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.
Wollny (Forsch. Agr. Phys. Vol. 12, 1889, p. 385), discusses the
influence of color on soils in relation to moisture and content of
carbonic acid. The results show in general simply the effects due
to increase of temperature when the soils are either darker-colored
throughout, or made so superficially.
_“Red” Soils._—Next to a black soil, a “red” one will usually command
the instinctive approval of farmers. The cause of this preference is
not as obvious as in the case of the black tints; but the general
consensus of opinion requires an examination of its claims. It is of
course easy enough to adduce examples of very poor “red” soils, derived
from ferruginous sandstones that supply little else than quartz and
ferric hydrate; the Cotton States supply cogent examples in point, as
do also the lower Foothills of the Sierra Nevada of California. It is
not, therefore, the iron rust or ferric hydrate that renders the land
productive; but its presence is a sign of some favorable conditions.
First among these is, that ferric hydrate cannot continue to exist in
badly drained soils; a “red” soil is therefore a well-drained one, and
this is probably one of the chief causes of the popular preference. The
“white land” sometimes seem in tracts otherwise colored with iron, is
distinctly inferior in production to the red lands; and examination
will generally show that from some cause, such white lands have been
subjected to the watery maceration which proves so injurious (see chap.
3, p. 46, chap. 12, p. 231).
That finely-diffused ferric hydrate has a very high power of absorbing
moisture as well as other gases of the atmosphere, has been shown in
the preceding chapter; it stands in this respect next to humus itself,
and hence highly ferruginous soils need not contain as much humus as
“white” soils from this point of view. Like humus, also, it renders
heavy clay soils more easily tillable.
_Origin of Red Tints._—Where crystalline rocks prevail, the red tint
usually indicates the derivation from the weathering of hornblende;
implying also, outside of the tropics, the presence of sufficient lime
in the land. Such lands are naturally preferred to those of lighter
tints derived from purely feldspathic rocks (see chap. 3, p. 32),
although they may be poorer in potash than the latter.
But the red tint has also its intrinsic advantages in the more ready
absorption of the sun’s heat by the colored than by a white surface.
This is probably the chief cause of the higher quality of wines
grown on red hillsides in the middle and northern vine districts of
Europe, where everything that aids earlier maturity is of the greatest
importance. The function of ferric oxid as a carrier of oxygen (chap.
4, p. 45) probably also aids nitrification.
“Yellow” lands owe their tint, of course, to smaller amounts of ferric
hydrate, but share more or less in the advantages of the “red.”
_White_ soils, or more properly those having very light gray tints, are
not usually looked upon with favor, especially in the humid region. The
causes of the unfavorable judgment current among farmers in respect to
white soils has already been partially explained in the discussion of
the black and red tints. The light color means the scarcity or absence
of both humus and ferric hydrate, and usually implies that the soil has
been subject to reductive maceration through the influence of stagnant
water; reducing the ferric hydrate to ferrous salts, oxidizing away
the humus, and accumulating in the form of inert concretions most or
all of the lime, iron and phosphoric acid of the soil mass (see chap.
3, p. 46, chap. 10, p. 184). The term “crawfishy,” so commonly applied
to white soils in the eastern United States, expresses well the usual
condition of the white soils of that region; which are very commonly
inhabited by crayfish, whose holes reach water a few feet below ground,
and are surrounded on the outside by piles of white subsoil mixed
with “black gravel” or concretions of bog iron ore. It is needless
to say why such lands cannot command the favorable consideration of
the farmer; they cannot as a rule be cultivated without previous
drainage, and even after that will usually prove unthrifty, “raw,” and
in immediate need of fertilization by green-manuring, and the use of
phosphates.
In the arid region, lands of this character are of rare occurrence,
while (as has been explained above, chap. 8, p. 135), the light gray
or “white” tints are there a very common characteristic of even the
very best soils. It is true that they are poor in humus and in finely
diffused ferric hydrate; but their “light” texture renders the presence
of humus for this purpose less needful, and as stated elsewhere (see
chap. 8, p. 135), the high nitrogen-content of arid humus renders
a smaller supply adequate for vegetative purposes. As to iron, its
presence being more important as a sign of good drainage and aeration
than directly, its absence from soils of great depth and loose texture
is of no consequence; especially when the heat-absorption which it
favors is not only not needed, but is usually already in undesirable
excess during the hot summers.
_White Alkali Spots._—In the valleys of the arid region, however,
_very_ white spots commonly indicate the prevalence of alkali salts,
and to that extent are an unfavorable indication; especially when
coupled with the occurrence of black rings or spots, which indicate the
presence of black alkali or carbonate of soda (see chap. 22).