xxvi TABLE OF CONTENTS 



CHAPTER XVII 

 MAINTAINING FERTILITY AND HUMUS IN DRY AREAS 



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1. SOILS IN DRY AREAS ARE USUALLY FERTILE 405 



(a) Because of inherent richness in the soil particles. 



(b) Because the subsoil is very similar in composition. 



(c) Because little has been lost by leaching. 



(d) Because of the moving of the soil by rodents. 



2. THE Loss OF FERTILITY IN DRY AREAS. 406 



(a) Many claim that it will never be lost. 



(b) Every crop grown and sold removes fertility. 



(c) The loss is less rapid than in humid climates. 



(d) The restoration when lost is more difficult. 



3. THE SOURCES OF FERTILITY IN DRY AREAS 408 



(a) Food furnished directly by the soil. 



(b) Food carried up from the subsoil. 



(c) Food liberated by cultivation. 



(d) Food furnished by legumes. 



4. How TO INCREASE FERTILITY IN DRY AREAS 410 



(a) By growing legumes. 



(b) By applying farm manures. 



(c) By applying artificial fertilizers. 



(d) How to increase the available fertility. 



5. THE SUPPLY OF HUMUS IN DRY AREAS. . 413 



(a) It is less than in humid areas . 



(b) Why it is less than in humid areas. 



(c) More difficult to increase it than in humid areas. 



(d) It is lost less readily than in humid areas. 



6. THE SOURCES OF HUMUS IN DRY AREAS 415 



(a) Organic matter that decays on the surface. 



(b) The roots and stubbles of plants. 



(c) Green crops grown and buried. 



(d) Farmyard manures. 



7. THE BENEFITS FROM HUMUS IN DRY AREAS 417 



(a) It improves the physical structure of soils. 



(b) It adds to their moisture-holding power. 



(c) It furnishes food to plants in a readily available 

 form. 



(d) It lessens the lifting of soil by winds. 



8. How TO INCREASE HUMUS IN DRY AREAS 420 



.(a) By growing and burying non-leguminous plants. 



(b) By growing alfalfa. 



(c) By growing and burying the Canadian field pea. 



(d) By growing and burying the sand vetch. 



(e) By'growing and burying sweet clover. 



