672 TRANSACTIONS OF SECTION M. 
the real thing on a large scale, where the second or even the third terms really 
dominate the issue. The farmer is fond of reproaching the scientific men with 
the discrepancy between theory and practice; there should be none if the theory 
is complete, but in such complex matters as the growth of plant and animal we 
are yet very far from being able to bring into account all the factors concerned. 
A shipbuilder, for instance, having built to a certain speed and measured off his 
distance on the map, may reckon on making his port on a certain day; he finds 
himself wrong, because of the existence of a current which takes a knot or 
more off his speed. His theory was not wrong, only incomplete. Fuller know- 
ledge may map the currents and their velocity, but even the new calculation may 
be put out by some unexpected weather factor. Now the growth of a plant is 
determined by an infinitely more numerous and less measurable series of factors 
than the speed of a ship: small wonder then that the calculations based upon 
them are apt to be so erroneous. 
Imperfect as is our knowledge, yet we have progressed far enough to see in 
what directions fruitful work may be done, and may plan our campaign of 
research. In connection with the soil, for example, the big problem is probably 
the prevention of the waste that goes on at an increasing rate as the soil becomes 
more enriched by the accumulation of organic matter. Many soil bacteria, as 
we know, deal with the compounds of nitrogen in the soil so as to set free 
nitrogen gas from them, all of which actions are sheer waste of the most 
valuable constituent of the soil, and to such an extent does this change take 
place that we cannot, as a rule, expect to recover in the crop more than one- 
half of the nitrogen contained in farmyard manure applied to the soil. Where 
the soil is rich, and a high level of production is being arrived at, the per- 
centage of waste may be even greater; for example, on the Rothamsted wheat 
plot, which has received 14 tons of dung every year, only about one-quarter 
of the nitrogen applied in the manure has been recovered in the crop, and less 
than a quarter remains stored in the soil. When a hundred pounds of nitrate 
of soda per acre is applied, nearly the whole of the nitrogen it contains will 
be recovered in the increased crop; with an application of 200 lbs. there may 
be a waste of 25 per cent. of the nitrogen, with still greater losses as the appli- 
cation is increased. The loss is not due to mere washing out of soluble 
materials, because it is greatest when the nitrogen is applied in organic manures. 
Under existing conditions, high productivity in the soil is associated with a 
high rate of waste, and nowhere is this more marked than'when cultivation is 
carried on under tropical conditions, so that one of the chief difficulties of 
tropical and semi-tropical agriculture is to maintain the stock of humus and 
nitrogen in the soil. An illustration of the waste that so often goes on in the 
soil is furnished in the practice of the cultivators under glass in England. For 
the growth of cucumbers and tomatoes they are in the habit of making up a 
very rich medium, half soil and half dung, but after a very few crops they are 
no longer able to use this mixture profitably, but must throw it away and renew 
their beds, though the rejected soil is still extremely rich in the elements of 
plant food. The recent investigations at Rothamsted have shown that the 
fertility of this ‘sick’ soil can be restored by merely heating it for an hour or 
-two to a temperature approaching that of boiling water, the cost of which 
operation is considerably less than that of renewing the soil. In this case the 
uselessness of the used soil appears not to be due to thé destruction of the 
nitrogen compounds, but to their retention in a condition unavailable for the 
plant. The nitrogen compounds have to be broken down to ammonia or nitrates 
before they can feed the plant; this process is effected by certain groups of 
bacteria, the numbers of which are limited in the sick soil by the excessive 
development of another group of soil organisms—protozoa, amcebe, &c., that 
feed upon the bacteria. 
We are only just beginning to take stock of all the changes in the soil 
materials that are effected by living organisms, some necessary, some com- 
petitors with the plant, some wasteful; the ultimate problem is to bring these 
processes under control in the field as well as in the laboratory. The antiseptic 
treatment of the land at large, in the way in which we can now clean up soils in 
pots, may seem an impossible dream, but not more impossible than the pro- 
duction of a heavily yielding weedless field of wheat would have seemed to 
