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.ToList
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.
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.ToList
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.
CHAPTER IVToC
Relation of Soils to Water
IMPORTANCE OF WATER TO PLANTS
We learned in a previous paragraph that plant roots take moisture from the soil. What becomes of this moisture? We will answer this question with an experiment.
Experiment.—Take a pot or tumbler in which a young plant is growing, also a piece of pasteboard large enough to cover the top of the pot or tumbler; cut a slit from the edge to the centre of the board, then place it on top of the pot, letting the stem of the plant enter the slit. Now close the slit with wax or tallow, making it perfectly tight about the stem. If the plant is not too large invert a tumbler over it (Fig. 21), letting the edge of the tumbler rest on the pasteboard; if a tumbler is not large enough use a glass jar. Place in a sunny window. Moisture will be seen collecting on the inner surface of the glass. Where does this come from? It is absorbed from the soil by the roots of the plant and is sent with its load of dissolved plant food up through the stem to the leaves. There most of the moisture is passed from the leaves to the air and some of it is condensed on the side of the glass.
By experiments at the Cornell University Agricultural Experiment Station, Ithaca, N.Y., it has been found that during the growth of a sixty bushel crop of corn the plants pump from the soil by means of their roots, and send into the air through their leaves over nine hundred tons of water. A twenty-five bushel crop of wheat uses over five hundred tons of water in the same way. This gives us some idea of the importance of water to the plant and the necessity of knowing something of the power of the soil to absorb and hold moisture for the use of the plant. Also the importance of knowing if we can in any way control or influence the water-holding power of the soil for the good of the plant.
SOURCES OF SOIL WATER
From what sources does the soil receive water? From the air above, in the form of rain, dew, hail and snow, falling on the surface, and from the lower soil. This water enters the soil more or less rapidly.
ATTITUDE OF THE SOILS TOWARDS WATER
Which soils have the greater power to take in the rain which falls on their surface?
FIG. 21.
To show what becomes of the water taken from the soil by roots.
Moisture, sent up from the roots, has been given off by the leaves and
has condensed on the glass.ToList
FIG. 22.—PERCOLATION EXPERIMENT.
To show the relative powers of soils to take in water falling on the
surface. A, sand; B, clay; C, humus; D, garden soil.ToList
Experiment.—Take four student-lamp chimneys. (In case the chimneys cannot be found get some slender bottles like salad oil bottles or wine bottles and cut the bottoms off with a hot rod. While the rod is heating make a shallow notch in the glass with the wet corner of a file in the direction you wish to make the cut. When the rod is hot lay the end of it lengthwise on the notch. Very soon a little crack will be seen to start from the notch. Lead this crack around the bottle with the hot rod and the bottom of the bottle will drop off.) (Fig. 23.) Make a rack to hold them. Tie a piece of cheese cloth or other thin cloth over the small ends of the chimneys. Then fill them nearly full respectively, of dry, sifted, coarse sand, clay, humus soil, and garden soil. Place them in the rack; place under them a pan or dish. Pour water in the upper ends of the tubes until it soaks through and drips from the lower end (Fig. 22). Ordinary sunburner lamp chimneys may be used for the experiment by tying the cloth over the tops; then invert them, fill them with soil and set in plates or pans. The sand will take the water in and let it run through quickly; the clay is very slow to take it in and let it run through; the humus soil takes the water in quite readily. Repeat the experiment with one of the soils, packing the soil tightly in one tube and leaving it loose in another. The water will be found to penetrate the loose soil more rapidly than the packed soil. We see then that the power of the soil to take in rainfall depends on its texture or the size and compactness of the particles.
If the soil of our farm is largely clay, what happens to the rain that falls on it? The clay takes the water in so slowly that most of it runs off and is lost. Very likely it carries with it some of the surface soil which it has soaked and loosened, and thus leaves the farm washed and gullied.
What can we do for our clay soils to help them to absorb the rain more rapidly? For immediate results we can plow them and keep them loose and open with the tillage tools. For more permanent results we may mix sand with them, but sand is not always to be obtained and is expensive to haul. The best method is to mix organic matter with them by plowing in stable manures, or woods soil, or decayed leaves, or by growing crops and turning them under. The organic matter not only loosens the soil but also adds plant food to it, and during its decay produces carbonic acid which helps to dissolve the mineral matter and make available the plant food that is in it.
Clay soils can also be made loose and open by applying lime to them.
