EFFECT OF INCREASING ONE MINERAL CONSTITUENT. 129 



potash, lime, and magnesia, to supply the four crops 

 stated above, but tliat it is deficient in the proper propor- 

 tion of sihcic acid — containing, for example, for 1 

 part by weight of phosphoric acid, only 2^ parts of 

 sihcic acid, in an available condition — this deficiency 

 will, in the first place, be felt in the crops of cereal 

 plants, Avhilst the potato and clover crops, on the contraiy, 

 will not be at all diminished. It will depend upon the 

 weather to determine whether this deficiency in the 

 crop of cereal plants extends both to corn and straw or 

 is confined to the straw alone. A want of potash, in 

 proportion to all the other constituents, will barely affect 

 the wheat and oat crops, but it will reduce the potato 

 crop ; in hke manner, a want of lime and magnesia will 

 impair the clover crop. 



If the ground can furnish one-tenth more potash, lime, 

 magnesia, and sihcic acid, than corresponds to the given 

 proportion of phosphoric acid — thus, if, 



Phc^ric p,,,3, I^me^nnd Silicic acid 



Instead of ... . 1 2-5 1-5 30 



The ground shoiild be able to 



furnish .... 1 2-75 1-G5 3-3 



the crops would not turn out larger than before. But if, 

 in such a field, the quantity of phosphoric acid is in- 

 creased, the produce wdll increase, until tlie right propor- 

 tion is restored between the phosphoric acid and the 

 other mineral constituents. The additional supply of 

 phosphoric acid serves in this case to increase the amount 

 of potash, hme, and sihcic acid in the produce ; but if 

 this additional supply exceeds one-tenth of the phosphoric 

 acid present in the soil, the quantity in excess remains 

 ineffective. Up to this limit, every pound — nay, every 

 ounce — of phosplioric acid supplied has, in this case, a 

 fully determinate action. 



K 



