vi.] RESERVES OF PLANT FOOD IN THE SOIL 173 



For example, a wheat crop on poor soil would often be 

 doubled by the use of 2 cwt. of nitrate of soda per acre, 

 i.e., by the addition of 35 lbs. of nitrogen in nitrate of 

 soda to a soil that already contained in the top 9 inches 

 more than 2000 lbs. per acre. Again, 4 cwt. per acre 

 of superphosphate, containing about 60 lbs. of phosphoric 

 acid, will be necessary in the usual rotation to secure 

 a good swede crop, though there may be already 2000 

 to 3000 lbs. of phosphoric acid in the soil. We are 

 then driven to conclude that the nitrogen, potash, and 

 phosphoric acid are present in the soil in some other 

 mode of combination than the form in which they exist 

 in manures : so that although they may be in the soil 

 they are in such a state as to be very partially of service 

 to the growing plant. Further evidence of the enormous 

 stores of plant food in the soil and the comparative slow- 

 ness with which they can be utilised may be obtained 

 by considering the results obtained at Rothamsted, 

 where on one plot wheat has been grown continuously 

 without manure for sixty-four years (to 1907). The 

 average yield from this plot was for the first twenty years, 

 1844-63, 16-3 bushels of grain and 15-1 cwt. of straw; 

 1 1-6 bushels of grain and 9-3 cwt. of straw for the next 

 twenty years, 1864-83 ; and 12-3 bushels of grain and 

 8-7 cwt. of straw for the third period of twenty years, 

 1884-1903. It is calculated that during the last fifty years 

 there have been removed from this plot about 900 lbs. 

 per acre of nitrogen, 470 of phosphoric acid, and 760 of 

 potash, i.e., about 18, 9, and 15 lbs. per acre per annum 

 respectively; yet from analyses of a sample taken in 1893 

 the surface soil to a depth of 9 inches still contained 

 on per cent, of nitrogen, 0-114 per cent, of phosphoric 

 acid, and 0-38 per cent, of potash soluble in strong 

 hydrochloric acid, or 2750, 2850, and 9500 lbs. per 

 acre respectively. The soil must therefore be re 



