132 THE SOIL. 



acid which the four crops have received from the soil, 

 we find for the wheat crop 26 kilogrammes of phos- 

 phoric acid, for the potato crop 25 kilogrammes, for 

 the oat crop ^7 kilogrammes, and for the clover crop 

 36 kilogrammes altogether, 114 kilogrammes ; multi- 

 plying the above proportional numbers by this num- 

 ber, we obtain the sum total of all the nutritive sub- 

 stances extracted from the soil by the four crops. 



With the help of these proportional numbers, we 

 are better able than before to give some more accurate 

 explanations. 



Suppose that the soil of a certain field contains, in 

 an available state, the requisite quantities of phosphoric 

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

 crops stated above, but that it is deficient in the proper 

 proportion of silicic acid containing, for example, for 

 1 part by weight of phosphoric acid, onj,y 2J parts of 

 silicic acid, in an available condition this deficiency 

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

 plants, whilst the potato and clover crops, on the con- 

 trary, 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 til one. 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 like manner, a want of lime and 

 magnesia will impair the clover crop. 



If the ground can furnish one-tenth more potash, 

 lime, magnesia, and silicic acid than corresponds to the 

 given proportion of phosphoric acid thus, if, 



Phosphoric p rt t ao1l Lime and c;^ n - n <* 

 acid. Potash ' Magnesia. Slllclc acid ' 



Instead of 1 2'5 T5 3-0 



The ground should be able 



to furnish 1 2'75 1-65 3-3 



the crops would not turn out larger than before. But 

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

 increased, the produce will increase, until the right 

 proportion is restored between the phosphoric acid and 



