174 SOILS AND MANUEES 



As the productiveness in respect of some of the crops 

 appears to be still diminishing, it is probable that the 

 surplus store of available plant foods is not yet entirely 

 exhausted, and that the estimate is too high. In other 

 words, it is probable that plant food does not become 

 available to these crops so rapidly as is indicated. In 

 any case, owing to the difference in the assimilative 

 capacities of the crops, it is impossible to arrive, by this 

 method, at any very satisfactory conclusion in regard 

 to the rate at which the plant foods become avail- 

 able. It must be remembered also that when nitrogen 

 alone is added larger amounts of phosphoric acid, etc., 

 are absorbed by the crops. Still it is not without 

 interest to notice that, at the lowest computation, the 

 potash is more than sufficient, and, at the highest, the 

 phosphoric acid is barely sufficient to make good the 

 losses of these ingredients by the sale of grain and meat. 

 Taking the average of the four crops, it will be seen that 

 the nitrogen is just equal to, and the phosphoric acid 

 is substantially less than, what is required. There are, 

 of course, other sources of loss, particularly of nitrogen, 

 incidental to the conduct of the operations, and which tend 

 to reduce the supply of available plant foods. In a general 

 way, therefore, these considerations confirm the conclu- 

 sions arrived at by practical experience, viz., that the 

 sale of grain and meat tends gradually to reduce the fer- 

 tility of the soil, but the deficiency is small and, on a 

 moderately rich soil, could probably be made good by occa- 

 sional fallowing and pasturing. Land has been farmed 

 for hundreds of years, and, in some districts, is still farmed 

 on these or similar lines, i.e., grain and meat, or their 

 equivalents, alone are sold ; the rest is put back, but 

 nothing is given to the land except what is produced 

 from it. 



Addition. The highest degree of productiveness cannot, 



