It follows that the best life-conditions of these organisms (even when anerobic) cannot be fulfilled below a certain limited depth in the soil; and all observations show that their number decreases very rapidly with increasing depth ([see chap. 9, p. 142]), varying with the perviousness of the soil, but rarely exceeding four or five feet in the humid regions; though doubtless found at greater depths in the arid climates. It is also obvious that the use of any antiseptic or poisonous materials on the field or in the manure pile will tend to disturb and restrain the useful activity of these organisms.
Putrefactive Processes.—Carbonic gas is formed also, but to a much more limited extent, in putrefactive processes, occurring in the absence, or with only limited access, of air or oxygen. These processes likewise are conditioned upon the presence or activity of (largely anerobic) bacteria; but they should not occur in normally constituted, and especially in tilled soils, being as a rule inimical to the growth of cultivated plants ([see chap. 9, p. 145]).
CHAPTER XV.
THE COLORS OF SOILS.
The natural coloration of soils forms a prominent part of the characters upon which farmers are wont to base their judgment of land quality; hence the origin and value of soil-colors deserve consideration.
Black Soils.—From the oldest times down to the present a “rich, black soil” has commanded attention and approval. The black and brown-black colors being almost invariably due to the presence of much humus (very rarely to an admixture of carbon [graphite], of magnetic oxid of iron, or sesquioxid of manganese), it is obvious that the farmers’ judgment coincides with a high estimate of the agricultural value of humus. A discussion of this point will be found in another place; but the popular judgment is based quite as much upon the experience had in the advantages that usually accompany the presence of humus. It largely characterizes low grounds, and therefore alluvial lands, whose richness is due to far more general causes. But the shade of the blackness seen in the soil deserves and usually receives close consideration. If tending toward brown, acid humus or “sour” land is indicated; unless indeed the surface soil should be bodily derived from decayed wood, as in the primeval forests. Forest soils in general are usually dark-tinted for some inches near the surface, owing to the presence of leaf mold, and mostly have an acid reaction.
But the black tint is equally welcome to the land-seeker when seen outside of alluvial and forest areas. Belts of “black lands” appear on hillsides and plateaus; and these lands, though clearly not alluvial, are also found to be preëminently productive; witness the upland prairies of the western and southern United States. These black soils are always characterized by the presence of a full supply of lime in the form of carbonate, under the influence of which the most deeply black humus is formed. In other words, the jet black tint is indicative of calcareous lands; and these, as will be more fully shown below, are almost always highly productive.
From both points of view, then, the favorable judgment passed upon black soils by practical men is justified.
But it is not necessarily true that soils showing no obvious black tint are poor in humus; for in strongly ferruginous or “red” soils its tint is frequently wholly obscured, though when still visible it gives rise to the laudatory name of “mahogany land,” which every farmer considers a prize.
Of course then it would be wholly incorrect to judge of the agricultural value of land from its humus-content alone; for its color may be entirely imperceptible and yet its amount and nitrogen content be fully adequate to the requirements of thrifty vegetation. Gray and even whitish soils very frequently fall within this category in the arid region.
The black tint is also favorable to the absorption of the sun’s heat, and is therefore conducive to earlier maturity than is to be looked for in light-tinted lands similarly located.