little left to benefit the soil, while weeds are gener- 

 ally present to cause future trouble. If the rotation is short and 

 the land plowed while the sod is heavy, the gain will be great. 



A systematic rotation possesses other advantages aside from 

 this, some of which may be mentioned here. i. Plants differ 

 in their power of extracting plant-food from the soil. Land 

 which will not produce a good crop of wheat may produce a 

 good crop of oats, although the latter requires more plant-food. 

 2. Plants differ in the demands made upon the soil. To grow 

 one crop continuously, therefore, tends to exhaust certain ele- 

 ments of plant-food. 3. Some plants send their roots deeper 

 than others, thereby bringing up food from the subsoil for their 

 own growth and the benefit of succeeding crops. 4. Some 

 plants can gather nitrogen from the air and leave a residue for 

 succeeding crops. 5. Humus can be introduced. 6. The 

 ground can be kept covered. 7. Insects are less likely to cause 

 injury. 8. Diseases are less likely to be troublesome. 9. 

 Weeds are more easily controlled. 10. Work may be better 

 distributed throughout the year. It is in maintaining the 

 humus supply, however, that a rotation plays its most important 

 part. 



Moisture constitutes the second factor in soil fertility. Its 

 importance may be realized from the fact that it has been esti- 

 mated that a crop of two hundred bushels of potatoes per acre 

 requires something like a million and a quarter pounds of water 

 for its growth, and a crop of com somewhat more. The soil 



