December 18, 1884. ] 
JOURNAL OF HORTICULTURE AND COTTAGE GARDENER. 
557 
pear, but is changed by the presence of a weak acid into a very stable 
brownish-green product which resists further change. The production of 
bright yellows or dull browns thus clearly depends on whether the chloro¬ 
phyll does or does not disappear before being modified by the action of acids, 
as may be verified experimentally by exposing suitable solutions to sunlight. 
It is, however, very clear that the manner in which it changes depends very 
much on the condition of the case. Thus, if chlorophyll is exposed to sun¬ 
light dissolved in bisulphide of carbon, a reddish-coloured product is formed, 
and though this differs very greatly from the red pigment met with in many 
autumnal leaves, it seems probable that under some conditions the chlorophyll 
in leaves is changed by the action of light into a red substance. By taking 
green Sorrel leaves and keeping them somewhat fresh by sticking the stalks 
into moist ground, I found that those exposed to the sun with the under aide 
upwards turned to a bright red, whereas those kept in the shade did not 
develope any fine colouring. We may often see that partially broken leaves 
or twigs undergo this change when all other parts of the tree remain green, 
and this and various other facts lead me to conclude that the change of 
chlorophyll into a red product depends on a certain amount of reduced 
vitality as well as on little-understood conditions varying in different kinds 
of plants. Though I fully admit that there are some facts not easy to under¬ 
stand, yet on the whole it seems to me that these principles fairly well 
explain why certain leaves turn red in autumn. Slight frosts reduce their 
vitality in such a manner that the chlorophyll is changed by the action of 
the light into a red product. Thus, according to the character of the season 
and the nature of the plants, the first effect of the reduced vitality in the) 
leaves is that the chlorophyll is removed so as to show their normal yellow 
colour, or is changed into a red pigment, or is altered into a comparatively 
stable duU brown green product. These are the three extreme changes, but 
in many cases intermediate mixed results give rise to such less perfect and 
well-marked tints as dirty yellows and reds. 
The next series of changes is best studied in the case of those leaves 
which in the first instance turn to a bright yellow, and it appears to me 
that they depend mainly, if not entirely, on the production of deeply 
coloured pigments by the oxidisation of tannic acid and other more or less 
colourless substances. The difference in the resulting tint seems to depend 
on the nature of these substances. Thus, for example, the tannic acid in the 
yellow Oak leaves changes into a brown substance, whereas the quinotannic 
acid in yellow Beech leaves changes into the fine orange-brown colour which 
makes those trees so ornamental in autumn. On the contrary, the bright 
yellow Poplar leaves rapidly pass to a dark dirty brown by the alteration of 
another constituent. Other kinds of leaves give rise to tints of an inter¬ 
mediate and less well-marked character. In many cases it is almost impos¬ 
sible to draw the line between the colour of this stage in the change and the 
final dark and dirty browns of dead and decaying leaves. For fine effect 
very much depends upon the production of each special tint in a fairly pure 
state, so as to show bright yellows, reds, and browns. This seems to be 
influenced by the character of the weather. It is also, of course, important 
that the half-dead leaves should hang long on the trees, so as to develope 
their full colouring before being blown off by the wind. 
Taking thus all the facts into consideration, it appears clear that all the 
bright and beautiful tints of autumn are merely the earliest stages of 
decomposition, and are due to the more or less considerable triumph of 
chemical forces over the weakened or destroyed vitality of the living plant. 
One cannot but feel that this is a very unpoetical way in which to regard the 
magnificent tints of a fine autumnal landscape, but it is no less true than 
that the coloured clouds of evening mark the departing day.—H. C. Sorbt 
(in Nature). 
ON SOME CHANGES WHICH NITROGENOUS MATTER 
UNDERGOES IN THE SOIL. 
A COURSE of lectures on agricultural science was delivered at South 
Kensington last season, and these are now published in book form by 
Messrs. Chapman & Hall. They comprise a number of subjects con¬ 
nected with agriculture, but the following by R. Warington, Esq., F.C.S., 
is of general interest to both gardeners and farmers :— 
“ The soil beneath our feet has been universally regarded as in some 
mysterious sense the mother of us all. To us, in the present day, the 
manner in which soil supports the life of plants and animals is still 
mysterious, in the sense that we are yet in the dark as to th^ nature of 
many of the substances contained in the soil, of the changes which they 
undergo, and of the part which they take in plant-nutrition. This is 
especially true with regard to the organic matters, consisting of carbon, 
nitrogen, hydrogen, and oxygen, which the soil contains. We may, I 
think, usefully spend an hour this evening in attempting to sketch the 
general history and course of change of this organic matter, though in so 
doing we may often have to speak rather of our ignorance than of our 
knowledge. I will ask you to fix your attmtion at present chiefly upon 
one constituent of this organic matter—its nitrogen, as this is the aspect 
of the question which has mod agricultural importance. Our subject, 
then, is the nitrogenous organic mdterof the soil: Whence comes it? 
What becomes of it ? 
“ In order to s'^art with definite notions on the subject, let us take as 
an example an ordinary arable field, of clay soil, in fair agricultural 
condition. Such a field, when all stubble and roots have been removed, 
will contain in the first 9 inches of the surface soil a quantity of organic 
matter containing about 3000 lbs. of nitrogen, and 30,000 lbs. of carbon 
per acre. This nitrogenous organic matter of the soil has been derived 
either entirely from the decay of the vegetable matter left in the land by 
preceding generations of plants, or possibly, to some extent, from past 
applications of farmyard or other organic manure. It is very important 
to bear alw.ays in mind that the nitrogenous capital of a soil, which 
represents to a considerable extant its agricultural condition, depends as a 
rule on the bulk and composition of the previous crop residues, and on 
the extent to which these have been subsequently destroyed by operaticms 
which we shall presently have to notice. The present fertility cf the soil 
is thus, in great measure, a consequence of its past fertility. 
