34 FIRST PRINCIPLES OF AGRICULTURE. 



best soils in large amounts ; they are formed slowly from 

 those that are dormant. Humus is not a direct food, 

 but is capable of being changed into food. 



Clay and substances containing phosphoric acid, by 

 weathering, or the action of frost, heat, and moisture, 

 are changed to such an extent as to give up, in time, 

 portions of their potash, phosphoric acid, and lime. 



The True Measure of Fertility. — The active constit- 

 uents, however, measure the true fertility of any soil. 

 The dormant substances may be rich in phosphates, pot- 

 ash, lime, and humus, and yet it may be impossible to 

 produce a single plant from them, because the surround- 

 ing conditions are never favorable for the activities that 

 cause their change into active substances. 



An analysis of the soil does not show true fertility, 

 unless it shows how much of the total constituents of 

 the soil are capable of being made active, and thus 

 useful to the crops : it simply shows the possibilities 

 that are lying dormant. 



One Element Cannot Substitute Another. — An- 

 other point is also important in this connection ; namely, 

 that, of the three active constituents, nitrogen, phos- 

 phoric acid, and potash, which exist in small quantities 

 in all soils, the one contained in minimum amounts in 

 the soil determines its power of producing plants; that 

 is, the crop cannot rise above the point measured by 

 the element existing in the smallest amount — one ele- 

 ment cannot be substituted for another. For example, 

 if we have in an acre of soil only sufficient nitrogen for 

 ten bushels of wheat, the crop could not be increased 

 to any considerable extent beyond that point, even though 



