CHEMISTEY OP SOILS .123 



by plants are relatively minute. Thus, a crop of mangels, 

 which requires more phosphoric acid and potash than any 

 other, removes from an acre of land only about 50 Ibs. of 

 the former and 300 Ibs. of the latter. If the total mass 

 of an acre of soil 9 inches deep be taken as 2,500,000 

 Ibs. which is a moderate estimate 0'002 per cent, of 

 phosphoric acid and 0'012 per cent, of potash would be 

 sufficient to provide these quantities. It is, of course, 

 essential that the soil should contain a considerable excess 

 of the various substances over and above what is actually 

 required by the crops, and the point may be, perhaps, better 

 illustrated by putting the case in another way. In a mass 

 of 2,500,000 Ibs. O'l per cent, is equal to 2,500 Ibs., which 

 is more than eight times the amount of potash and fifty 

 times the amount of phosphoric acid required by a mangel 

 crop. The ordinary grain crops require only about h,alf 

 the quantity of phosphoric acid and about a tenth part of 

 the potash that mangels require. 



Available Plant Foods. The total acid extract is, how- 

 ever, a matter of secondary interest. A large proportion 

 of the phosphoric acid and other ingredients dissolved by 

 concentrated acids is present in the soil in a non-available 

 state, i.e., in such a state of combination that it cannot 

 be assimilated by plants. The total acid extract does not, 

 therefore, afford a reliable indication of the capacity of 

 soils to provide for the requirements of the. crops. For 

 example, it has been found that soils which, experience 

 shows, stand in need of potash manures often contain as 

 much potash, soluble in concentrated acids, as soils which 

 do not. Exactly what this so-called " .available state " may 

 be is not known. That it is closely connected with the solu- 

 bility of the compounds is obvious, and solubility depends 

 partly on physical and partly on chemical conditions. For 

 example, there is a great difference between the solu- 

 bility of crystalline apatite and that of freshly precipitated 



