80 THE ESSENTIALS OF AGRICULTURE 



deposits are the elements of plant food contained in our soils. 

 The soil is the only mine known which under proper manage- 

 ment will not run out. But too many farmers are yet " min- 

 ing " their soil by taking everything out and putting nothing 

 back. A city built near a mine is abandoned when the mine 

 ceases to yield its mineral. A nation built upon agriculture will 

 perish when the soil can no longer yield its harvest. 



QUESTIONS AND PROBLEMS 



1. What are the three elements of plant food in which the farmer is 

 most interested ? Why are these of such importance ? 



2. How do you explain the fact that soils are sometimes said to contain 

 large quantities of unavailable plant food? 



3. How may the farmer increase the amount of available plant food in 

 the soil ? 



4. Of the three elements, nitrogen, phosphorus, and potassium, which is 

 likely to be exhausted first? How may this element be replenished? 



5. Describe the processes which vegetable matter must undergo before 

 it is of value to the land and crops. 



6. What are the four essential conditions in the soil for successful 

 ammonification and nitrification ? 



7. As organic matter undergoes these processes, how does it help the 

 soil? 



8. Secure the records from some agricultural experiment station where 

 they have grown the same kind of crop for many years upon the same land, 

 and see what results followed. 



9. Why is a subsoil usually infertile for farm crops? 



EXERCISES 



1. The effect of organic matter upon soil fertility. 

 MATERIAL : Two one-gallon pots. 



Fill two one-gallon flower pots with a finely sifted clay subsoil until, 

 after being settled well by jarring, the subsoil comes to within an inch 

 and a half of the top. Empty each pot separately upon a piece of 

 oilcloth or paper. 



Secure a quantity of leaf mold and rub through a one-fourth 

 inch sieve. To one batch of subsoil add about half its bulk of this 



