PENNSYLVANIA FIELD EXPERIMENTS 425 



$13.21 (average $16.83); an d with 48 pounds of nitrogen in addi- 

 tion, the increase is $12.87, ^S-Sy, $20.71, and $17.52 (average 

 $18.49). Here we have an average increase of $1.66 resulting from 

 the application of $7.20 in 48 pounds of nitrogen. Further addition 

 of nitrogen produces some additional increases, but always far 

 below the cost of the nitrogen applied. 



After subtracting the cost of treatment (counting nitrogen at 

 15 cents a pound, phosphorus at 12 cents in acid bone black and 

 at 10 cents in ground bone, and potassium at 6 cents a pound), 

 we find the greatest net profit from commercial plant food is in 

 the use of phosphorus alone. 



While $5.04 worth of phosphorus used alone produced $12.17 

 increase (plot 3), when applied in addition to other treatment, the 

 same amount of phosphorus produced $14.28 over nitrogen (plot 5 

 over plot 2), $17.64 over potassium (plot 7 over plot 4), and $16.48 

 over nitrogen and potassium (plot 9 over plot 6). Plots 12 and 35 

 also show marked increases from the use of ground bone. Plot 17 

 appears to give too low results compared with the general averages 

 or with plot 15, although the increase from plot 17 (NPK) is $8.48 

 more than that from plot 6 (NK). Thus, under every condition 

 phosphorus has much more than paid its cost, the average effect 

 being a net profit of about 200 per cent for phosphorus if we dis- 

 regard the cost of the other elements. While phosphorus and nitro- 

 gen together more than paid the combined cost and produced dis- 

 tinctly better crops, this system yields less net profit than the 

 phosphorus alone. Similarly, phosphorus and potassium gave 

 larger increases, but less profit than phosphorus alone. In no case 

 has either nitrogen or potassium paid their cost. 



It is noted that $9.96 worth of potassium alone produced only 

 $2.68 increase, but when applied with phosphorus the average 

 increase ($16.83) was $4-66 more than that from phosphorus alone 

 ($12.17). Surely we should try to secure this increase by some 

 means. If kainit at one third the cost would produce the same 

 increase, it could be used with profit, and if farm manure or clover 

 as green manure would produce still greater increase at still less 

 cost, we should plan accordingly. 



Where manure was applied at the rate of 12 tons per acre in four 

 years (6 tons for corn and 6 for wheat) , the value of the increase is 



