Soil Fertility and Its Preservation 41 



that that river carries sediment and dissolved mineral 

 matter enough into the Gulf of Mexico every year to make 

 a block of earth a mile square and about three hundred 

 feet high, if it could all be collected in one place. 



The sod of the prairies is an ideal place for the forma- The loss of 

 tion of humus. But when the prairies are cultivated, j^ us> 

 the sod is destroyed ; and if manure or other humus- avoided 

 making material is not added, the humus will gradually 

 be used up, thus reducing the soil fertility. An excellent 

 practice is to raise a crop of clover or timothy hay every 

 few years, thus letting the land form a sod and so regain, 

 to some extent, the condition it had in its wild state. 



Cultivated fields suffer a loss of fertility that does not An 



j i 



occur in wild lands, through the annual removal of crops. foJJ 00 

 The most fertile lands become worn out by cropping 

 unless vegetable matter and food materials are returned 

 to them. In time the vegetable matter that new soil 

 usually has in abundance disappears, and the supply of 

 mineral food materials that the plants must have becomes 

 almost exhausted. This loss cannot be entirely avoided, 

 and it becomes necessary to add fertilizers. These will 

 be specially considered in Chapter Twelve. 



Keeping up fertility. With a little care the farmer Soil 

 can return to his soil a large part of the annual crop. 

 Straw is almost valueless as feed and is easily given not be 

 back by plowing under. Leaves that fall about a tree 

 or under a rosebush should be kept moist and be dug 

 into the soil. They will give back what the plant drew 

 from the ground in making them, and will, besides, 

 greatly improve the physical condition of the soil by in- 

 creasing its humus. 