“ It is quite true that besides the residues of crops soils receive certain 
amounts of nitrogen from the atmosphere in the form of ammonia and 
nitric acid ; but the quantity of these substances contributed annually by 
rain is apparently not more than 3 to 4 lbs. of nitrogen per acre ; and 
though the amount of ammonia directly absorbed by the soil from the 
atmosphere may in some soils be much larger than this, the total nitrogen 
thus acquired, though most important as tending to counterbalance the 
losses of nitrogen which the soil annually sutlers, will have little effect 
on the present fertility in comparison with the large accumulations of 
nitrogenous matter resulting from previous crop residues. 
“ The nitrogenous organic matter of the soil has its origin in the various 
vegetable substances left in the soil as residues from preceding crops, to 
which in some cases we must add the residues from dressings of organic 
manures. A recognition of this fact is of vital importance if we are to 
have accurate notions as to the influence of different crops in maintaining 
or exhausting the fertility of the land. It is evidently the crop which 
leaves behind the largest amount of roots, stubble, and leaves, which will 
best maintain or increase the nitrogenous capital of the soil ; while the 
crop leaving the smallest residue in the soil will be most exhausting in its 
effect. Permanent grass and Clovers will thus stand at the head of the 
list as conservers of soil nitrogen , while root crops carted from the land 
will be placed at the opposite end of the scale. 
“ We may now ask—What becomes of the organic matter in the soil ? 
What course of change does it undergo ? 
“ Before saying anything as to the stages of this course of change, or 
about the means whereby the transformations are effected, it will be well 
to state at once that the organic matter in a fertile soil is contipually 
undergoing oxidation by various agents, the general result being its con¬ 
version into three simple substances—^water, carbonic acid, and nitric 
acid. The vegetable residues left by crops are thus reconverted into 
plant-food, and made fit to support the life of a new generation of 
plants. 
“ That carbonic acid gas is formed in large quantities in soil has 
been abundantly proved by Boussingault and Lewy, and by a number of 
more recent experimenters. The quantity of carbonic acid produced is 
greater according to the richness of the soil in vegetable matter, and is 
much increased when farmyard manure has been applied. The carbonic 
acid is formed in largest quantity in summer time ; the amount is also 
generally much increased by applications of chalk or lime. 
“ That nitrates are produced in soil has been known from very early 
times. Many examples of the quantities of nitrates existing in agricul¬ 
tural soils of various history are given in the tables illustrating this 
lecture. The facts connected with this part of the subject are, however, 
of so much practical importance that they will be best considered by 
themselves after the general sketch of the course of change of the organic 
matter has been completed. 
“ The first stage in the oxidation of a crop residue is marked by a 
rapid disappearance of carbon, doubtless evolved as carbonic acid, the 
nitrogen apparently remaining still in organic combination. The mean 
proportion of nitrogen to carbon in seven analyses is about 1 : 19, the 
extremes being 1:15 and 1:23. We are thus fairly well acquainted with 
the ratio of nitrogen to carbon in the crop residues and manure, from 
which the organic matter of soil is derived. If we now compare these 
ratios with the ratio shown by the organic matter of the soil, the dis¬ 
appearance of carbon becomes very striking. In the first 9 inches of the 
old pasture land at Rothamsted, with roots as far as possible removed, the 
ratio of nitrogen to carbon is about 1 : 13, while in the same depth cf 
arable soil the ratio is about 1 : 10, and does not reach 1 : 12 even where 
14 tons of farmyard manure per acre have been annually applied for more 
than thirty years. 
“ What is the true chemical nature of the nitrogenous organic matter 
forming the so-called humus of soils we do not know, nor even if it 
consists mainly of one substance, or of a variety of more and less nitro¬ 
genous bodies. The relation of nitrogen to carbon observed in the clay 
subsoils, and in the organic matter held in solution by the drainage waters 
from the experimental fields at Rothamsted, seems, however, to point to 
the formation of some highly nitrogenous organic matter capable cf 
diffusion into the subsoil. 
“ We may now pass to a further stage of the subject, and consider the 
agents by which the oxidation of organic matter in soil is effected. Our 
knowledge on this branch of the subject has certainly made great strides 
in recent years. At the time when Lieb’g’s writings directed so much 
attention to the subject of agriculture it was assumed that the oxidation 
of organic matter took place by mere contact with the oxygen of the air. 
The active oxidation taking place in soil was referred to the fact that soil 
is a porous substance ; it was assumed that the oxygen of the air became 
condensed within these pores, and was hence capable of exerting an 
increased power. We now know that the oxidation of organic matter 
generally requires something more than the presence of oxygen. Oxida¬ 
tion in nature is, in fact, nearly always performed by living agents, either 
by colourless plant cells, or by means of animal organisms. Ourv'ew of 
the nature of fertile soil has also enlarged, and instead of regarding it 
simply as a porous mass of clay, sand, and humus, we now look on it as 
a medium full of life. The soil beneath our feet is in fact not dead, but 
thickly peopled with a variety of organisms, with the particular functions 
of which we are only gradually becoming acquainted. As to whether 
any oxidisation takes place in soil without the intervention of life we 
can hardly perhaps state quite definitely at present, but it seems probable 
that this is the case. We cannot at present deny that some of the car- 