Experiment.—Take two bottles or jars, put therein a few spoonsful of clay soil, fill with water, put a little lime in one of them, shake both and set them on the table. It will be noticed that the clay in the bottle containing lime settles in flakes or crumbs, and much faster than in the other bottle. In the same manner, lime applied to a field of clay has a tendency to collect the very fine particles of soil into flakes or crumbs and give it somewhat the open texture of a sandy soil. Lime is applied to soil for this purpose at the rate of twenty bushels per acre once in four or five years.
Which soils have the greater power to absorb or pump moisture from below?
Experiment.—Use the same or a similar set of tubes as in the experiment illustrated in Fig. 23. Fill the tubes with the same kinds of dry sifted soils. Then pour water into the pan or dish beneath the tubes until it rises a quarter of an inch above the lower end of the tubes (Fig. 24). Watch the water rise in the soils. The water will be found to rise rapidly in the sand about two or three inches and then stop or continue very slowly a short distance further. In the clay it starts very slowly, but after several hours is finally carried to the top of the soil. The organic matter takes it up less rapidly than the sand, faster than the clay, and finally carries it to the top. By this and further experiments it will be found that the power of soils to take moisture from below depends on their texture or the size and closeness of their particles.
We found the sand pumped the water only a short distance and then stopped.
What can we do for our sandy soils to give them greater power to take moisture from below? For immediate results we can compact them by rolling or packing. This brings the particles closer together, makes the spaces between them smaller, and therefore allows the water to climb higher. For more lasting results we can fill them with organic matter in the shape of stable manures or crops turned under. Clay may be used, but is expensive to haul.
Which soils have greatest power to hold the water which enters them?
Experiment.—Use the same or similar apparatus as for the last experiment. After placing the cloth caps over the ends of the tubes label and carefully weigh each one, keeping a record of each; then fill them with the dry soils and weigh again. Now place the tubes in the rack and pour water in the upper ends until the entire soil is wet; cover the tops and allow the surplus water to drain out; when the dripping stops, weigh the tubes again, and by subtraction find the amount of water held by the soil in each tube; compute the percentage. It will be found that the organic matter will hold a much larger percentage of water than the other soils; and the clay more than the sand. The tube of organic soil will actually hold a larger amount of water than the other tubes. (See also Fig. 25.)
In the experiment on page 40 we noticed that the sand took in the water poured on its surface and let it run through very quickly. This is a fault of sandy soils.
What can we do for our sandy soils to help them to hold better the moisture which falls on them and tends to leach through them? For immediate effect we can close the pores somewhat by compacting the soil with the roller. For more lasting effects, we can fill them with organic matter.
Which soils will hold longest the water which they have absorbed? Or which soils will keep moist longest in dry weather?
Experiment.—Fill a pan or bucket with moist sand, another with moist clay, and a third with moist organic matter; set them in the sun to dry and notice which dries last. The organic matter will be found to hold moisture much longer than the other soils. The power of the other soils to hold moisture through dry weather can be improved by mixing organic matter with them.
We find then that the power of soils to absorb and hold moisture depends on the amount of sand, clay, or humus which they contain, and the compactness of the particles. We see also how useful organic matter is in improving sandy and clayey soils.
THE EFFECT OF WORKING SOILS WHEN WET
By this time the soils we left in the pans (see page 26), sand, clay, humus and garden soil, must be dry. If so, examine them. We find that the clay which was stirred when wet has dried into an almost bricklike mass, while that which was not stirred is not so hard, though it has a thick, hard crust. The sand is not much affected by stirring when wet. The organic matter which was stirred when wet has perhaps stiffened a little, but very easily crumbles; the unstirred part was not much affected by the wetting and drying.
The garden soil after drying is not as stiff as the clay nor as loose as the sand and humus. This is because it is very likely a mixture of all three, the sand and the humus checking the baking. This teaches us that it is not a good plan to work soils when they are wet if they are stiff and sticky; and that our stiff clay soils can be kept from drying hard or baking by the use of organic matter. "And that's a witness" for organic matter.
The relation of the soil to moisture is very important, for moisture is one of the greatest factors if not the greatest in the growth of the crop.
The power to absorb or soak up moisture from any source is greatest in those soils whose particles are smaller and fit closer together.
It is for this reason that strong loams and clay soils absorb and hold three times as much water as sandy soils do, while peaty or humus soils absorb a still larger proportion.
The reason why crops burn up so quickly on sandy soils during dry seasons is because of their weak power to hold water.
