Popular Science Monthly/Volume 26/February 1885/Field Experiments in Agriculture
THE field experiment is both the oldest, the most common, and the most popular form of agricultural experiment. So soon as agriculture passed beyond the rudest and most primitive stages, the idea of testing the value of different manures, or of different modes of culture and treatment, or of divers kinds or varieties of plants by means of comparative trials on adjacent plots of ground, must have suggested itself, and so the agricultural field experiment was initiated. In its beginning it must have been of the rudest character, and yet the fundamental idea was essentially scientific, viz., to place the things to be compared under the same conditions, and let each bring forth its results; and while the details of such experiments have been gradually refined, and errors of method eliminated, they are still the same in purpose and essence.
Such experiments appeal powerfully to popular interest; and the reasons for this are not difficult to perceive. Field trials deal with subjects of constant and absorbing interest to the farmer in a way readily comprehended; they employ processes with which he is familiar through daily use of them; above all, they seem to promise results directly applicable to practice. It is not to be wondered at, then, that in a time when agricultural experimentation is attracting attention as never before, field experiments should be multiplied on every hand, and that the public should regard the making of them as one of the chief ends, if not the chief end, of experiment stations and experimental farms. Both in Europe and America a vast deal of time and money has been devoted to their execution, and not only have organizations undertaken them, but private farmers have been urged to experiment on their own farms, both with a view to obtaining a better knowledge of the needs and capacities of their soils, and in the hope of advancing the science of agriculture.
The literature of the subject is voluminous, particularly in relation to the use of manures and fertilizers, and it might be expected that by this time our knowledge of these matters would be tolerably complete.
When, however, we come to look for the results of all this work, we find them surprisingly meager in comparison with the expenditure of time and labor which they have cost. That many valuable results have been reached goes without saying; but relatively their number is small, while the number of uncertainties and contradictions is remarkably great. The weakest portion of agricultural chemistry is that relating to fertilizers and manures; that is, precisely that part which we should expect to find well developed.
This state of affairs could not fail to impress thoughtful students of agricultural science, and cause them to seek out the reason why a method, which is apparently based upon a correct principle, and which has been executed with so much labor and care, has yielded, on the whole, such unsatisfactory returns.
Quite recently two German investigators, Professor Paul Wagner in Darmstadt, and Professor G. Drechsler in Göttingen, have given especial attention to this question, and have reached some interesting and important results, a brief account of which may not be uninstructive at a time when such general attention is being given to agricultural experimentation. These two experimenters have worked quite independently of each other, and their substantial agreement is strong evidence of the correctness of their conclusions.
We have said that the fundamental idea of the field experiment is essentially scientific; but, while this is true, a more critical examination shows that the way in which this idea has been carried into execution has been far from scientific. The scientific method of experiment requires two things: 1. All the conditions of the experiment must be identical, with the exception of the one whose action is to be tested, and that must vary to a known extent. 2. The limits of error of the methods of weighing, measuring, etc., used, must be known, to the end that we may know whether any difference which may be observed is accidental or significant.
The field experiment, as commonly executed, has sinned against both these requirements, but particularly the first, in that it has assumed 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 sufficient 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 convince 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 between adjacent plots are frequently found to invalidate the results of such trials. These are not only differences in the amount of available plant-food present; the physical properties of the soil play a prominent rôle. Even slight differences in the depth or texture of soil or subsoil, a greater or less proportion of organic matter, a difference 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 preliminary 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 differently affected by the same manuring. As a control on the natural variation of the soil, Drechsler depends chiefly upon duplication of manurings, the same treatment being applied to a number of plots scattered 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 manuring 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 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, such a case could not arise, the water-supply being uniform and in excess of the needs of the plants. The same considerations apply to other conditions, of course, though less markedly than to the water supply. In carefully conducted pot experiments it is possible to have practically all the conditions controllable, while duplicate trials will show the degree of accuracy obtained.
What, now, is the value of this method, as compared with properly conducted field experiments, in the study of agricultural questions? Can it replace them either partially or wholly? An intelligent reply to these questions must distinguish between the various kinds of problems which present themselves for solution.
In the first place, many purely scientific problems demand attention. These are of the first importance, for^ until we can master them, all attempts to apply science to practice will have but partial and uncertain success. Such problems are, for example, the most suitable form in which certain fertilizing substances may be applied (sulphate or chloride of potassium, nitrates or ammonium salts, soluble or reverted phosphoric acid, etc.), the effect of differing degrees of fineness, or of a more or less uniform distribution at different depths in the soil, the effect of different manurings upon the chemical composition and feeding value of the plants produced, the specific needs of different plants as regards fertilizers, etc., etc.
Such problems as these can be solved only by scientific methods of experiment, in which all the conditions are under control. Just as the question, what substances are essential to plant-growth, was not solved by field experiments, but by the method of water-culture, in which no soil at all is used, so questions such as were just mentioned seem likely to reach their solution by a method almost equally removed from the conditions of practice. But while the method of pot experiments appears well adapted to resolve scientific questions, and while its results (if reached legitimately, and tested carefully) are true independently of any extraneous considerations, those results need to be tested under actual working conditions; not as to their truth—that is settled—but as to their applicability to practice. It is true, as a scientific fact, that certain varieties of feldspar contain several per cent of potash, and it is also true that potash is an indispensable element of plant-food; but he who should therefore try to supply potash to his crops by means of ground feldspar, would be likely to meet with very indifferent success. He would not thereby disprove the fact that feldspar contains potash, or that potash is indispensable to plants. He would simply show that to these two facts there must be added some information as to the availability of feldspathic potash as plant-food, and so his field experiment would be the starting-point of a new series of scientific investigations, which should show whether the first-named facts were capable of any useful application.
The method of exact field experiments, then, as developed by Drechsler, has for its proper object the testing of facts obtained by pot experiments or by other scientific methods, as to their direct applicability to practice. It can never be a means of investigation itself, but it is indispensable to a proper utilization of the results of investigation, as well as in suggesting new directions for research. For these purposes it can not be too exact, and it would be well if those who are called to the conduct of such experiments would make themselves thoroughly acquainted with the difficulty of obtaining results which will endure careful criticism, and with the almost numberless precautions necessary thereto.
Other and ruder forms of the field experiment are omitted here. They are, or may be, of much practical value when carefully made and rightly interpreted, but their contributions to the science of agriculture are nil. The two methods whose general features have been described, however, are really means of scientific research. They are laborious because the subject is a difficult and complicated one, but by their conjoined aid we may hope to make sure if slow progress. The thing of prime importance is a clear recognition of the possibilities and of the limitations of each method.