years can eliminate this source of uncertainty in the interpretation of the results.
As regards the practicability of the method of exact field experiments as developed by Drechsler, it is worthy of remark that, while results which conform to its criteria are trustworthy, a considerable proportion of his own experiments have simply succeeded in demonstrating that the soil was too unequal to admit of successful field trials.
Wagner has attacked the problem in a different way His first attempt was to make field experiments upon very small plots, only two or three square metres in area, separated from each other by walls of masonry, and to compensate for the small size of the plots by the care with which they were treated. He also adopted the plan of repeating each manuring several times, as described above. The results were not satisfactory, however, owing largely to the unequal distribution of water among the plots, and after numerous experiments he has abandoned this method and adopted that of pot experiments. His pots are cylindrical zinc vessels, fifty centimetres (nineteen inches and a half) high and twenty-five centimetres (nine inches and three quarters) in diameter. These are uniformly filled with the carefully mixed soil, and are provided with an arrangement by which the water of the soil is automatically replaced as fast as it evaporates. The small size of the pots permits the use of pure materials as fertilizers, while for the same reason duplicate trials can easily be multiplied. The method in Wagner's hands has proved a practical one, and has already yielded some very interesting results.
It may seem that such a method as this is too far removed from the conditions of actual practice to afford results of any practical value. There is a degree of truth in this criticism. The conditions in such an experiment are different from those in the field. Wagner's method has one inestimable advantage, however, viz., that all the conditions of the experiment are under control. The importance of this is strikingly shown by considering the effects of a deficient supply of water, such as is liable to occur in any field experiment.
It is a well-known law of vegetable growth that that factor which is present in the least quantity in comparison to the amount needed—i. e., which is present in relatively the minimum quantity—is the one which chiefly regulates the amount of production. If, in a field trial, the supply of water holds this position, as it easily may, it and not the diverse manuring will determine the amount of crop. Moreover, as the plants grow larger and expose more leaf-surface, they exhale more water, and it might very well happen that a supply of water which was sufficient for a plot lightly manured might not be enough to supply the exhalation from the more luxuriant plants on a better-manured plot. The result would be, that the growth on the latter plot would be hindered, and the manure would not have a chance to show its full value. In pot experiments conducted according to Wagner's plan,
