THE ROTHAMSTED EXPERIMENTS 369 



Thus, the average increase of 22.6 bushels resulting from the 

 annual application of 15.7 tons of farm manure (14 tons of 2240 

 pounds) would be worth $15.82 at 70 cents a bushel. In other 

 words, manure is worth $i per ton for use at this rate in continu- 

 ous wheat culture. 



In no case has the total application of commercial plant food 

 paid for its cost at standard prices; and rarely has any addition 

 paid for itself in increase produced, even though the cost of other 

 materials be disregarded. We may reckon $6.45 as the cost of 

 43 pounds of nitrogen, $3.48 for 29 pounds of phosphorus, $5.10 

 for 85 pounds of potassium, and $8.98 for the full minerals (assum- 

 ing that the magnesium and sodium salts can be bought as cheaply 

 as in kainit at $15 a ton when used in connection with sufficient 

 potassium). 



Thus the minerals alone on plot 5 produced an average increase 

 of 2 bushels, worth $1.40, at a cost of $8.98, and of course any 

 application made in addition to minerals must pay for this deffcit 

 of $7.58 as well as for its own cost before there could be any profit. 

 But if we disregard this deficit, we find that $6.45 in nitrogen on 

 plot 6 produced 8.9 bushels increase, worth only $6.23; that a 

 second $6.45 in nitrogen on plot 7 produced a further increase of 

 9 bushels, worth $6.30; and that the third $6.45 invested in nitro- 

 gen on plot 8 produced 4.3 bushels of wheat, worth $3.01. 



We may also begin our computations with the 400 pounds of 

 ammonium salts applied to plot 10, on which 86 pounds of nitro- 

 gen (only 12 pounds more than would be contained in a 5o-bushel 

 crop), costing $12.90, produced 7.5 bushels increase, worth $5.25, 

 thus placing a deficit of $7.65 against any additional treatment. 

 The increase from minerals alone was worth only $1.40, but with 

 nitrogen provided (on plot 7) the minerals costing $8.98 produced 

 $8.68 increase in the value of the crop, thus adding only 30 cents 

 to the deficit; while acid phosphate on plot n added $1.31, mak- 

 ing a total deficit of $8.96 for nitrogen and phosphorus, which, 

 however, was reduced to $8.60 by the potassium on plot 13. The 

 plan of the Broadbalk experiment affords no answer to the question 

 as to how much effect would be produced by potassium salt by 

 itself, but some extensive American experiments hereinafter dis- 

 cussed indicate that by itself the potassium sulfate would have 



