PENNSYLVANIA FIELD EXPERIMENTS 



437 



approximately 10 per cent in 10 years, and if this rate of decrease 

 continues, we may expect the average values to drop during suc- 

 cessive lo-year periods from $80 to $72, to $65 to $59, to $53, and 

 to $48, in the next 70 years. 



It will be noted that the dividing point between the two zo-year 

 periods in Table 8iP is exactly the same as between the two 12- 

 year periods referred to in Tables 79 and 80; and it may also be 

 noted that, as an average of the four phosphorus-potassium plots, 

 the average yields during the second 1 2-year period show 6.4 

 bushels less corn, 8.3 bushels less oats, and 25 pounds less hay, but 

 with 3.5 bushels more wheat, than during the first 12 years. These 

 figures mean that for each rotation (four years) the yields have 

 decreased by 2.1 bushels of corn, 2.8 bushels of oats, and 8 pounds 

 of hay, while the yield of wheat shows an increase of 1.2 

 bushels. The algebraic sum shows, as an average, that each re- 

 curring rotation produces $2.36 lower crop values from an acre of 

 land than during the preceding four years. 



All this must remind us of the mineral plot on Agdell field, where 

 the yields of turnips and legumes are still well maintained, and the 

 wheat yield has appreciably increased, while only the barley has 

 very markedly decreased. 



Mathematically, it is not possible for the roots and stubble of 

 the clover crop to furnish sufficient nitrogen for the other four 

 crops, timothy (associated with the clover), corn, oats, and 

 wheat; but the question again arises, whether important amounts 

 of atmospheric nitrogen may not be fixed that are not thus ac- 

 counted for. It is fully established that the azotobacter (and 

 possibly other similar bacteria) fixes measurable quantities of 

 free nitrogen under favorable conditions; and it is also fully es- 

 tablished that the bacteria which commonly live in symbiotic 

 relationship with legume plants can fix appreciable amounts of 

 free nitrogen, under suitable artificial conditions, and entirely 

 independent of legume plants. It is thus conceivable that these 

 may fix nitrogen to a greater or less extent while they continue to 

 live, not in the tubercles of growing clover, but upon the dead and 

 decaying residues; and, if such is the case, it is exceedingly 

 probable that the presence of carbohydrate matter (as in plant 

 residues) and a liberal supply of available mineral plant food in a 



