98 



CIVIC BIOLOGY 



practically the entire crop. The Agricultural Department at Madison 

 was appealed to for help, and the experts, on visiting the infested terri- 

 tory, found here and there a cabbage plant that had not been attacked. 

 Seed was saved from these specimens and a resistant strain secured. 

 Another example is the resistance of mazzard stock to cherry gumraosis. 



Problem of soil fertility. Fifteen chemical elements com- 

 monly enter into or constitute the plant body. They are natu- 

 rally the most abundant elements of air, ^yater, and earth. 

 Take, for example, the composition of corn : 



Elements obtained in abundance from air 

 and water. 



Elements that the corn plant must get from 

 the soil, and that we must buy if they are 

 deficient. 



Elements seldom lacking in the sgil in the 

 small amounts required, except calcium, 

 which in regions free from limestone is 

 often added to " sweeten," or correct acid- 

 ity in, soils. 



An acre of soil 6| inches deep weighs 2,000,000 pounds, 

 and if we analyze this and determine how many pounds of 

 the necessary elements it contains, and if we know how many 

 pounds of these elements are removed in a given crop, we can 

 figure roughly how long the soil will '' last," that is, be able 

 to produce the crop. Hopkins has done this in the table on 

 the following page.^ 



1 Cf . Hopkins, Soil Fertility and Permanent Agriculture, p. 13. 



2 Ibid., p. 59. 



