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 coa.r.s.e 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, coa.r.s.e, 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 coa.r.s.e 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 cla.s.sification of soils.

[Ill.u.s.tration: 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.]

[Ill.u.s.tration: FIG. 17.

Bottle _A_ contains fresh water, bottle _B_ contains boiled water.

Notice the air bubbles in bottle _A._]

[Ill.u.s.tration: 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.]

CLa.s.sIFICATION OF SOILS

Soil materials and soils are cla.s.sified as follows:

_Stones._--Coa.r.s.e, irregular or rounded rock fragments or pieces of rock.

_Gravel._--Coa.r.s.e 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.

Coa.r.s.e 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 ill.u.s.tration 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 ill.u.s.trations 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 cla.s.s belongs the so-called Norfolk Sand. This is a coa.r.s.e 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, gra.s.s, 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 gra.s.s.

_Gravelly loams_ are from one-fourth to two-thirds coa.r.s.e 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 quant.i.ties, 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.