4 24 INVESTIGATION BY CULTURE EXPERIMENTS 



to eliminate so far as possible the influence of seasonal variation' 

 .in individual crops and to simplify comparison, the aggregate value 

 of the four crops has been computed, so that in all cases the finan- 

 cial statement refers to values for four acres. No value is allowed for 

 the corn stover or straw, and the prices used are 35 cents a bushel 

 for corn, 30 cents for oats, 70 cents a bushel for wheat, and $3 per 

 1000 pounds for hay. While these prices should be modified to suit 

 local conditions, they are as high as can safely be used as a basis 

 for planning profitable systems in the center of the principal grain- 

 growing section of the United States, especially if we must allow 

 for some shrinkage (particularly in the yield of hay) and for 

 occasional unavoidable losses from damaging storms. 



In the column headed " Value of the four crops," it will be seen 

 that the figures range from $31.69 (plot i, untreated) to $60.01 

 (plot 28, receiving phosphorus, potassium, and the heaviest appli- 

 cation of sodium nitrate). The four untreated plots show $31.69, 

 $37.27, $36.52, and $39.15, making a very considerable variation; 

 and the problem presents itself, How shall we determine the increase 

 produced by the different kinds of treatment? Manifestly, we must 

 adopt some method of estimating what would have been the yield 

 of the fertilized plots if they had not been fertilized. The average 

 of the four untreated plots would be the most satisfactory under 

 some conditions, but plainly this is not correct for these conditions, 

 because this would show an injurious effect from the nitrogen 

 alone, whereas positive and very appreciable gains are produced 

 in every crop on plot 2 in comparison with the immediately ad- 

 joining unfertilized plot (No. i). In the absence of specific objec- 

 tions it seems best to assume that the productive power of the land, 

 if unfertilized, would vary in uniform graduation from one check 

 plot to the next, and the figures given in the column headed 

 "Value if unfertilized" are computed on this basis. While this 

 seems fair to plots near No. i, a comparison of duplicate plots shows 

 some marked differences in " Value of increase," especially be- 

 tween plots 9 ($21.87) and 17 ($13.87), and between 7 ($20.32) and 

 29 ($13.21), although in the main the duplication is sufficiently 

 harmonious to justify full confidence in important average results. 

 Thus, the four plots receiving phosphorus and potassium show 

 "Value of increase" amounting to $20.32, $16.20, $17.59, and 



