306 Our Surroundings 



soil whose surface is kept broken up by frequent cultivation. 

 In practicing dry farming, a plot of land, though required to pro- 

 duce a crop only every other year, is tilled after each rain- 

 fall, in order to keep the soil loose and thus conserve the moisture 

 by retarding evaporation. In this way the soil gathers and holds 

 sufficient moisture during one year to supply and mature a crop 

 during the following year. Of course, this system is used only 

 in regions where there is an insufficient rainfall. 



Soil Exhaustion. Soil remains productive only when it re- 

 ceives constant supplies of the substances taken from it by grow- 

 ing plants. If these plants are left to die and decay, the soil re- 

 ceives back the nutrients, or foods, they took from it in growth 

 and the soil remains rich. But when crops are raised and removed 

 without replacement of these essentials for plant growth, the soil 

 'becomes exhausted and can no longer produce a good crop. Such 

 a condition is called soil exhaustion. Farms are sometimes aban- 

 doned as useless because the soil no longer contains the com- 

 pounds that render it productive. Other reasons that cause loss 

 of productive power are the washing away, or erosion, of the 

 surface soil by heavy rains, an inadequate supply of water, and a 

 lack of humus in the soil. 



Lack of Humus. The lack of humus is usually the main 

 cause for any decrease in the yield of farm crops, since it leads 

 to the development of other unfavorable conditions in the soil. 

 Without humus, the soil loses its water-holding power more easily, 

 and, when it hardens, prevents the admission of air. Both air 

 and water are necessary to insure the development of plant 

 life. 



Keeping the Soil Fertile. Soil will continue to produce 

 crops in sufficient quantity to reward the farmer for the labor 

 involved in cultivation only when it receives enrichment in return 

 for that which a growing crop removes. This may be brought 

 about by the use of fertilizers and, to some extent, by crop rota- 

 tion. 



Fertilizers. Fertilizers are substances containing the ele- 

 ments needed by the soil to replace those taken from it by grow- 

 ing crops. In soil where there is sufficient moisture the elements 



