SOILS
The soil considered agriculturally, is that part of the earth's crust
which is occupied by the roots of plants and from which they absorb
food and moisture.
RELATION OF SOIL TO PLANTS
We have learned that plant roots penetrate the soil to hold the plant
in a firm and stable position, to absorb moisture and with it plant
food. We learned also that for roots to do these things well, the soil
in which they grow must be mellow and firm, and must contain moisture
and plant food, air must circulate in its pores and it must be warm.
How can we bring about these conditions? To answer this question
intelligently it will be necessary for us to study the soil to find
out something about its structure, its composition, its
characteristics; also, how it was made and what forces or agencies
were active in making it. Are these forces acting on the soil at the
present time? Do they have any influence over the conditions which are
favorable or unfavorable to plant growth? If so, can we control them
in their action for the benefit or injury of plants?
We will begin this soil study with an excursion and a few experiments.
Go to the field. Examine the soil in the holes dug for the root
lessons, noticing the difference between the upper or surface soil and
the under or subsoil. Examine as many kinds of surface soils and
subsoils as possible, also decayed leaf mould, the black soil of the
woods, etc. If there are in the neighborhood any exposed embankments
where a road has been cut through a hill, or where a river or the sea
water has cut into a bank of soil, visit them and examine the exposed
soils.
=Experiment.=--Place in separate pans, dishes, plates, boxes, or on
boards, one or two pints each of sand, clay, decayed vegetable matter
or leaf mould or woods soil, and garden soil. The soil should be fresh
from the field. Examine the sand, clay and leaf mould, comparing them
as to color; are they light or dark, are they moist or not? Test the
soils for comparative size of particles by rubbing between the fingers
(Fig. 19), noticing if they are coarse or fine, and for stickiness by
squeezing in the hand and noting whether or not they easily crumble
afterwards.
=Experiment.=--Take samples, about a teaspoonful, of sand, clay and
leaf mould. Dry them and then place each in an iron spoon or on a
small coal shovel and heat in stove to redness. It will be found that
the leaf mould will smoke and burn, and will diminish in amount,
while the sand and clay will not.
=Experiment.=--Take two wide-mouthed bottles; fill both nearly full of
water. Into one put about a teaspoonful of clay and into the other the
same amount of sand; shake both bottles thoroughly and set on table to
settle (Fig. 20). It will be found that the sand settles very quickly
and the clay very slowly.
As the result of our three experiments we will find something as
follows:
Sand is light in color, moist, coarse, not sticky, settles quickly in
water, and will not burn.
Clay is darker in color, moist, very fine, quite sticky, settles
slowly in water, and will not burn.
Leaf mould or humus is very dark in color, moist, very fine, slightly
sticky, and burns when placed in the fire.
=Experiment.=--We now have knowledge and means for making simple tests
of soils. Repeat the last three experiments with the garden soil. We
will find, perhaps, that it is dark in color and some of it burns away
when placed in the fire, therefore it contains organic matter or
decaying vegetable matter or humus, as it is called. This sample has
perhaps fine particles and coarse particles; part of it will settle
quickly in water while part settles very slowly, and it is sticky.
Therefore we conclude that there are both clay and sand in it. If we
shake a sample of it in a bottle of water and let it settle for
several days, we can tell roughly from the layers of soil in the
bottom of the bottle the relative amounts of sand and clay in the
soil. Also if we weigh a sample before and after burning we can tell
roughly the amount of organic matter in the soil. Test a number of
soils and determine roughly the proportions of sand, clay and organic
matter in them.
=Experiment.=--Take the pans of soil used in our first soil experiment
and separate the soils in the pans into two parts by a trench across
the centre on the pan. Now wet the soil in one side of the pan and
stir it with a stick or a spoon, carefully smooth the surface of the
soil in the other side of the pan and pour or sprinkle some water on
it, but do not stir it. Set the pans aside till the soils are dry.
This drying may take several days and in the meantime we will study
the classification of soils.
[Illustration: FIG. 16.
To show that roots need air. Bottle _A_ was supplied with fresh water,
and bottle _B_ with water that had been boiled to drive the air out
and then cooled.]
[Illustration: FIG. 17.
Bottle _A_ contains fresh water, bottle _B_ contains boiled water.
Notice the air bubbles in bottle _A._]
[Illustration: FIG. 18.
Tumblers _A_ and _C_ contained moist sand, _B_ and _D_ contained
puddled clay. Cuttings in _B_ and _D_ died, because there was not
sufficient ventilation in the clay for root-development.]
CLASSIFICATION OF SOILS
Soil materials and soils are classified as follows:
_Stones._--Coarse, irregular or rounded rock fragments or pieces of
rock.
_Gravel._--Coarse fragments and pebbles ranging in size from several
inches in diameter down to 1/25 inch.
