Relation of Plant to Soil 
WHENEVER subsoil is left exposed to the air it 
begins to cover itself with vegetation. The first 
plants that come up draw some of their food 
material from the soil, and they build up their leaf and 
stem tissues partly out of this and partly out of the car- 
bon-dioxide in the air. The process is in one important 
respect very much like rolling a ball up a hill — energy has 
to be put into it ; and in this case the energy comes from 
the sunshine. But neither energy nor matter is ever de- 
stroyed in Nature, and, in consequence, when the plants 
die and their leaves and stems become mingled with the 
soil, they add to its mineral matter both organic matter 
and energy. 
Direct experimentation shows that this addition of 
plant residues is beneficial to plant growth. Other evi- 
dence all tends the same way, and the general conclu- 
sion is that the difference between the surface and the 
subsoil lies largely in the presence of the residues left 
by the generations of plants that have lived and died 
there. The problem now is to find why the plant resi- 
dues are so beneficial. 
This is as far as observation will take us ; it is now 
necessary to bring the problem into the laboratory, in 
■order to investigate it further. 
The ]ilant residues consist mainly of four elements : 
carbon and oxygen in large proportions ; hydrogen and 
nitrogen in smaller proportions. In addition, there are 
lesser quantities of phosphorus ; calcium, magnesium, po- 
tassium, and other substances. The chief reaction in the 
soil is soon found to be an oxidation : oxygen is absorbed 
in great quantities, and carbon-dio.xide is given out in 
approximate equal volume. The carbohydrates of the 
plant disappear very rapidly ; some of the cellulose takes 
longer and gives rise to the black humus familiar to all 
gardeners. The nitrogen appears as nitrate. This is 
not quite what one would expect. In the decomposition 
of protein as it has been studied in the laboratory — and 
a prodigious amount of work has been done on the sub- 
ject — the result is always a remarkable variety of acid, 
known as amino-acids. Under the action of putrefactive 
bacteria, the decomposition is carried a stage further, and 
ammonia and other bases are produced, which are largely 
responsible for the very strong odor of decaying sub- 
stances — but nitrates are not found by the processes of 
the chemist. At first sight, therefore, it looks as if the 
process of the chemist was quite distinct from that of 
the soil, but closer study shows that this is not so. All 
the substances isolated in the chemist's decomposition can 
he found represented in the soil, and, what is still more 
to the point, if a trace of chloroform or toluene is added to 
soil no nitrate is formed, but ammonia accumulates in- 
stead. When a trace of untreated soil is added, the 
jirocess starts again, and nitrate is found as usual. Thus, 
it appears that ammonia is the precursor of nitrates, and 
is itself preceded by the amino-acids of the chemist. The 
difference between the laboratory decomposition and the 
soil decomposition is simply that the latter is carried 
several stages further ; up to the point reached in 
the laboratory the two, processes appear to be substan- 
tially the same. Thus Nature operates in the same way, 
whether in the laboratory or the field: the dift'erences are 
onlv in the lengths to which things go. 
This decomposition is absolutely indispensable to the 
plant ; the initial products — the proteins — are useless for 
plant nutrition ; the intermediate products are not much 
good ; the ammonia is considerably better, while the final 
stage — the nitrate — is the best of all. 
During this decomposition energy stored up by the 
plant during its lifetime is run down, so that there is a 
a transformation, both of material and of energy. We are 
accustomed to think of Nature as somewhat prodigal, 
the vast number of seeds produced by certain plants, the 
hosts of spores produced by fungi to ensure survival, all 
indicate this. But in the soil Nature is in a far more 
economical mood. The energy and material are not 
wasted ; they go to support a vast population of the most 
varied kind, ranging from the microscopic bacteria to the 
earth worm. All these depend on plant residues for their 
food and their energy. But theirs is no case of taking 
all and giving nothing in return. Their work is nothing 
less than the production of food for the plant; preparing 
new plant food out of old plant residues. 
Thus we have a great cycle going on in the soil; dead 
plant residues mingle with it and give life to countless 
micro-organisms, which in turn convert them into food 
for a new generation of plants. 
It is necessary to set some limits to the enquiry, and so 
we restrict ourselves to the production of nitrates. This 
process is the work of a great number of organisms, some 
of which carry out the first stages, and others the later 
stages. It resembles the process of making munitions, 
in that the first stages can be brought about by a large 
variety of workers, while later stages are much more 
specialized and can be effected only by one or two special 
workers. Indeed, in the eighteenth century. Nature's 
process of manufacturing nitrates was actually under the 
Ministry of jMunitions of the time. Great wars were 
going on, which consumed vast quantities of nitrate ; 
there were no Chilean deposits available, and no artificial 
nitrates ; all that was wanted had to be made by this soil 
process. Nitrate beds were made up, much like cucumber 
borders ; they were kept moistened by liquid manure, etc., 
and in course of time great quantities of nitrates were 
formed, which were afterwards washed out. 
The process is not free from waste ; starting with 100 
parts of nitrogen as protein, one never recovers 100 
parts of nitrogen as nitrate ; there is always a loss. But 
the fault does not appear to be with the special organ- 
isms that carry out the last stages of the process, for at 
least 96 per cent of the ammoniacal nitrogen is recovered 
as nitrate. It is not clear that it lies with the organisms 
producing ammonia ; at any rate, they can work without 
loss. The probability is that the loss arises from some of 
the nitrate that has been actually formed. 
However it arises, this loss, as well as the leaching 
out of nitrate by rain, would in natural conditions bring 
the stock of soil nitrogen to a very low level, if there 
was no counter-balancing process, and for the last fifty 
years chemists and bacteriologists have been searching the 
soil very thoroughly to find out how these gains are 
brought about. Two sources are now known ; the organ- 
isms associated with clovers and other leguminosae, and 
the free living nitrogen-fixing organisms. They differ 
very much in appearance and mode of life, but for their 
work they both require a source of energy ; for the process 
of nitrogen fixation, like that of plant growth, is like roll- 
ing a stone up a hill. In place of sunlight, these organisms 
get their energy from the combustion of sugar. 
It must not be supposed, however, that the organisms 
bringing about these changes are the only ones in the 
soil, or that they lead their lives quite independently of the 
rest of the soil population. Indeed, they could hardlv do 
that in any case, for there is only a limited store of food 
and energy, and whatever is not helping them is hindering 
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