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SCIENCE. 



[N. S. Vol. XXII. No. 5G3. 



figures will be of a very indecisive char- 

 acter, and will lead to little information of 

 practical value. This is because the pro- 

 ductivity 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 ex- 

 ample, 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 cul- 

 tivation ; 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 item 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 mini- 

 mum' 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 con- 

 sistent with the water-supply, or if the 

 growth of the plant is being limited by 

 insufficient root range caused by bad tex- 

 ture 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 consid- 

 eration of the data supplied by the analysis. 

 The function, then, of soil analysis is not 

 to make absolute deductions from the re- 

 sults, but by a comparison of the unknown 

 soil under examination with other soils al- 

 ready known to interpret the divergences 

 and similarities in the light of previous 

 experience. That a given soil contains one 

 tenth per cent, of phosphoric acid or one 

 fiftieth per cent, of the same constituent 

 soluble in a dilute citric-acid solution is in 

 itself meaningless information ; but it be- 



comes of great value when we know that 

 the normal soils of that particular type 

 contain less than this proportion of phos- 

 phoric acid as a rule, and yet show no par- 

 ticular response to phosphatic manuring. 



What, then, the soil analyst can do is to 

 characterize the type, ascertain its normal 

 structure and composition, and correlate its 

 behavior under cultivation, its suitability 

 for particular crops and its response to 

 manuring in various directions. Thus an 

 unknown soil may by analysis be allotted 

 to its known type, deviations from the type 

 can be recognized and conclusions may be 

 drawn as to the connection of these defects. 



Valuable as recent development of soil 

 analysis may have been (and I allude in 

 particular to the improvements in the 

 methods of mechanical analysis which have 

 been worked out in the United States De- 

 partment of Agriculture, to the many in- 

 vestigations that have been made on the 

 measurement of 'available' plant-food by 

 attack with weak acid solvents, to the de- 

 terminations of the bacterial activity of the 

 soil), the results they yield can only be 

 truly interpreted when they can be com- 

 pared with a mass of data accumulated by 

 the use of the same methods on known soils. 



One of the services, then, which the 

 farmers in every country may very prop- 

 erly expect from the scientific man is such 

 a survey of the principal soil types, afford- 

 ing the necessary datum lines by which the 

 comparative richness and poverty of any 

 particular soil may be gauged. In an old 

 settled country like the United Kingdom 

 such a survey would guide the farmer in 

 his selection of manures ; in a new country 

 the advantages would be even more appar- 

 ent, as the areas appropriate to particular 

 crops would be indicated, and settlers 

 would be saved from many expensive at- 

 ' tempts to introduce things for which their 

 land was unsuited. 



It would also be possible to indicate the 



