398 ANNUAL REPORT OF THE Off. Doc. 



the application made to tlie corn or wheat. In these experiments 

 which deal with the maintenance of soil fertility the most important 

 question is not the inlluence of a given fertilizer on a given crop, 

 but the total result obtained during a rotation or a series of rota- 

 tions. The results will be presented, therefore, by rotation periods 

 of four years. The weights or values given will be the total weight 

 or total value of the seven farm products obtained during one year 

 on the four tiers of plats. In computing values the following prices 

 have been used — corn, 50c; oats, 32e; and wheat, 80c per bushel. 

 Corn stover, oat straw and wheat straw, %'IXA) per ton, and hay $10 

 per ton. The values placed upon the grains are approximately the 

 average; December farm prices in Pennsylvania during the decade 

 ending December 1, 1906. 



COMPARATIVE EFFECT OF SINGLE FERTILIZER INGREDIENTS. 



In all discussion on the influence of the effect of different fertilizer 

 ingredients, it must be borne in mind that the results were obtained 

 in a rotation in which nitrogen was added to the soil from the nodule 

 producing bacteria of clover as well as by other soil bacteria. 

 Where single fertilizer ingredients have been applied the effect has 

 been to add to the soil the particular ingredient, plus nitrogen. 

 AA^'here nitrogen was the ingredient applied the effect has been 

 merely to increase that ingredient. In reality, therefore, the nitro- 

 gen plats are the only ones in which a single ingredient has been 

 applied. Keeping this in mind, the effect of an application of phos- 

 phoric acid, potash and nitrogen when applied singly is shown in 

 the table for twenty-five four years' rotation. 



Single Fertilizer Ingredients. 



Average value of products from twenty-five four years' rotations, 

 Pennsylvania Experiment Station, State College, 



Alternate Years. Lb. Value. 



None, $63 03 



Phosphoric acid 48 71 70 



Potash, 100 60 16 



Nitrogen, 24 58 84 



The application of potash and nitrogen singly has had no material 

 effect upon the yield. The actual yields have been slightly less upon 

 the plats where potash or nitrogen have been applied singly than 

 upon the average of the five untreated plats. It is probable, how- 

 ever, that the differences are within the limits of natural soil varia- 

 tion. 



Phosphoric acid, when applied singly, has had a distinct influence 

 in maintaining the crop producing power of the soil. The influence 

 of phosphoric acid in keping up the crop producing power is best 

 understood by a comparison with the untreated plats by five year 

 periods. During the first five years, while the land was in a good 

 state of fertility, no material influence was produced by adding 

 phosphoric acid. During the next five years there was a slight in- 

 crease in yield where phosphoric acid was applied. During the last 

 three five year periods the increase in yield on plats receiving phos- 

 phoric acid was marked compared with the untreated plats. While 



