CLOVER AS A PREPARATORY CROP FOR WHEAT. 475 



The propoi'tion of phosphoric acid in six inches of surface soil, 

 it will be seen, amounted to about two-tenths per cent.; a propor- 

 tion of the whole soil, so small that it may appear insufficient for 

 the production of a good corn crop. However, when calculated 

 to the acre, we find that six inches of surface soil, in an acre of 

 land actually contains over two tons of phosphoric acid. An 

 average crop of wheat, assumed to be 25 bushels of grain, at 60 

 pounds per bushel, and 3000 pounds of straw, removes from the 

 land on which it is grown 20 pounds of phosphoric acid. The 

 clover soil, analyzed by me, consequently contains an amount of 

 phosphoric acid in a depth of only six inches, which is equal to 

 that present in 24T| average crops of wheat ; or supposing that 

 by good cultivation, and in favorable seasons, the average yield of 

 wheat could be doubled, and 50 bushels of grain at 60 pounds a 

 bushel and 6000 pounds of straw could be raised, 124 of such 

 heavy wheat crops would contain no more phosphoric acid than 

 actually occurred in six inches of this clover ^oil per acre. 



The mere presence of such an amount of phosphoric acid in a 

 soil, however, by no means proves its sufficiency for the produc- 

 tion of so many crops of wheat ; for, in the first place, it cannot 

 be shown that the whole of the phosphoric acid found by analysis 

 occurs in the soil in a readily available combination : and, in the 

 second place, it is quite certain that the root-fibres of the wheat- 

 plant cannot reach and pick up, so to speak, every particle of phos- 

 phoric acid, even supposing it to occur in the soil in a form most 

 conducive to "ready assimilation by the plant." 



The calculation is not given in proof of a conclusion which 

 would be manifestly absurd, but simply as an illustration of the 

 enormous quantity, in an acre of soil six inches deep, of a constit- 

 uent of which only a fractional part of a per cent, is found in the 

 soil. This fractional percentage of phosphoric acid in an acre of 

 land six inches deep amounts to a very large quantity which may 

 be rendered available for the use of plants. 



We have here an indication which clearly points out the pro- 

 priety of rendering available to plants the natural resources of the 

 soil in plant-food ; to draw, in fact, up the mineral wealth of the 

 soil by thoroughly working the land, and not leaving it unutilized 

 as so much dead capital. 



The exact determination of phosphoric acid in a soil, it may be 

 observed in passing, is attended with no difficulty, if certain pre- 

 cautious, which it is feared are sometimes neglected by chemists, 



