430 INVESTIGATION BY CULTURE EXPERIMENTS 



could be withdrawn at any one time. (Two per cent per annum is 

 approximately the rate of decrease in crop values from the 20 un- 

 fertilized plots in these experiments.) 



There are only four combinations of commercial plant food that 

 have maintained the productive power of the soil, and these are all 

 combinations of the three elements; plot n (with dried blood, 

 144 Ib. N), plots 27 and 28 (with sodium nitrate, 96 and 144 Ib. N), 

 and plots 12 and 35 (with bone and blood, 60 Ib. N). However, as 

 an average of the 24 years, the increase was not sufficient to pay 

 for the cost of treatment on any of these plots. It appears, however, 

 that during the second 1 2-year period plots 12 and 35 have, as an 

 average, paid the cost of the plant food and left a net profit of 

 $3.29 from the four acres. Thus it will be noted that the only 

 plots receiving commercial plant food that has paid its cost even 

 for the second 1 2-year period and that has also fully maintained the 

 crop yields are those treated with ground bone. The difference in 

 favor of ground bone is not sufficient to show that it is distinctly 

 better than acid phosphate, but we may surely conclude that the in- 

 soluble bone is at least as good as the acidulated form. 



During the first 1 2-year period the net profit from the use of 

 manure decreased as the amount of manure increased above 12 

 tons, but during the second 1 2-year period the value of the 1 2-ton 

 application is more than twice as much as during the previous 12 

 years, and the greatest net profit per acre is where the heaviest 

 applications are made (counting 30 cents a ton for manure), but 

 the value per ton is still greatly in favor of the lighter application, 

 the first 12 tons being worth $2.14 a ton and the next 8 tons only 

 72 cents, compared with $1.14 and 4 cents, respectively, for the 

 first 12 years. While the larger amounts of manure show a distinct 

 cumulative effect ($56.31 to $62.78, or $6.47 a year from the four 

 crops), the lightest application has but little more than maintained 

 the earlier crop yields, the markedly greater apparent profit dur- 

 ing the second 12 years being due to the decreased yields of the 

 unfertilized land. 



In Table 81 is given a summary of the effect of treatment over 

 the 24-year period, and also a concise statement showing the actual 

 or absolute profit or loss from every system, based upon the aver- 

 age yields secured during the second 1 2-year period in comparison 



