The organic matter, as already stated, is derived from plants and therefore at the outset is composed of cellulose, lignin, protein, carbohydrates, waxes, oils and other plant constituents. In the soil it rapidly undergoes changes, being at tacked by the vast population of micro-organisms which use it for food and energy supply. The general direction of the change, therefore, is a decomposition to simple compounds such as CO2, water and ammonia, which is rapidly oxidised by specific micro organisms first to nitrite then to nitrate. The reverse changes, however, go on. The micro-organisms during their growth build up their own tissue, which, in the aggregate forms an appreciable part of the soil organic matter. Some of the organisms, e.g., the nitrifying organisms and the algae on the surface, assimilate
like plants, building it up in new organic matter ; others, like Clos tridiunt and Azotobacter may (though it is not known how far they actually do) assimilate gaseous nitrogen, converting it into pro tein; others again assimilate the decomposition products and re convert them into complex organic substances, while still others such as the active amoebae, feed on members of the soil popula tion. The net result is that the soil always contains the whole range of substances : the starting products mentioned alone : the simple end products CO2, nitrate and inorganic compounds of phosphorus, sulphur, potassium, etc. ; and a number of intermediate products in constant state of change. Among these the most characteristic are the black, sticky compounds collectively known as humus, which like clay, are colloidal, and therefore have great power of absorbing water and soluble substances but differ from clay in that they do not make the soil sticky but on the contrary render it more pliable. It is a complex mixture which has been resolved into the following groups:— Most investigators have confined their studies to humic acid although there is no evidence that it is any more important than the humin. Humic acid can be (and in the soil apparently is) formed in three different ways (r) from cellulose by micro organisms (2) from lignin by chemical changes (3) from the my celium of fungi : there are differences between all three, but all agree in general physical properties and also in their general effects in the soil. Humic acid from soil contains about 5% of nitrogen apparently as part of its molecule : it is therefore not entirely a cellulose or lignin condensation product. Oden regards
it as a tetrabasic acid with an equivalent weight of about 300.
These two substances are of vital importance to the plant. Calcium carbonate is derived from three sources (I) from the decomposition of plant residues e.g., from calcium oxalate (2) from the weathering of rocks (3) from the calcareous deposits forming the chalk and limestone. As it slowly dissolves from the soil water it tends to wash out from the surface soil in humid and semi-arid regions and is either carried away in the drainage water, or if the rainfall be insufficient, is deposited lower down. Its valuable property in the soil is that it keeps the calcium clay stable, neutralises the humic acid, and maintains both in a flocculated physical state suitable for plant growth. Calcium phosphate is derived from fish and other marine animals, which are sometimes segregated as vast deposits many feet in thickness, as in North Africa and parts of the United States, sometimes disseminated through the mass of the formation as in the chalk. Some also occur as crystalline apa tite. In the soil it is probably in the form of hydroxyapatite
but some of the phosphorus occurs in organic combination.
Soil usually contains some 8 to 15 per cent of its weight of water distributed over its particles and this contains some of all the soluble substances present : the amounts being much influenced by the changes effected with the clay and by the absorption by the colloids. In general it contains in agricultural soils about o•05 to 0.2 per cent of total solid mat ter. It appears to contain all the chloride and nitrate present in the soil, along with the equivalent amount of calcium. It seems to be saturated with phosphate (about i to 3 parts per million of
being present whatever the moisture content of the soil). The concentration of potassium, however, increases as the moisture content of the soil decreases, but not proportionately like the chlo ride and the nitrate : this suggests an equilibrium between the potassium in the liquid and that in the solid phase. The chief constituents of the soil solution are calcium nitrate, bicarbonate and sulphate; its composition, however, varies with the growth of the crop, the season and the activity of the micro-organisms. Its agricultural and ecological significance is that it is the culture solution for the growth of plants. (E. J. R.)