860 



SCIENCE 



[N. S. Vol. XXVIII. No. 729 



to the acre, in the tobacco lands of Florida and 

 of the Connecticut Valley, where 2,000 or 3,000 

 pounds of high-grade fertilizers carrying 10 per 

 cent, of potash are used, even where these applica- 

 tions have been continued year after year for a 

 considerable period of time, the dissolved salt con- 

 tent of the soil as shown by this method is not 

 essentially different from that in surrounding 

 fields that have been under extensive cultivation. 

 In England and in Scotland it is customary to 

 make an allowance to tenants giving up their 

 farms for the unused fertilizers applied in the 

 previous seasons. The basis of this is usually 

 taken at 30 to 50 per cent, for the first year, and 

 at 10 to 20 per cent, for the second year after 

 application, but in the experience of this bureau 

 there is no such apparent continuous effect of 

 fertilizers on the chemical constitution of the soil. 



Again, on pages 21 and 22, Farmers' 

 Bulletin 257, published in 1906, we have 

 the following definite statements from Pro- 

 fessor Whitney : 



There is another way in which the fertility of 

 the soil can be maintained, viz., by arranging a 

 system of rotation and growing each year a crop 

 that is not injured by the excreta of the preceding 

 crop. ... In other experiments of Laws and 

 Gilbert they have maintained for fifty years a 

 yield of about 30 bushels of wheat continuously 

 on the same soil where a complete fertilizer has 

 been used. . . . With a rotation of crops without 

 fertilizers they have also maintained their yield 

 for fifty years at 30 bushels, so that the effect of 

 rotation has in such case been identical with that 

 of fertilization. 



It is not my purpose in this paper to 

 discuss the work^ and theories and eon- 

 elusions of the Bureau of Soils, except so 

 far as seems necessary in fixing upon some 

 chemical principles fundamental to main- 

 tenance of the fertility of American soils. 



Aside from negative factors, including 

 the prevention of injury by disease, insects, 

 weeds, etc., we must recognize six essential 

 and positive factors in crop production: 



First, the seed, whose value is governed 

 by kind or variety, by previous selection or 



* See Illinois Experiment Station circulars 72 

 and 105, and the Norfolk report (1907) of the 

 committee of seven on the president's address of 

 1906, Association of OSicial Agricultural Chemists. 



breeding, and by inherent vitality and the 

 vigor of growth to be imparted to the 

 young plant. 



Second, the home of the plant, or the 

 physical character of the soil, including 

 structure, texture and tilth. 



Next, the heat, light and moisture, which 

 influence so markedly the rate of growth, 

 and which can be controlled to a greater or 

 less extent beyond what is done under the 

 normal conditions of crop production. 



And lastly, the plant food, a factor of 

 no less importance to crop production than 

 is animal food to the growth of animals. 



It can not be said that any one of these 

 factors is the most important, because 

 every one is absolutely essential ; but it can 

 be said that of the factors that may be con- 

 trolled plant food is certainly the most 

 neglected and possibly the least under- 

 stood, not only by practising farmers, but 

 also by many agricultural teachers and in- 

 vestigators. 



Failure to appreciate the importance of 

 the plant-food factor is due in part to the 

 short-sighted view too commonly given to 

 the problem. 



The great question that stands before the 

 soil investigator, and before the American 

 people is not how to grow good crops for 

 the next year or even the next generation 

 alone, but how to permanently maintain 

 the fertility of American soils. As soon 

 as we try to plan for permanent systems 

 then we begin to realize the limitations of 

 our plant-food supplies. 



Another matter that has led to much 

 confusion and misunderstanding is the 

 common talk of available plant food, as 

 distinct from the total supply, when as a 

 matter of fact there is no definite line of 

 distinction. The question as to the amount 

 of available plant food contained in the soil 

 at any given time is very insignificant in 

 comparison with the question how to make 

 plant food available. The plant food re- 