The clay and humus soils carry crops through dry weather better because of their power to hold moisture and to absorb or soak up moisture from below. It is for this reason also that clay and peaty soils more often need draining than sandy soils.
When rain falls on a sandy soil it enters readily, but it is apt to pass rapidly down and be, to a great extent, lost in the subsoil, for the sand has not sufficient power to hold much of it.
When rain falls on a clay soil it enters less readily because of the closeness of the particles, and during long rains or heavy showers some of the water may run off the surface. If the surface has been recently broken and softened with the plow or cultivator the rain enters more readily. What does enter is held and is not allowed to run through as in the case of the sand.
Humus soil absorbs the rain as readily as the sand and holds it with a firmer grip than clay.
This fact gives us a hint as to how we may improve the sand and clay.
Organic matter mixed with these soils by applying manures or plowing under green crops will cause the sand to hold the rain better and the clay to absorb it more readily.
CHAPTER VToC
Forms of Soil Water
Water which comes to the soil and is absorbed exists in the soil principally in two forms: Free water and capillary water.
FREE WATER
Free water is that form of water which fills our wells, is found in the bottoms of holes dug in the ground during wet seasons and is often found standing on the surface of the soil after heavy or long continued rains. It is sometimes called ground water or standing water and flows under the influence of gravity.
Is free water good for the roots of farm plants? If we remember how the root takes its food and moisture, namely through the delicate root hairs; and also remember the experiment which showed us that roots need air, we can readily see that free water would give the root hairs enough moisture, but it would at the same time drown them by cutting off the air. Therefore free water is not directly useful to the roots of house plants or farm plants, excepting such as are naturally swamp plants, like rice, which grows part of the time with its roots covered with free water.
FIG. 26.—CAPILLARY TUBES.
To show how water rises in small tubes or is drawn into small spaces.ToList
FIG. 27.—CAPILLARY PLATES
Water is drawn to the highest point where the glass plates are closest
together.ToList
FIG. 28.
A cone of soil to show capillarity. Water poured about the base of
this cone of soil has been drawn by capillary force half-way to the
top.ToList
FIG. 29.
To show the relative amounts of film-moisture held by coarse and fine
soils. The colored water in bottle A represents the amount of water
required to cover the half pound of pebbles in the tumbler B with a
film of moisture. The colored water in bottle C shows the amount
required to cover the soil grains in the half pound of sand in tumbler
D.ToList
CAPILLARY WATER
If you will take a number of glass tubes of different sizes, the largest not more than one-fourth of an inch in diameter, and hold them with one end of each in water or some colored liquid, you will notice that the water rises in the tubes (Fig. 26), and that it rises highest in the smallest tube. The force which causes the water to rise in these tubes is called the capillary force, from the old Latin word capillum (a hair), because it is most marked in hair-like tubes, the smaller the tube the higher the water will rise. The water which rises in the tubes is called capillary water.
Another method of illustrating capillary water is to tie or hold together two flat pieces of glass, keeping two of the edges close together and separating the opposite two about one-eighth of an inch with a sliver of wood. Then set them in a plate of water or colored liquid and notice how the water rises between the pieces of glass, rising higher the smaller the space (Fig. 27). It is the capillary force which causes water to rise in a piece of cloth or paper dipped in water.
Take a plate and pour onto it a cone-shaped pile of dry sand or fine soil; then pour water around the base of the pile and note how the water is drawn up into the soil by capillary force (Fig. 28).
Capillary water is the other important form of water in the soil. This is moisture which is drawn by capillary force or soaks into the spaces between the soil particles and covers each particle with a thin film of moisture.
FILM WATER
Take a marble or a pebble, dip it into water and notice the thin layer or film of water that clings to it. This is a form of capillary water and is sometimes called film water or film moisture. Take a handful of soil that is moist but not wet, notice that it does not wet the hand, and yet there is moisture all through it; each particle is covered with a very thin film of water.
Now this film water is just the form of water that can supply the very slender root hairs without drowning them, that is, without keeping the air from them. And the plant grower should see to it that the roots of his plants are well supplied with film water and are not drowned by the presence of free water. Capillary water may sometimes completely fill the spaces between the soil particles; when this occurs the roots are drowned just as in the case of free water as we saw when cuttings were placed in the puddled clay (see Fig. 18). Free water is indirectly of use to the plant because it serves as a supply for capillary and film moisture.
