VI.] RESERVES OF PLANT FOOD IN THE SOIL 153 



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, 

 1 884- 1 903. 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 

 oil 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- 

 garded as possessing most of its plant food in states 

 of combination that cannot be utilised by the plant, 

 and these forms slowly pass, by weathering and other 

 changes, into material which is available for the crop. 

 The plant food of the soil represents so much capital, 

 and, as in many another business, but a small proportion 

 of the capital is liquid at any given time : it is largely 

 the object of cultivation to effect such a turnover of the 

 capital as will liquidate some of it in a form available 

 for the nutrition of the crop. 



In the old systems of agriculture, before the land was 

 enclosed, the whole crop was grown out of capital, 

 nothing but labour was put into the soil : in which con- 



