300 NATURE STUDY AND AGRICULTURE 



increases the depth of the soil. How does it do this? 

 Which is better, shallow plowing or deep ? Why ? If year 

 after year only the upper three or four inches of soil are 

 turned over, this will become so depleted of its plant foods 

 that it will be 1 1 worn out." Besides, the lower layer of soil 

 will become so packed and sour that it will be utterly unfit 

 for the plant roots. How will deeper plowing prevent this ? 

 Tillage also aids in the saving of moisture. How? It 

 also loosens the soil so that it will hold more air and be 

 better ventilated. It kills out the weeds, and thus prevents 

 a loss of plant foods and water. By breaking up the soil 

 particles it renders the plant foods more available. It 

 turns under vegetation and thus increases the amount of 

 humus in the soil. 



The second thing to consider in helping the soil to 

 supply the needs of plants is drainage. We have already 

 seen some of the advantages of under drainage. It ren- 

 ders the soil more porous. It increases the temperature 

 in the spring. It gives an opportunity for better ventila- 

 tion. It results in a deeper root system. Proper tillage 

 and drainage then are two very important forces in main- 

 taining good physical conditions of the soil. 



The rotation of crops is another important considera- 

 tion. Agriculturists are coming to believe more and more 

 that to grow the same kind of crop in a field year after year 

 will result in absolute ruin to the soil. There are several 

 reasons for this. One is that certain kinds of crops use 

 more of one kind of plant material than of others. After a 

 number of years the soil is so lacking in this particular 

 compound that it is difficult to grow any kind of a crop in 

 it. Another probable reason is that each plant gives out 