Now I think we can answer the question which was asked when we were studying the habit of growth of roots but was left unanswered at the time (see page 14). The question was this: Of what value is it to the farmer to know that roots enter the soil to a depth of three to six feet? We know that roots will not grow without air. We also know that if the soil is full of free water there is no air in it, and, therefore, roots of most plants will not grow in it. It is, therefore, of interest to the farmer to see that free water does not come within at least three or four feet of the surface of the soil so that the roots of his crops may have plenty of well ventilated soil in which to develop. If there is a tendency for free water to fill the soil a large part of the time, the farmer can get rid of it by draining the land. We get here a lesson for the grower of house plants also. It is that we must be careful that the soil in the pots or boxes in which our plants are growing is always supplied with film water and not wet and soggy with free water. Water should not be left standing long in the saucer under the pot of a growing plant. It is best to water the pot from the top and let the surplus water drain into the saucer and then empty it out.
Which soils have the greatest capacity for film water?
Experiment.—Place in a tumbler or bottle one-half pound of pebbles about the size of a pea or bean; pour a few drops of water on them and shake them; continue adding water and shaking them till every pebble is covered with a film of water; let any surplus water drain off. Then weigh again; the difference in the two weights will be approximately the weight of the film water that the pebbles can carry. Repeat this with sand and compare the two amounts of water. A striking illustration can be made by taking two slender bottles and placing in them amounts of colored water equal to the amounts of film water held by the pebbles and sand respectively. In the accompanying illustration (Fig. 29), A represents the amount of water that was found necessary to cover the pebbles in tumbler B with a film of moisture. C is the amount that was necessary to cover with a film the particles of sand in D. The finer soil has the greater area for film moisture. It has been estimated that the particles of a cubic foot of clay loam have a possible aggregate film surface of three-fourths of an acre.
CHAPTER VIToC
Loss of Soil Water
LOSS OF SOIL WATER AND MEANS OF CHECKING THE LOSS
We noticed in previous paragraphs that soil might at times have too much water in it for proper ventilation and so check the growth of the roots of the plant. Now is it possible that soil water may be lost or wasted and if so can we check the loss?
In the experiment to find out how well the soils would take in the rainfall (page 40) we noticed that the clay soil took in the water very slowly and that on a field of clay soil part of the rain water would be likely to run off over the surface and be lost. Free water may be lost then, by surface wash.
We noticed methods of checking this loss, namely, pulverizing the soil with the tillage tools and putting organic matter into it to make it absorb the rain more readily.
We noticed that water poured on the sand ran through it very quickly and was apt to be lost by leaching or percolation. This we found could be checked by rolling the soil and by putting organic matter into it to close the pores.
We learned that roots take water from the soil for the use of the plant and send it up to the leaves, which in turn send it out into the air, or transpire it, as this process is called. We learned also that the amount transpired is very great. Now water that is pumped up and transpired by the crops we are growing we consider properly used. But when weeds grow with the crop and pump and transpire water we consider this water as lost or wasted.
Water may be lost then by being pumped up and transpired by weeds. And this is the way weeds do their greatest injury to crops during dry weather. The remedy is easily pointed out. Kill the weeds or do not let them get a start.
There is another way, which we are not apt to notice, by which water may be lost from the soil. When the soil in the pans in a previous experiment (page 26) had been wet and set aside a few days it became very dry. How did the water get out of this soil? That at the surface of the soil evaporated or was changed into vapor and passed into the air. Then water from below the surface was pumped up by capillary force to take its place just as the water was pumped up in the tubes of soil. This in turn was evaporated and the process repeated till all of the water in the soil had passed into the air. Now this process is going on in the field whenever it is not raining or the ground is not frozen very hard.
Water then may be lost by evaporation.
How can we check this loss?
Suppose we try the experiment of covering the soil with some material that cannot pump water readily.
Experiment.—Take four glass fruit jars, two-quart size, with straight sides. If you cannot get them with straight sides cut off the tops with a hot iron just below the shoulder; tin pails will do if the glass jars cannot be had. Fill these with moist soil from the field or garden, packing it till it is as hard as the unplowed or unspaded soil. Leave one of them in this condition; from two of them remove an inch or two of soil and replace it in the case of one with clean, dry, coarse sand, and in the case of the other with chaff or straw cut into half-inch lengths. Stir the soil in the fourth one to a depth of one inch, leaving it light and crumbly. Now weigh the jars and set them aside. Weigh each day for several days. The four jars illustrated in Fig. 30 were prepared in this way and allowed to stand seven days. In that time they lost the following amounts of water: