out of the quagmire of doubt. The formulas might not be accurate ; 

 in some cases they might supply excessive amounts of plant food 

 elements and apparently be very wasteful, yet he believed that in the 

 end it was better economy to apply too much and insure a crop, than 

 use too little and lose a crop. Nevertheless, as Professor Stockbridge 

 anticipated would be the case, the fertilizers based on his formulas 

 were modified from time to time as we gained light, chiefly by the 

 reduction of nitrogen and the increase of phosphoric acid, as it was 

 found that many crops were able to gather from natural sources, 

 through bacterial action or otherwise, some part of the required 

 nitrogen, and that an excess of available phosphoric acid would has- 

 ten maturity. It was also found that to supply the full complement 

 of nitrogen in addition to what the crop would assimilate for itself 

 tended in many cases to produce an unbalanced growth ; yet, on the 

 other hand, it was found that in some cases, especially where a 

 forced growth or a tender leaf was required, an excess of nitrogen 

 was desirable. Thus it will be seen that the crop was both the start- 

 ing and the objective point. Not only its chemical needs, but its 

 habits and conditions of growth, the object for which it was grown, 

 and its market qualities, were all factors which influenced the compo- 

 sition or modification of the fertilizers ; and the same factors are as 

 potent to-day. Since, then, it was the crop that chiefly concerned 

 Professor Stockbridge, how natural and sensible was his ques- 

 tion : " What shall I supply you to make you a perfect and profita- 

 ble crop ? " 



Let us now consider for a moment another phase of the subject, 

 namely, the potential fertility of the soil, or " the natural yield," to 

 which Professor Stockbridge frequently referred. It has been known 

 for a long time that practically all tillable soils are rich in plant food 

 elements, and yet many of them are barren, and most of them will 

 not produce profitable crops without the aid of manure or fertilizer.* 



* Professor Frederick D. Chester, of Delaware, states in an able bulletin recently pub- 

 lished: 



'"An average of the results of 49 analyses of the typical soils of the United States 

 showed per acre for the first eight inches of surface 2600 pounds of nitrogen, 4800 pounds of 

 phosphoric acid and 13,400 pounds of potash. The average yield of wheat in the United 

 States is 14 bushels per acre. Such a crop will remove 29.7 pounds of nitrogen, 9.5 pounds 

 of phosphoric acid and 13.7 pounds of potash. 



" Now, if all the potential nitrogen, phosphoric acid and potash could be rendered 

 available, there is present in such an average soil, in the first eight inches, enough nitrogen 

 to last 90 years, enough phosphoric acid for 500 years, and enough potash for 1000 years. 



" This is what is meant by potential soil fertility, and yet such a soil possessing this 

 same high potential fertility may, under certain conditions, be so actually barren of results 

 to the farmer as to lead him to believe it absolutely devoid of plant food.'' 



