PENNSYLVANIA FIELD EXPERIMENTS 



427 



land. It should be kept in mind, too, that this soil is not very acid, 

 and, consequently, neither burned lime nor ground limestone would 

 be expected to produce marked effects. 



Since phosphorus and manure were both used separately with 

 marked effect and profit, it seems probable that phosphorus and 

 manure together would have produced still more satisfactory re- 

 sults; and, if the action of the ground limestone were modified 

 by being used with manure as much as was that of caustic lime, 

 then it, too, would have produced increases above its cost. At least, 

 the facts suggest that manure, phosphorus, and limestone .would 

 make a very profitable combination ; and green manures and other 

 crop residues could of course be used in place of animal manures. 

 - In Tables 79 and 80 are recorded the 24 years' data arranged in 

 two periods of 12 years each. While 24 years is too short a period 

 to furnish very trustworthy data concerning the tendency of a sys- 

 tem of farming toward increase or decrease in crop yields on one 

 piece of land or with one crop, probably the average results from 

 all crops on the four series of plots in these Pennsylvania Experi- 

 ments furnish almost, if not quite, as satisfactory information 

 along this line as any of the Rothamsted fields. Such results are 

 recorded in the columns headed " Value of the four crops." It 

 should be remembered that a poor year for oats may be a very good 

 year for winter wheat, corn, or hay, and that four crops every year 

 for 27 years furnish almost twice as much data as the single system 

 on Agdell field, even though continued for 60 years. 



As an average of the 20 plots that have received no treatment 

 since 1882 (including the No. 8 plots), the yields have decreased as 

 shown below: 



AVERAGE YIELDS PER ACRE ON 20 UNFERTILIZED PLOTS 



