66 Agricultural Chemistry. 



super-phosphate or nitrogen as nitrate of sodium, may greatly 

 increase its productiveness. If we compare the above table with 

 the table in the appendix, showing the amount of plant food re- 

 moved by various farm crops, it will be seen that the clay loam 

 soils show the presence of sufficient nitrogen for 95 crops of 

 wheat yielding 30 bushels per acre; phosphoric acid for 233 

 crops ; sulphur trioxide for 254 crops ; and potash enough to sup- 

 ply 1555 such crops. There is, in addition, nearly as much phos- 

 phoric acid and potash in the second and third foot, so that as 

 far as the latter substance is concerned, the supply seems almost 

 inexhaustible. The other two substances, nitrogen and phos- 

 phoric acid, and probably a third, sulphur, must be considered 

 as limited in quantity in many of our soils. In peat soils, potash 

 may also be very low. 



While chemical analysis will often disclose a large total amount 

 of plant food sufficient for many crops, nevertheless experience 

 has demonstrated that long before the theoretical number of 

 crops have been produced the yield will have decreased so mate- 

 rially as to become unprofitable. 



Available plant food. Chemical analysis gives the total 

 amount of nitrogen, phosphoric acid and potash in a soil, but 

 it does not indicate what part of these materials is available to 

 the plant. It takes an inventory of our stock on hand but does 

 not measure the crop-producing power of the soil. A large pro- 

 portion of this plant food is locked up in insoluble compounds, in 

 which form the plant is unable to use it. Food can be taken up 

 by the roots of plants only when in solution or in a condition 

 capable of being dissolved by contact with the acid sap of the 

 root hairs. 



The agencies operative in the soil and which we have already 

 considered are continually changing these insoluble compounds 

 to forms available to the plant; most of the soil ingredients are 

 in an insoluble form and this fact is really of the greatest im- 

 portance, for if it were not so soils would then lose fertility by 

 heavy rains. The unavailable or " potential" plant food is grad- 



