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
