Brush harrows.

Plank or drag harrows.

These types vary in the depth to which they cut, and the degree to which they pulverize the soil.

_Rolling cutter harrows._ Harrows of this type (see Fig. 54) consist of one or more revolving shafts on which are arranged a number of concave disks. These disks are either entire, notched, or made of several pieces fastened together. Examples of these are the disk, cutaway and spading harrows. These harrows cut and move the soil deeper than the other types. They are especially adapted to work on heavy clay soils.

The value of this type of harrow as moisture preservers depends on the manner in which they are used. If the disks are so set that they cover but a portion of the surface with a mulch of fine earth they leave a ridge exposed to the action of the wind and sun and the rate of evaporation is greatly increased. The disks should be set at such an angle that the whole surface shall be stirred or covered. Soils which need the disk harrow should generally be gone over again with some shallower working tool to smooth the surface. An objection to the rolling cutters is that unless great care is taken they will leave the land in ridges and valleys.

The two gangs of disks throw the earth in opposite directions. They are generally set to throw it from the centre and the result is a shallow double furrow the width of the machine. By lapping each time the furrow is partially filled, but to get the land smooth a smoothing harrow should be used after the rolling cutter.

_Spring-toothed harrows_ (Fig. 55). Spring-toothed harrows with their curved spring teeth enter the soil readily, draw moderately easy and pa.s.s over obstructions without much difficulty. They are very useful in new land that is full of roots and stumps and also stony land. They pulverize the soil to an average depth. They leave the soil in ridges.

The ridges can be leveled by a smoother in the shape of a piece of plank attached to the rear of the harrow. On newly plowed gra.s.s land they tend to tear up the sod and leave it on the surface. They also tend to drag out coa.r.s.e manures when plowed in.

The original and more common form of the spring-toothed harrow is a floating harrow when at work. That is, it rests on the points of the teeth and is dragged or floated over the ground. A newer form of spring-toothed harrow, sometimes called the fallow cultivator, is mounted on high wheels and its action is largely controlled by them.

This form of harrow is claimed to do much better work than the floating harrow and may in a large measure displace the rolling cutter. The weight of this harrow is entirely taken from the soil except in the wheel tracks, and the entire action is that of pulverizing and lightening the soil.

_Spike-toothed harrows_ (Fig. 56). The teeth of these harrows are round, square or diamond-shaped spikes fastened into a wood or iron frame. The teeth are set in a vertical position or are inclined to the rear. These harrows are shallow in their action; they run easily but tend to compact the soil more than the other types and are therefore better adapted to loose soils and to finishing off after the work of the deep cutting harrows. They are also used for covering seeds.

[Ill.u.s.tration: FIG. 56.--SPIKE-TOOTHED HARROWS.]

[Ill.u.s.tration: FIG. 57.--A COULTER-TOOTHED HARROW.]

[Ill.u.s.tration: FIG. 58.--A PLANK HARROW.]

_Coulter-toothed harrows._ The coulter-toothed harrows (Fig. 57) have teeth resembling the coulter of a plow twisted or bent into various shapes. The Acme is a good example of this cla.s.s of harrow. It cuts, turns and pulverizes the surface soil somewhat after the manner of the plow. It prepares a fine mulch and leaves an excellent seed bed. It is an excellent harrow to finish off with after using a rolling cutter.

_Chain harrows._ The chain harrow consists of a web of chains linked together. They have a wonderful power for breaking clods and are useful for collecting weeds. They shake the dirt from the weeds and roll them into heaps. Chain harrows tend to compact the soil.

_Brush harrows._ The brush harrow is a primitive form made by fastening brush to a long pole. Brush harrows are quite useful for brushing in seed and for pulverizing manure broadcasted on gra.s.s lands.

_Plank harrows._ The plank harrow (see Fig. 58) is made of several planks fastened together so that each plank overlaps the next one to it, like the clapboards of a house. This harrow is as good as a roller in fining and smoothing the surface soil. It is an excellent tool to use alternately with a spike or coulter-toothed harrow on lumpy soil.

This tool rasps or grinds many of the lumps or clods which slip by the harrow teeth and presses others into the ground so that the harrow following can get a grip on them. It is a harrow that can be made on any farm. This planker is an excellent tool to smooth the surface, for broadcasting small seeds and for planting truck crops.

