ON RECENT DEVELOPMEN‘'S IN AGRICULTURAL SCIENCE. 975 
stricture can afterwards be worked out in the laboratory. Geological 
formations are constantly showing lithological changes as one passes 
along their outcrop either in a vertical sense or in their lateral extension ; 
and these changes are often reflected by corresponding changes in the 
character of the soil which are of commercial importance. 
But while the mechanical analysis of the soil has been of late the basis 
upon which all soil surveys are constructed, it is of equal importance, at 
any rate in the older countries under intensive cultivation, to undertake 
certain chemical determinations, which come to possess a new value when 
taken in connection with a soil survey. It has been generally demon- 
strated that an analysis, physical and chemical alike, of the soil of a 
particular field, taken by itself, possesses but little value. The physical 
analysis will indicate roughly the character of the soil; but very little 
better than could have been learnt by walking over the soil and digging 
in it for five minutes ; the chemical analysis will disclose any glaring 
deficiencies ; but, as a rule, the analytical figures will be of a very 
indecisive character, and will lead to little information of practical value. 
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 aération 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 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 aération 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 divergences 
and similarities in-the light of previous experience. That a given soil 
contains ;'; per cent. of phosphoric acid or ;4; per cent. of the same 
constituent soluble in a dilute citric-acid solution is in itself meaning- 
less information ; but it becomes of great value when we know that the 
normal soils of that particular type contain less than this proportion of 
phosphoric acid as a rule, and yet show no particular response to phos- 
phatic manuring. 
What, then, the soil analyst can do is to characterise the type, ascer- 
tain its normal structure and composition, and correlate its behaviour 
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 
_ recognised, and conclusions may be drawn as to the connection of these- 
defects. i 
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