38 
CHEMISTRY OF SOILS AND MANURES. 
the larger pieces of roots, stems, &c. The soluble organic matters remain in solution, the finer with 
the mud of No. 2, and the coarser with the sand and gravel. If we wish, however, to separate the 
organic matters completely from the other constituents, we heat a portion of the soil in an iron spoon 
or platinum dish, to redness. The organic matter will first blacken the soil, and disappear entirely 
when the heat is increased and continued for some time, leaving the incombustible matter behind, 
generally coloured red by oxide of iron, which is present more or less in every soil. The organic part 
of the soil, or that which burns away when exposed to heat, is likewise called the combustible part of 
the soil. We shall see presently how it is formed, what its composition and its functions in the soil 
are. But first we must examine more particularly the nature of the substances which we find in the 
three portions into which we have separated the soil by the simple process of washing as it is called. 
By this process we have obtained:—1. Stony fragments, sand and gravel, and coarser organic matters. 
2. An impalpable powder, and finer organic matters. 3. Soluble organic and inorganic matters. 
1. The sand, gravel, and larger stony fragments are easily separated from each other by a small 
sieve, after having been first dried. They result from the decomposition of the solid rocks, and as the 
variety of rocks is very great, we find the characters of the sand and gravel varying very much. For 
instance, granitic and some of the primitive rocks yield fragments composed of mica, felspar, quartz. 
Oolitic rocks, fragments of limestones, sandstones furnish grains of sand, &c. 
2. The impalpable powder, on examination, will be found to be a mixture of clay with finer frag¬ 
ments of the stones and gravel and organic matter. In it the chemist will distinguish alumina com¬ 
bined with silica, free alumina, free silica, more or less oxide of iron, oxide of manganese, lime, 
magnesia, potash, and soda, and traces of phosphoric, sulphuric, and carbonic acid. Of the above 
substances, silica, a substance called in common day life sand, preponderates in all soils. The presence 
of organic matter is recognised by the black colour the powder assumes when heated over a spirit-lamp. 
3. The watery solution of the soil, evaporated to dryness, leaves behind an inconsiderable residue, 
generally coloured brown by organic matters, which may be driven off by heat. In most cases a few 
grains only will remain, even if a large quantity of soil has been used. In the combustible or the 
organic portion of this residue the chemist can easily detect the presence of ammonia, of fiumic, ulmic, 
crenic, and apocrenic acids (substances known under the common name of soluble humus). In the 
incombustible portion he will generally find, by means of chemical tests, potash, soda, lime, magnesia, 
phosphoric acid, sulphuric acid, silicic acid, chlorine, and occasionally oxide of iron and manganese, 
nitric, iodine, and bromine. The two latter substances, iodine and bromine, are of rare occurrence, and 
only met with in soils inundated by the sea or by the waters of some salt springs. 
All cultivated soils present a great similarity of composition; they all contain the enumerated 
chemical constituents. This similarity becomes still more apparent, when we burn the soils, in which 
case they, with the exception of chalk soils, assume a red colour, which is due to the presence of oxide 
of iron. At first sight this might be regarded as opposed to the great diversity of soils; but if we 
examine the relative proportions in which the above-named substances are mixed, their state of com¬ 
bination, and the manner in which the different soils are formed, we shall find that diversity is perfectly 
compatible with a certain similarity of the elementary composition. The fact is, an analysis of a soil 
which represents nothing else but the percentage of silica, oxide of iron, alumina, potash, and other 
substances which enter into the composition of soils, is incapable of throwing light on its fertility, 
agricultural capabilities, and general nature. The state of combination in which the inorganic matters 
are found, their origin, physical characters, the nature of the organic matters and other circumstances, 
to which we shall afterwards refer, must be taken into account, before we can reasonably form an 
opinion as to the fertility of a soil. 
Having given the general composition of cultivated soils, we now proceed to examine the origin and 
chemical nature of the combustible part or the organic matters of the soil. Soils differ much in their 
character, agricultural capabilities, immediate origin, yet they all agree in containing organic matter in 
larger or smaller quantities. This organic matter appears essential to a healthy growth of plants; in no 
fertile soil it is wanting ; all cultivated soils contain an appreciable quantity, varying from ~ per cent, to 
10-12 per cent. Hich black garden mould often contains 20-24 per cent, of its own weight of organic 
matter, and in peaty or boggy soils the proportion frequently amounts to 60-70 per cent. 
In good garden land the organic matter amounts to 10-12 per cent, on an average ; the best soils of 
our fields seldom contain more than 6 per cent. Although a certain proportion of organic matter is always 
found in fertile soils, yet the relative larger or smaller amount must by no means be taken as a criterion 
of their capabilities, simply because there are other substances which have a direct influence on their 
fertility. Thus a soil containing 6 or 10 per cent, of organic matter may be much inferior to one 
which contains but 2 per cent. In soils celebrated as good wheat soils I found myself not more than 
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