ROLLING

The objects of rolling are:

To compress the surface soil so that the harrow will do its work more efficiently, also to break clods or lumps that may have resisted the action of the harrow.

To smooth the surface of the soil for an even distribution of small seeds, and to firm the soil around such seeds after they are planted so that they will keep moist and sprout readily.

To give compactness to soils that are light and loose and thus enable them to hold moisture and plant food better.

To press into the ground the roots of plants partly dislodged by the frost.

To remove the conditions favorable to the development of many kinds of insects.

To sink surface stones so that they will not interfere with harvesting the crop.

Light porous soils may be rolled at any time, but clay soils can be rolled to advantage only when they are stiff and cloddy.

Spring-sown grain is often rolled as soon as sown. This is all right in ordinary spring weather, but if showers are frequent and the soil is quite moist the rolling should be omitted till after the grain is up. The same practice will apply to autumn-sown grain also. If the soil is dry the rolling helps it to pump water up to the seeds. But if it is moist and showers are frequent the combined action of the roller and the rain is to make so thick a crust that many of the seeds will not be able to force their way through it or will be smothered by poor ventilation. After the grain is up the rolling may be done to advantage, as it then makes a firm soil about the roots of the plants, a condition of benefit to grain crops.

The most simple form of roller is a solid or hollow cylinder of wood fastened into a frame by which it is drawn. Some rollers have spikes or blunt attachments fastened to their surfaces for breaking clods. A roller that is quite popular consists of a cylinder of pressed steel.

CHAPTER XIII

LEAVES

FACTS ABOUT LEAVES

We found in an earlier lesson that all of our farm plants have roots, stems, leaves, flowers, fruit and seeds. We studied the root first as being the most important part of the plant to the farmer. The seed was the next part studied, for that was considered the next most important, because the seed is the main reliance for new plants. The part next in importance is the leaf and that we will now study.

If you will go into the field and observe the leaves on a number of plants, you will find that the following facts are true:

They are all green.

They are flat and thin.

Many of them are very broad.

Some of the leaves on a single branch are larger than others on the same branch, and some have longer stems than others.

Most of them have a rather dark glossy upper surface and a lighter rougher under surface.

[Ill.u.s.tration: FIG. 59.

To show transpiration. Plant _A_ was set in the sunlight, plant _B_ was left in the darker part of the room. _A_ has transpired much more than _B_, showing that sunlight is necessary for this work.]

[Ill.u.s.tration: FIG. 60.--AMOUNT OF TRANSPIRATION This plant transpired within 48 hours an amount of water equal to the colored liquid in the bottle standing on the jar, more than 6 ounces.]

The leaves on the lower branches of the trees are spread out in a more or less flat layer and have their glossy surfaces all turned up, while those on branches in the tops of trees or shrubs are arranged all around the branch, the glossy surface being turned up.

What are the reasons for these facts?

A study of the work of the leaves and the conditions necessary for them to perform their work will help us to answer this question.

THE USES OF LEAVES TO PLANTS

=Experiment.=--(See Fig. 59). 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; cut a slit from the edge to the centre of the pasteboard, then place it on the top of the pot, letting 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, letting the edge of the tumbler rest on the pasteboard; if a tumbler is not large enough use a gla.s.s jar. If a potted plant is not convenient a slip or a seedling bean or pea placed in a tumbler of water will serve the purpose. Prepare several and place some in a sunny window and leave others in the room where it is darker, and observe them from time to time. In the case of those plants that were set in the sunny window moisture will be seen collecting on the inner surface of the tumbler. Where does this come from? It is absorbed from the soil by the roots and is sent with its load of dissolved plant food up through the stems to the leaves.

There most of the water is pa.s.sed from the leaves to the air and is condensed on the sides of the gla.s.s. A work of leaves then is to throw off or to transpire moisture and thus make room for a new supply of food-laden moisture. This water is thrown off through little pores or mouths or stomata which are very small and very numerous on the under side of the leaf. It will be noticed that the plant not placed in the sunlight transpires very little moisture, showing that sunlight helps the leaves in this work of transpiration.

How much water does a plant transpire or throw off from its leaves?