able crops without the aid of manure or fertihzer. Prof. Fred- 

 erick D. Chester, of the Delaware Agricultural College, states: 



"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, 2,600 

 lbs. of nitrogen, 4,800 lbs. of phosphoric acid and 13,400 lbs. of potash. 

 The average yield of wheat in the United States is 14 bushels per acre. 

 Such a crop will remove 29.7 lbs. of nitrogen, 9.5 lbs. of phosphoric acid 

 and 13.7 lbs. 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 phos- 

 phoric 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 con- 

 ditions, be so actually barren of results to the farmer as to lead him to 

 believe it absolutely devoid of plant food." 



In a word, potential fertility represents plant food which 

 is so tightly locked up that it is not available for present needs, 

 and becomes available only through the process of decay and 

 disintegration, which is too uncertain to meet the require- 

 ments of modern, intensive farming. 



^*The Little Balance'* 



Amount 

 of Plant 

 Food in 

 Soil 



Therefore, in modem practice, instead of asking the soil 

 how much of the potential fertility can be depended upon for 

 each crop, or what the "natural yield will be" (a question 

 which will never be satisfactorily answered), we now apply 

 what we beheve to be necessary to produce the maximum yield 

 over and above the natural yield of the land. In all cases we 

 find that the actual requirements of plant food for various crops 

 are very small indeed, in many cases less than a grain of 

 available plant food to each pound of soil — so little to pro- 

 duce so much, and yet if it is absent, the crop will be a 

 failure. It is this little, essential balance of available plant 

 food which stands between success and failure and which con- 

 cerns the modern farmer today. 



Fertilize 

 for 



Maximum 

 Yield 



27 



