248 



SOIL SURVEYS. 



This varying distribution of calcium carbonate in soils suggests 



the undertaking of a systematic series of soil analyses in any 



district, with a view to making soil maps that shall be of service 



to the agriculturist. Various foreign Governments have long been 



executing such a soil survey, but in the United Kingdom the 



matter has only excited one or two local attempts. While the 



basis of such work must always be the geological survey of the 



district, a geological survey in which, however, the thin " drift" 



formations are of greater importance than the solid geology, there 



are certain other items of information required by the farmer 



which would have to be supplied by the agricultural specialist. In 



the first place, the farmer wants to be told the thickness of the 



superficial deposits; he requires frequent "ground profiles," so 



that he can construct an imaginary section through the upper 10 



feet or so of his ground. The proximity, and, if near the surface, 



the direction of flow of the ground water are also matters on which 



there could be given to the farmer information of great importance 



when questions of drainage or water supply have to be considered. 



For the examination of the soil the field surveyor will procure 



typical samples of which the texture and physical structure can 



afterwards be worked out in the laboratory. But analytical 



figures are liable to be deceptive. This is because the productivity 



of a given piece of land depends upon a large number of agencies, 



any one of which may be the limiting factor in the crop yield. 



We may enumerate, for example, temperature and water supply, 



both determined by the climate, by the natural physical structure 



of the soil, and by the modifications in its texture induced by 



cultivation ; there are further, the aeration and the actual texture 



of the soil, the initial supply of plant-food of various kinds, and, 



again, the rate at which this last itejn is rendered available to the 



plant by bacterial action or by purely physical agencies. All these 



factors interact upon one another, to all of them, and not merely 



to the nutrient constituents, does Liebig's law of the minimum 



apply ; so that any one may become the limiting factor and alone 



determine the yield. It is of no use, for example, to increase the 



phosphoric acid content of a soil, however deficient it may be, if 



the maximum crop is being grown that is consistent with the 



water supply, or if the growth of the plant is being limited by 



insufficient root range caused by bad texture and the lack of 



aeration in the soil. However much we may refine our methods 



of analysis, we may take it as certain that we shall never be able 



to deduce a priori the productivity of the soil from a consideration 



of the data supplied by the analysis. The function, then, of soil 



analysis is not to make absolute deductions from the results, but 



by a comparison of the unknown soil under examination with other 



soils already known, to interpret the divergencies and similarities 



in the light of previous experience. What, then, the soil analyst 



can do is to characterize the type, ascertain its normal structure 



and composition, and correlate its behaviour under cultivation, its 



