M.—AGRICULTURE 249 
system of extreme complexity greatly increases the difficulties. The 
chemical methods generally employed involve the extraction of the soil 
with water or some dilute solvent and the estimation of the phosphates 
and potassium which come into solution under standard conditions. 
When the results can be interpretated in the light of field experiments 
or experience, they are a very useful guide in advisory work. The fact 
that the method is an empirical one is a great drawback, but the more 
serious objection is that the results give a measure of the condition of a 
soil at a particular time only and obviously cannot apply to its condition at 
different times throughout the year. There is probably no hard and fast 
line between the ‘ non-available’’ and the ‘ available’ constituents, the 
one set gradually merging into the other. 
These fundamental difficulties have suggested the idea of making use of 
the plant itself as an index to the available plant nutrients in the soil. 
Much work from this point of view has been carried out and two methods 
based on these principles have been in use on the Continent for several 
years. These are the well-known methods of Mitscherlich and Neubauer. 
These methods, as well as Wiesemann’s modification of Mitscherlich’s 
method, have been very ably and critically reviewed by Dr. R. Stewart in 
a recent publication of the Imperial Bureau of Soil Science (Technical 
Communication No. 25, 1932). 
Mitscherlich’s method is based on his claim to have discovered a Law of 
Plant Growth which is applicable to all plant species ; he claims to have 
established that the plant yield can be increased by each single growth 
factor, even when it is not present in minimum, so long as it is not present 
in maximum. In its original form it was given by Mitscherlich as follows : 
The increase of crop produced by unit increment of the lacking factor is 
proportional to the decrement from the maximum. This can be expressed 
mathematically as follows : 
dy 
oY —C(A—y) 
where y is the yield, A is the maximum yield and x the growth factor. 
(It is interesting to observe that this equation is identical with that for 
a mono-molecular chemical change.) 
Mitscherlich developed an elaborate technique for applying his theory 
to the testing of soils by means of pot experiments. Here again, certain 
arbitrary assumptions had to be made, e.g. as regards the depth of 
sampling, the effects of the subsoil and the permeability of the subsoil. 
These assumptions regarding the sampling factor undoubtedly raise 
serious difficulties and have been the subject of adverse criticism. 
__ From his estimation of the manurial content of the soil, Mitscherlich 
then calculates the manurial requirements, the calculation being greatly 
simplified by the assumption of the constancy of the effect factor, which 
means that the manurial requirements for any one soil are the same for all 
crops. 
Mitscherlich’s method has been subjected to severe criticism, first on 
his theoretical assumptions, his treatment of his experimental data and 
the applicability of the Logarithmic Law ; and secondly, on the constancy 