_Sand._--Soil particles ranging from 1/25 of an inch down to 1/500 of
an inch in diameter. Sand is divided into several grades or sizes.
Coarse sand 1/25 to 1/50 of an inch.
Medium sand 1/50 to 1/100 of an inch.
Fine sand 1/100 to 1/250 of an inch.
Very fine sand 1/250 to 1/500 of an inch.
These grades of sand correspond very nearly with the grains of
granulated and soft sugar and fine table salt.
_Silt._--Fine soil particles ranging from 1/500 to 1/5000 of an inch
in diameter. It feels very fine and smooth when rubbed between the
fingers, especially when moist. A good illustration of silt is the
silicon used for cleaning knives, a small amount of which can be
obtained at most any grocery store. By rubbing some of this between
the fingers, both dry and wet, one can get a fair idea of how a silty
soil should feel. Silt when wet is sticky like clay.
_Clay._--The finest of rock particles, 1/5000 to 1/250000 of an inch
in diameter, too small to imagine. Clay when wet is very soft,
slippery and very sticky. Yellow ochre and whiting from the paint shop
are good illustrations of clay.
_Humus_, or decaying vegetable and animal matter. This is dark brown
or almost black in color--decaying leaves and woods soil are examples.
Soils composed of the above materials:
_Sands or Sandy Soils._--These soils are mixtures of the different
grades of sand and small amounts of silt, clay and organic matter.
They are light, loose and easy to work. They produce early crops, and
are particularly adapted to early truck, fruit and bright tobacco, but
are too light for general farm crops. To this class belongs the
so-called Norfolk Sand. This is a coarse to medium, yellow or brown
sand averaging about five-sixths sand and one-sixth silt and clay and
is a typical early truck soil found all along the eastern coast of the
United States.
"It is a mealy, porous, warm sand, well drained and easily cultivated.
In regions where trucking forms an important part of agriculture, this
soil is sought out as best adapted to the production of watermelons,
canteloupes, sweet potatoes, early Irish potatoes, strawberries, early
tomatoes, early peas, peppers, egg plant, rhubarb and even cabbage and
cauliflower, though the latter crops produce better yields on a
heavier soil."
A very similar sand in the central part of the country is called Miami
Sand and, on the Pacific Coast, Fresno Sand. These names are given to
these type soils by the Bureau of Soils of the United States
Department of Agriculture.
_Loams or Loamy Soils_, consist of mixtures of the sands, silt and
clay with some organic matter. The term loam is applied to a soil
which, from its appearance in the field and the feeling when handled,
appears to be about one-half sand and the other half silt and clay
with more or less organic matter. These are naturally fine in texture
and quite sticky when wet. They would be called clay by many on
account of their stickiness. They are good soils for general farming
and produce good grain, grass, corn, potatoes, cotton, vegetables,
etc.
_Sandy Loams_, averaging about three-fifths sand and two-fifths silt
and clay. These soils are tilled easily and are the lightest desirable
soil for general farming. They are particularly adapted to corn and
cotton and in some instances are used for small fruits and truck
crops.
_Silt Loam_ consists largely of silt with a small amount of sand,
clay, and organic matter. These soils are some of the most difficult
to till, but when well drained they are with careful management good
general farming soils, producing good corn, wheat, oats, potatoes,
alfalfa and fair cotton.
_Clay Loams._--These soils contain more clay than the silt loams. They
are stiff, sticky soils, and some of them are difficult to till. They
are generally considered the strongest soils for general farming. They
are particularly adapted to wheat, hay, corn and grass.
_Gravelly loams_ are from one-fourth to two-thirds coarse grained; the
remaining fine soil may be sandy loam, silt or clay loam. They are
adapted to various crops according to the character of the fine soil.
Some of them are best planted to fruit and forest.
_Stony Loam._--Like the gravelly loam the stony loams are one-fourth
to three-fourths sandy, silty or clay loam, the remainder being rock
fragments of larger size than the gravel. These fragments are
sometimes rough and irregular and sometimes rounded. The stones
interfere seriously with tillage, and naturally the soils are best
planted with forest or fruit.
_Clay Soils._--Clay soils are mixtures of sand, silt, clay and humus,
the clay existing in quite large quantities, there being a greater
preponderance of the clay characteristics than in the clay loams; they
are very heavy, sticky, and difficult to manage. Some clay soils are
not worth farming. Those that can be profitably tilled are adapted to
wheat, corn, hay and pasture.
_Adobe Soils._--These are peculiar soils of the dry West. They are
mixtures of clay, silt, some sand and large amounts of humus. Their
peculiar characteristic is that they are very sticky when wet and bake
very hard when dry and are, therefore, very difficult to manage,
though they are generally very productive when they are moist enough
to support crops.
