500 THE POPULAR SCIENCE MONTHLY. 



The field experiment, as commonly executed, has sinned against 

 both these requirements, but particularly the first, in that it has as- 

 sumed uniformity of conditions instead of proving it. One of the 

 most important of these conditions is the soil. It has too frequently 

 been assumed that simple inspection is suflicient to assure one of the 

 uniformity of this factor, but this is far from being the case. 



Uniformity of soil over any considerable area is by no means an 

 easy thing to attain. In our Northern States, or in any drift-region, 

 one has only to examine the sides of the nearest ditch in order to con- 

 vince himself that the character of the soil varies from rod to rod, and 

 almost from foot to foot, and to cause serious doubts as to the value 

 of comparative field trials to arise in his mind. But even in localities 

 where such striking variations do not occur, sufficient differences be- 

 tween adjacent plots are frequently found to invalidate the results of 

 such trials. These are not only differences in the amount of avail- 

 able plant-food present ; the physical properties of the soil play a 

 prominent role. Even slight differences in the depth or texture of 

 soil or subsoil, a greater or less proportion of organic matter, a dif- 

 ference in surface color, a variation in the moisture of the soil, may 

 have a decided effect on the crop. Repeated trials have shown that it 

 is practically impossible to prepare a series of plots whose natural crop- 

 producing power shall be uniform. 



Under these circumstances trustworthy results in field trials can be 

 expected only if the amount of probable variation between different 

 plots is known. A preliminary cropping without manure naturally 

 suggests itself as adapted to furnish this information, and such pre- 

 liminary trials are of great value. At the same time they are not of 

 themselves sufficient. A different season may cause the relative yield 

 of two plots to vary considerably in different years* Moreover, two 

 plots might show the same yield when unmanured, and yet be dif- 

 ferently affected by the same manuring. As a control on the natural 

 variation of the soil, Drechsler depends chiefly upon duplication of ma- 

 nurings, the same treatment being applied to a number of plots scat- 

 tered over the field. By this he aims to accomplish two things : 1. 

 The average yield of these several plots is more likely to correspond to 

 the response which the field as a whole would make to the same ma- 

 nuring than is the yield of any single plot. 2. The variations of the 

 several plots from the average furnish a measure of the uniformity 

 of the soil, and serve to show whether a given difference in the final 

 results of two kinds of manuring is significant, or is simply accidental 

 and less than the errors of experiment. This is not the place to enter 

 into a description of all the precautions required in the conduct of such 

 trials. Those interested can consult Drechsler's original papers. One 

 point may be noted, however, viz., that differences in the season, 

 whether wet or dry, e. g., may have a deciding influence on the action 

 of manures, and that only continuing the experiments for a number of 



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