266 SECTIONAL ADDRESSES. 
him to countermand the order. So far from the subject being exhausted, 
it still bristles with problems. The new nitrogenous fertilisers, resulting 
from war-time activities in nitrogen fixation; the need for reducing the 
cost of superphosphate ; the change in character of basic slag; and the 
Alsatian development in potash production are producing changes in the 
fertiliser industry the full effects of which are not easy to foresee. Hconomic 
pressure is driving the farmer to derive the maximum benefit from his 
expenditure on fertilisers, lime, farmyard manure and other ameliorating 
agents, and is compelling a more careful study of possibilities hitherto 
disregarded, such as the use of magnesium salts, silicates, and sulphur as 
fertilisers, and, above all, a much more precise diagnosis of soil deficiencies 
than was thought necessary in pre-war days. 
But there are more fundamental problems awaiting solution. It 
is by no means certain that we know even yet all the plant nutrients. 
The list compiled by Sachs many years ago includes all needed in relatively 
large amounts, but Gabriel Bertrand has shown that it is not complete 
and that certain substances—he studied especially manganese—are essen- 
tial, although only in very small amounts. Miss Katherine Warington, 
working with Dr. Brenchley at Rothamsted, has shown that leguminous 
plants fail to develop in the so-called complete culture solution unless a 
trace of boric acid is added. Mazé has indicated other elements needed in 
small amounts. 
Another problem needing elucidation is the relationship between the 
quantity of nutrients supplied and the amount of dry matter produced. 
Is dry matter production simply proportional to nutrient supply, as Liebig 
argued, with the tailing off beyond a certain point, as demonstrated by 
Lawes and Gilbert, or is it always less than this, as indicated by 
Mitscherlich’s logarithmic curve, or is the relationship expressed by 
one of the more complex sigmoid curves as there is some reason to 
suppose ? We do not know; and the problem is by no means simple, 
yet it governs the ‘diminishing returns’ about which farmers now hear 
so much. Again, very little is known of the relationship between 
nutrition and period of growth. One and the same quantity of a 
nitrogenous fertiliser, for example, may have very different effects 
on the plant according as it is given early or late in life ; not only is there 
a difference in quantity of growth, but also in the character of the growth. 
Late dressings cause the characteristic dark-green colour to appear late 
in the season, and thus affect the liability to fungoid diseases ; they increase 
the percentage of nitrogen in the grain and they may give larger increases 
of crop than early dressings. 
Investigations are needed to find the best methods of increasing the 
supply of organic matter in the soil and its value for the different crops 
in the rotation. 
All these problems will sooner or later find some solution. But there 
remains a greater problem of more importance than any of them: the 
linking-up of plant nutrition studies with those of the soil solution. As 
our cousins in the United States werethe first toemphasise, the fundamental 
agent in the nutrition of the plant is the soil solution, and they have 
made a remarkable series of investigations into what appeared at one time 
a hopeless proposition—the physico-chemical interactions between the 
soil and the soil water. Whitney and Cameron began the work, and it 
has gone on with much controversy—as important scientific investigations 
