PRESIDENTIAL ADDRESS. 639 
nitrogen derived from the atmosphere, in which process the carbonaceous part 
of the annual crop supplies the motive power. 
The other leading case to be found at Rothamsted is that of certain grass-~ 
plots which have artificially been brought into an acid condition by the 
continued application of sulphate of ammonia. In these soils nitrification is 
suspended, the nitrification organisms have even disappeared, though the 
herbage still obtains nitrogen because most plants are able to utilise ammoniacal 
nitrogen as well as nitrates. The interesting feature, however, is that the 
decaying grass on these acid soils passes into the form of peat, a layer of 
which is forming upon the surface of the soil, though nothing of the kind is 
found on adjacent plots where the use of lime or of alkaline manures has 
prevented the development of acidity. From this we may learn that the 
development of a surface layer of peat, independent of waterlogging (when 
another kind of peat forms even under alkaline conditions), is determined by 
the acidity of the soil, when certain of the bacterial processes of decay are 
replaced by changes due to micro-fungi which do not carry the breaking-down 
of organic matter to the destructive stage. This affords us a clue 
to the origin of many areas of upland peat in the British Isles, where the 
remains of ancient forest roots and stumps of trees are found on the 
true soil surface below the layer of peat, but where there is no water- 
logging to bring about the death of the trees and the formation of peat. We 
may suppose that when the land-surface became fit for vegetation at the 
close of the glacial epoch it covered itself with a normal vegetation, chiefly 
dwarf forest, because of the rainfall and temperature. The soil, however, 
being without carbonate of lime, would in time become acid with the pro- 
ducts of decay of the vegetable matter falling to the ground, and as soon 
as this acid condition was set up peat would begin to form from the grassy 
surface vegetation. The process would continue until the acid conditions 
and the depth of the accumulating layer of peat would kill the trees, the 
stumps of which would remain sealed up below the peat. I am far from 
thinking that this explanation is complete, but at least we have facts in 
sight which could lead one to suppose that a non-calcareous soil originally 
neutral and carrying a normal vegetation can naturally become acid, alter 
the character of its vegetation and clothe itself with a layer of peat. The 
point of economic importance is that these peaty acid soils are of very little 
value as long as they are acid, though they take on a quite different aspect if 
they are limed and made neutral. 
Of all the soil factors making for fertility I should put lime the first; upon 
its presence depend both the processes which produce available plant food in 
quantities adequate for crop-production at a high level and those which naturally 
regenerate and maintain the resources of the soil; it is, moreover, the factor 
which is most easily under the control of the agriculturist. 
I need say little about those cases in which infertility is due to the 
presence in the soil of some substance which is actually injurious to plant- 
growth, because such substances are nearly always due to the physical environ- 
ment of the soil, to too much or too little water. In waterlogged situations 
we may find in the soil peaty acids, iron salts, sulphides, &c., inhibiting the 
growth of plants; in arid regions the soil may still be charged with an excess 
of soluble compounds of the alkalis and alkaline earths, resulting from the 
decomposition of the rocks that have been broken down to form the soil, but 
which through the inadequate rainfall have never been washed out. ‘The 
establishment of normal conditions of growth, irrigation in the one case, 
drainage in the other, will speedily result in the removal of the deleterious 
substances. Practically, only bodies that are soluble can get into.a plant to 
injure it, hence such bodies can be removed from the soil by water, provided 
that the water can find its way through the soil and escape. 
Let us now consider the various methods by which land suffering from 
one or other of the disabilities we have just discussed is nowadays being brought 
into cultivation. The most important, if we consider the area affected, is the 
extension of cropping into regions of deficient rainfall by means of what has 
been termed dry-farming. As far as its immediate methods go, dry-farming 
consists in nothing more than the application of the principles of husbandry 
