172 



SOILS AND MANURES 



Effect of the Loss of Plant Foods on the Fertility of the 

 Soil. It is obviously of great importance to determine 

 whether this practice is theoretically justified, i.e., to com- 

 pare the rate of loss of plant food by the sale of produce, 

 with the rate at which it is converted into an available 

 state. If the former exceed the latter, the fertility must 

 depreciate. Unfortunately it is not known at what rate 

 plant food is converted into an available state in any soil 

 under any conditions. It cannot be exactly determined 

 because if for no other reason the term available is not 

 definable. Substances that are assimilated by one kind of 

 crop cannot be assimilated by others. Some idea, may, how- 

 ever, be formed from the yield of the unmanured plots at 

 Kothamsted. Crops have been grown on these plots and 

 removed year after year for more than fifty years. During 

 that time nothing has been put back and nothing has been 

 added, but the land has been carefully cleaned and cul- 

 tivated all the time. The records show that the yield from 

 these plots is now much smaller than it was when the 

 experiments were begun. If it be assumed that the surplus 

 of available plant foods has been exhausted, the quantities 

 contained in the produce of these plots should correspond, 



