110 SOILS AND FERTILIZERS 



of the atmosphere. The last of these is inexhaustible, 

 and the exhaustion of the soil nitrogen supply, which a few 

 years ago was thought by some to be a matter of less than 

 half a century, has now ceased to cause any apprehension. 

 The conservation or extension of the supply of mineral 

 nutrients is now of supreme importance. The utilization 

 of city refuse and the discovery of new mineral deposits 

 are developments well within the range of possibility, but 

 neither of these promises to afford more than partial relief. 

 The utilization of the subsoil through the gradual removal 

 by natural agencies of the topsoil will, without doubt, tend 

 to constantly renew the supply. The removal of topsoil 

 by wind and erosion is, even on level land, a very considerable 

 factor. The large amount of sediment carried in streams im- 

 mediately after a rain, especially in summer, gives some idea of 

 the extent of this shifting. This affects chiefly the surface soil, 

 and thereby brings the subsoil into the range of root action. 

 There is little doubt that a moderate supply of plant- 

 food materials will always be available in most soils, but for 

 progressive agriculture manures must be used. 



QUESTIONS 



1. How does the total quantity of plant-food materials in soils 

 compare with the total weight of soil ? 



2. Are the percentages of nitrogen, phosphoric acid and potash 

 uniform in different soils, or do they differ ? 



3. Is there a direct relation between the productiveness of a soil 

 and its content of plant-food materials ? 



4. What is meant by available and unavailable plant nutrients ? 



5. Name some of the factors that influence the availability of 

 plant nutrients in soils. 



6. Why is it not always possible to determine by chemical analy- 

 sis the degree of productiveness of a soil ? 



7. Explain what is meant by the absorptive properties of soil for 

 soluble fertilizers. 



8. Explain what is meant by selective absorption. 



