28 



FERTILITY DEPENDS ON THE MINIMUM QUANTITY OF ANY NECES- 

 SARY CONSTITUENT PRESENT. 



As the soil is the source wbence plants derive their mineral food, all 

 the elements required for this nourishment have, in a certain sense, 

 equal value ; for if one of them is wanting in the soil, or is present in a 

 form not readily available by the roots, the plant will not flourish; and 

 so its fertility depends on the minimum quantity of any necessary con- 

 stituent present. 



WEIGHT OF A SOIL PEE ACRE. 



The weight of soil on an acre of land is so enormous that even small 

 proportions of plant food may amount to very considerable quantities. 

 The weight varies with the porosity and the amount of sand and gravel 

 the soil contains. 



The following table gives the specific gravity, the weight of 1 cubic 

 foot of different soils, dry and wet, taken from the table in Professor 

 Schiibler's article "On the Physical Properties of Soil," in the Journal 

 of the Royal Agricultural Society, vol. 1, p. 210 ; together with their 

 approximate weight per acre to a depth of 9 inches, equal to 32,670 cu- 

 bic feet, in tons of 2,000 pounds : 



TABLE III. Table showing the specific gravity, theiveight of one cubic foot of different soils, 

 dry and wet, according Schnblcr, and the approximate weight per acre to a depth of 9 

 inches (32,670 cubic feet). 



Thus 0.10 per cent, of any constituent, such as phosphoric acid, potash, 

 &c., would amount to from 2,250 to 3,500 pounds in 1 acre of soil 9 

 inches deep. 



