LAND AND OTHER NATURAL AGENTS n 



8. What is meant by a " marginal" climate ? What factors enter 

 into the establishment of the margin of cultivation ? Is its location 

 to be regarded as permanent ? Why ? 



9. Does rainfall impose a more or a less rigid limitation than does 

 temperature upon the productivity of land ? Explain. 



10. Show how moisture is a limiting factor even in so-called 

 humid regions. Is there also a surplus of moisture in the same 

 regions ? At the same time ? 



n. Give a brief discussion of the methods of dry farming. Are 

 similar methods applicable during droughts in sections ordinarily 

 sufficiently watered ? Which would bring the larger returns, taking 

 more dry land into cultivation or handling the old lands better during 

 dry periods ? Which is more difficult to achieve ? 



12. Show the nature of the economic problem resulting from the 

 presence of irrigable lands, sub-humid lands, lands subject to drought, 

 lands occasionally overflowed or waterlogged, small and scattered bits 

 of swamp, and large tracts permanently too wet for cultivation. 

 Where can we best push out the margin of production ? 



13. Explain how judicious and scientific selection and breeding 

 of plants may enable us to use more lands in their natural state. Is 

 this cheaper than trying to change the character of the land ? How 

 about the value of the product ? 



14. What proportion of the area of the United States is now cul- 

 tivated ? Is any part of our area unavailable because of temperature 

 conditions ? How much may be added by reclamation of lands too 

 wet or too dry at the present time ? Why have we not already done 

 this ? When shall we do it ?' 



15. What are the chemical elements which are the limiting factors 

 in most agricultural soils ? Is it economically practicable to main- 

 tain this source of productivity through an indefinite period of agri- 

 cultural use of land ? Explain. 



16. Which physical factors in productivity are inherent in a soil 

 and which are the result of tillage methods? Show the economic 

 aspect of these facts. May intelligent farming secure a virtual 

 increase in our land resources at no expense ? 



17. Does the present trend of our biological studies seem to hold 

 out a hope that in the future we may maintain the productivity of our 

 lands more completely and more economically than in the past ? How ? 



18. Is agricultural productivity an " original and indestructible 

 power of the soil"? In your answer relate the physical law 01 the 

 conservation of matter, the question of biological condition and the 

 needs of plants, and the economic issue involved. 



19. Point out both the technical and the economic sense in which 

 rough topography constitutes a brake upon the productivity of land. 



