252 SECTIONAL ADDRESSES 
agreement, especially for potassium. ‘The richer the soil as estimated by 
the Neubauer method, the greater the growth of Aspergillus. For phos- 
phates the agreement between the two methods is very good as far as 
poor soils are concerned, but is only moderately good for the intermediate 
or richer soils ; one difficulty is that the absorption of phosphates by the 
fungus is not constant for different types of phosphates. 
The Aspergillus method is therefore likely to be valuable in estimating 
the potassium and phosphate requirements of a soil. The results, as 
might be expected, are more reliable for potassium than for phosphates, 
and while not rigidly quantitative, give information as to whether the soil 
is rich or poor in these constituents. It has the advantage of being rapid 
and requiring no expensive apparatus (A. M. Smith and R. Coull, Scot. 
Journ. Agr. vol. xv (1932), p. 262). 
The whole question of available plant food is necessarily bound up 
with the complex relationships which exist between plant and soil, and it 
is unlikely that any simple or single method will be devised to overcome 
the inherent difficulties of the problem and be generally applicable to 
different sets of conditions. ‘The admitted lack of agreement obtained 
with the various methods at present in use is undoubtedly due, to a large 
extent, to the variety of factors involved, as well as to the fundamental 
objections which may be raised to any one method. We are still very 
ignorant of the process of assimilation by the growing plant, and until 
we have more information on this subject, methods of estimating availa- 
bility must continue to be largely empirical and the results merely first 
approximations. f 
The usual method of approach to the problem has been to study the 
effect of the soil or plant medium on the plant. In Edinburgh attention 
has in recent years been directed in the opposite direction—namely, to a 
study of the effect of the plant on the soil. ‘The alterations to be observed 
are, of course, small, but by applying methods which might almost be 
described as analogous to modern micro-methods of analysis, measurable 
changes can be followed with considerable accuracy. The results which 
have been obtained are interesting and sometimes rather unexpected, and 
although it is scarcely to be supposed that they will furnish a complete 
picture of the relationship between soil and plants, one feels that any 
contribution to the subject from a new angle may be of value in the study 
of such a complex problem. 
FERTILISERS. 
Turning next to the progress which has been made in the manufacture 
and use of fertilisers since the time of Gilbert’s address, there are one or 
two notable dates and achievements to be mentioned. 
In 1878 Thomas and Gilbert introduced their new basic process for 
the manufacture of steel which resulted in the production of basic slag as a 
by-product. It was a few years before the value of the slag as a source of 
phosphates for plants was discovered. ‘The importance of the new slag 
in agriculture was first realised in Germany. ‘The earliest experiments 
in this country were carried out in England by Wrightson and Munro in 
1885, and by A. P. Aitken in Scotland about the same time ; a year.or 
