ISO CHEMICAL COMPOSITION OF THE SOIL [chap. 



account for the great differences in fertility which we 

 know to exist between one soil and another, differences 

 which are reflected in a rent of 5s. an acre in one case 

 and £$ in another. The next point that requires 

 consideration is the comparatively large quantities of 

 plant food present in even the poorest soils, a quantity 

 which is enormous in comparison with the amount 

 required by ordinary crops. For example, Table VI. 

 shows that few crops take more than 100 lb. of nitrogen 

 per acre from the soil, or 50 lb. of phosphoric acid, or 

 1 50 lb. of potash ; only heavy root crops will remove as 

 much as this, crops of cereals require about one-half 

 As the layer of soil 9 inches deep over an acre weighs 

 about two and a half million pounds, the one-tenth per 

 cent, of nitrogen which is to be found in all but the very 

 poorest of soils would still represent as much as 2500 lb. 

 per acre, or sufficient for twenty-five full crops of roots 

 or fifty full crops of corn. 



Yet at Rothamsted, where an attempt has been made 

 to grow both roots and corn continuously without 

 manure, in a very few years the crop of wheat fell to 

 about 13 bushels per acre of corn and 10 cwt. of straw, 

 while the mangold crop averaged no more than 4 tons 

 per acre. Nor does growing crops in a rotation instead 

 of continuously help matters much : on the Agdell field 

 at Rothamsted, where the Norfolk four-course system is 

 followed, the average production of the unmanured plot 

 has been 16 cwt. swedes, 16 bushels of barley, 9 cwt. of 

 clover hay, and 26 bushels of wheat. When grown in a 

 rotation, the cereal crops have maintained their yield at 

 a much better level than when grown continously on the 

 same land, but the preceding crops of clover or svv'edes 

 are so poor that the land is practically lying fallow for 

 the year before the corn. Some clue to the explanation 

 of the difficulty is afforded by the facts which have been 