_Swamp Muck_ is a dark brown or black swamp soil consisting of large
amounts of humus or decaying organic matter mixed with some fine sand
and clay. It is found in low wet places.
_Peat_ is also largely vegetable matter, consisting of tough roots,
partially decayed leaves, moss, etc. It is quite dense and compact and
in some regions is used for fuel.
HOW WERE SOILS MADE?
As a help in finding the answer to this question collect and examine a
number of the following or similar specimens:
_Brick._--Take pieces of brick and rub them together. A fine powder or
dust will be the result.
_Stones._--Rub together pieces of stone; the same result will follow,
except that the dust will be finer and will be produced with greater
difficulty because the stones are harder. Some stones will be found
which will grind others without being much affected themselves.
_Rock Salt or Cattle Salt._--This is a soft rock, easily broken. Place
on a slate or platter one or two pieces about the size of an egg or
the size of your fist. Slowly drop water on them till it runs down and
partly covers the slate, then set away till the water dries up. Fine
particles of salt will be found on the slate wherever the water ran
and dried. This is because the water dissolved some of the rock.
_Lime Stone._--This is harder. Crush two samples to a fine powder and
place one in water and the other in vinegar. Water has apparently no
effect on it, but small bubbles are seen to rise from the sample in
vinegar. The vinegar which is a weak acid is slowly dissolving the
rock. The chemists tell us water will also dissolve the limestone, but
very slowly. There are large areas of soil which are the refuse from
the dissolving of great masses of limestone.
We find that the rocks about us differ in hardness: they are ground to
powder when rubbed together, some are easily dissolved in water,
others are dissolved by weak acids.
Geologists tell us that the whole crust of the earth was at one time
made up of rocks, part of which have been broken down into coarse and
fine particles which form the gravel, sand and clay of our soils. The
organic matter of our soils has been added by the decay of plants and
animals. Several agencies have been active in this work of breaking
down the rocks and making soils of them. If we look about we can
perhaps see some of this work going on now.
_Work of the Sun._--Examine a crockery plate or dish that has been
many times in and out of a hot oven, noticing the little cracks all
over its surface. Most substances expand when they are heated and
contract when they are cooled. When the plate is placed in the oven
the surface heats faster than the inner parts, and cools faster when
taken out of the oven. The result is that there is unequal expansion
and contraction in the plate and consequently tension or pulling of
its parts against each other. The weaker part gives way and a crack
appears. If hot water is put into a thick glass tumbler or bottle, the
inner surface heats and expands faster than the outer parts and the
result is tension and cracking. If cold water be poured on a warm
bottle or piece of warm glass, it cracks, because there is unequal
contraction. In the early part of a bright sunny afternoon feel of the
surface of exposed rocks, bricks, boards, or buildings on which the
sun has been shining. Examine them in the same way early the next
morning. You will find that the rocks are heated by the sun just as
the plate was heated when put into the oven, and when the sun goes
down the rocks cool again. This causes tension in the rocks and
little cracks and checks appear in them just as in the heated plate,
only more slowly. This checking may also be brought about by a cool
shower falling on the sun heated rocks just as the cool water cracked
the warm glass. Many rocks if examined closely will be found to be
composed of several materials. These materials do not expand and
contract alike when heated and cooled and the tendency for them to
check is greater even than that of the plate. This is the case with
most rocks.
[Illustration: FIG. 19.--COMPARING SOILS.]
[Illustration: FIG. 20.--WATER TEST OF SOILS.
Bottle _A_ contains sand and water, bottle _B_ clay and water. The
sand settles quickly, the clay very slowly.]
_Work of Rain._--Rain falling on the rocks may dissolve a part of them
just as it dissolved the rock salt; or, working into the small cracks
made by the sun, may wash out loosened particles; or, during cold
weather it may freeze in the cracks and by its expansion chip off
small pieces; or, getting into large cracks and freezing, may split
the rock just as freezing water splits a water pitcher or the water
pipes.
_Work of Moving Water._--Visit some neighboring beach or the banks of
some rapid stream. See how the waves are rolling the sand and pebbles
up and down the beach, grinding them together, rounding their corners
and edges, throwing them up into sand beds, and carrying off the finer
particles to deposit elsewhere. Now visit a quiet cove or inlet and
see how the quiet water is laying down the fine particles, making a
clay bed. Notice also how the water plants along the border are
helping. They act as an immense strainer, collecting the suspended
particles from the water, and with them and their bodies building beds
of soil rich in organic matter or humus.
