PRINCIPLES OF MANURING 103 



food that is naturally present in soil. If 

 we regard only the top 12 inches, we have 

 to deal with 43,560 cubic feet of soil per acre ; 

 and, assuming a weight of 100 Ib. per cubic 

 foot, the top 12 inches of soil works out 

 at 4,356,000 Ib. per acre. Frequently the 

 percentage of nitrogen is about O'l, the per- 

 centage of phosphoric acid about 0*2, and 

 the percentage of potash about 0*4 ; and on 

 this assumption the surface soil contains 

 4356 Ib. of nitrogen, 8712 Ib. of phosphoric 

 acid, and 17,424 Ib. of potash per acre. On 

 the basis of this estimate, therefore, an acre 

 of ordinary land contains enough nitrogen 

 to satisfy the requirements of a cereal crop 

 for nearly a hundred years, while the 

 phosphoric acid and potash, if available, 

 would meet the demands of the crop for 

 more than four hundred and five hundred 

 years respectively. And yet with these 

 enormous supplies of plant food in the 

 neighbourhood of the roots, it is a matter of 

 everyday experience that the use of, say, 

 1 cwt. of nitrate of soda, supplying less than 

 20 Ib. of nitrogen per acre, is followed by 

 marked effects. The reason for this result 

 is, of course, that the plant food naturally 

 present in the soil is not, for the most part, 

 immediately available, whereas the material 



