i3 



In a word, potential fertility represents plant food which is so 

 tightly locked up that it is not available for present needs, and be- 

 comes available only through the process of decay and disintegra- 

 tion, which is too slow to meet the requirements of the commercial 

 farmer. Stockbridge realized the situation, but instead of asking 

 the soil how much of the potential fertility could be depended upon 

 for each crop (a question which will never be satisfactorily an- 

 swered), he went to the crop and asked it how much it was necessary 

 to supply for a stated yield over and above the natural yield of the 

 land. In all cases he found it to be a very small quantity. For the 

 corn crop not over 200 pounds of nitrogen, potash and phosphoric 

 acid was necessary, which the crop would return fifty fold (at least 

 five tons in stalk and grain) — so little to produce so much — and yet 

 if this little quantity of 200 pounds was not supplied the crop would 

 be a failure. 



It was this little essential balance of available plant food which 

 stood between success and failure that concerned Professor Stock- 

 bridge, as it concerns every farmer to-day. Although it was small, 

 he did not deem it wise to depend upon the potential fertility of the 

 soil to supply it, or even any considerable part of it. For the com- 

 mercial farmer it was too risky and uncertain. To insure a crop, as 

 far as one was able, was a cardinal principle with him ; not to do it 

 was in his eyes almost a crime. But he felt that all these things 

 would right themselves as we came to know more about farm crops 

 and their environment. 



As bearing on the economy of his system of plant feeding, I want 

 to quote here one of his apt illustrations. He said in effect : 



" In a sense the farmer is a manufacturer and the soil is his ma- 

 chine, into which he puts plant food, and out of which, by the aid of 

 Nature and his own efforts, he takes his product at harvest time. If 

 the soil machine is a good one, so much the better. If it has a 

 balance of crop-producing power to its credit, let us preserve that 

 balance for an emergency. Let us not draw on it for present needs." 



He had no patience with the so-called single-element doctrine, 

 which depends for its success on the potential fertility — no patience 

 with the farmer who was trying to find out for himself if he could 

 leave out any one of the three leading elements of plant nutrition 

 (nitrogen, potash and phosphoric acid), or how little of each he could 

 get along with. That was a proper subject for the scientific worker 



