20 MASSACHUSETTS HORTICULTURAL SOCIETY. 



conditions, 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 be- 

 comes available only through the process of decay and disintegration, 

 which is too uncertain to meet the requirements of modern, intensive 

 farming. 



The Little Bal.vxce. 



Therefore, in modern practice, instead of asking the soil how 

 much of the potential fertility can be depended upon for each crop, 

 or what the "natural \ield will be" (a question which will never be 

 satisfactorily answered) we now apply what we believe to be neces- 

 sary 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 

 not over 200 lbs. of actual plant food (nitrogen, potash, and phos- 

 phoric acid) — so little to produce so much — and yet if it is absent 

 the crop will be a failure. It is this little, essential balance of avail- 

 able plant food which stands between success and failure and which 

 concerns the modern farmer today. Thus our problem is to supply 

 the "little balance of ready plant food" for the growing crop, as 

 milk or prepared food is supplied to the growing child, and later, 

 as cooked or digestible foods are supplied to man. 



To illustrate: We apply say 1,000 lbs. of concentrated fertilizer, 

 carrying about 200 lbs. in all, of nitrogen, potash, and phosphoric 

 acid, to an acre of corn. An acre of land contains 43,560 square 

 feet. If we take the average depth of agricultural soil to be tweh'e 

 inches, then the acre contains 43,560 cubic feet of soil, through which 

 this 1,000 lbs. of fertilizer, or 200 lbs. of actual plant food, is dis- 

 tributed. In short, it gives to each cubic foot of soil 160 grains of 

 mixed fertilizer, or 32 grains of actual plant food. A cubic foot of 

 arable soil, according to the best authorities, weighs from 80 to 90 

 lbs. Taking it at 85 lbs., an acre of soil weighs in round numbers, 

 about 2,000 tons; and therefore we have only 2 grains of mixed 

 fertilizer to each pound of soil, or seven-tenths of a grain of actual 

 plant food. 