The sun, besides expanding and cracking the rocks by its heat, helps
in another way to make soils. It warms the water that has been
grinding soil on the beach or along the river banks and causes some of
it to evaporate. This vapor rises, forms a cloud and floats away in
the air. By and by the vapor forms into rain drops which may fall on
the top of some mountain. These rain drops may wash loosened particles
from the surface or crevices of exposed rocks. These drops are joined
by others until, by and by, they form a little stream which carries
its small burden of rock dust down the slope, now dropping some
particles, now taking up others. Other little streams join this one
until they form a brook which increases in size and power as it
descends the mountain side. As it grows by the addition of other
streams it picks up larger pieces, grinds them together, grinds at its
banks and loads itself with rocks, pebbles, sand and clay. As the
stream reaches the lower part of the mountain where the slope is less
steep, it is checked in its course and the larger stones and pebbles
are dropped while the sand and finer particles are carried on and
deposited on the bottom of some broad quiet river farther down, and
when the river overflows its banks, are distributed over the
neighboring meadows, giving them a new coating of soil and often
adding to their fertility. What a river does not leave along its
course it carries out to sea to help build the sand bars and mud flats
there. The rain drops have now gotten back to the beach where they
take up again the work of grinding the soil.
The work of moving water can be seen in almost any road or cultivated
field during or just after a rain, and particularly on the hillsides,
where often the soil is loosened and carried from higher to lower
parts, making barren sand and clay banks of fertile hillsides and
destroying the fertility of the bottom lands below.
We have already noticed the work of freezing water in splitting small
and large fragments from the rocks. Water moving over the surface of
the earth in a solid form, or ice, was at an earlier period in the
history of the earth one of the most powerful agencies in soil
formation. Away up in Greenland and on the northern border of this
continent the temperature is so low that most if not all of the
moisture that falls on the earth falls as snow. This snow has piled up
until it has become very deep and very heavy. The great weight has
packed the bottom of this great snow bank to ice. On the mountains
where the land was not level the masses of snow and ice, centuries
ago, began to slide down the slopes and finally formed great rivers of
solid water or moving ice.
The geologists tell us that at one time a great river of ice extended
from the Arctic region as far south as central Pennsylvania and from
New England to the Rocky Mountains. This vast river was very deep and
very heavy and into its under surface were frozen sand, pebbles,
larger stones and even great rocks. Thus it acted as a great rasp or
file and did an immense amount of work grinding rocks and making
soils. It ground down mountains and carried great beds of soil from
one place to another. When this great ice river melted, it dropped its
load of rocks and soils, and as a result we find in that region of the
country great boulders and beds of sand and clay scattered over the
land.
_Work of the Air._--The air has helped in the work of wearing down the
rocks and making soils. If a piece of iron be exposed to moist air a
part of the air unites with part of the iron and forms iron rust. In
the same way when moist air comes in contact with some rocks part of
the air unites with part of the rock and forms rock rust which
crumbles off or is washed away by water. Thus the air helps to break
down the rocks. Moving air or wind picks up dust particles and carries
them from one field to another. On sandy beaches the wind often blows
the sand along like snow and piles it into drifts. The entire surface
of sandy regions is sometimes changed in this way. Sands blown from
deserts sometimes bury forests which with their foliage sift the fatal
winding sheet from the dust-laden winds.
_The Work of Plants._--Living plants sometimes send their roots into
rock crevices; there they grow, expand, and split off rock fragments.
Certain kinds of plants live on the surface of rocks. They feed on
the rocks and when they die and decay they keep the surface of the
rocks moist and also produce carbonic acid which dissolves the rocks
slowly just as the vinegar dissolved the limestone in our experiment.
Dead decaying roots, stems, and leaves of plants form largely the
organic matter of the soil. When organic matter has undergone a
certain amount of decay it is called humus, and these soils are called
organic soils or humus soils. The black soils of the woods, swamps and
prairies, contain large amounts of humus.
_Work of Animals._--Earth worms and the larvæ of insects which burrow
in the soil eat soil particles which pass through their bodies and are
partially dissolved. These particles are generally cast out on the
surface of the soil. Thus these little animals help to move soil, to
dissolve soil, and to open up passages for the entrance of air and
rain.
SOIL TEXTURE
We have seen that the soil particles vary in size and that for the
best development of the plant the particles of the soil must be so
arranged that the delicate rootlets can readily push their way about
in search of food, or, in other words, that the soil must have a
certain texture. By the texture of the soil we mean the size of its
particles and their relation to each other. The following terms are
used in describing soil textures: Coarse, fine, open, close, loose,
hard, stiff, compact, soft, mellow, porous, leachy, retentive, cloddy,
lumpy, light, heavy. Which of these terms will apply to the texture of
sand, which to clay, which to humus, which to the garden soil, which
to a soil that plant roots can easily penetrate? We find then that
texture of the soil depends largely on the relative amounts of sand,
silt, clay and humus that it contains.