In the same manner a wet soil loses heat by the evaporation of water from its surface.
=Experiment.=--Heat an iron rod, take it from the fire and hold it near your face or hand. You will feel the heat without touching the rod. The heat is radiated from the rod through the air to your body and the rod gradually cools. In the same way the soil may lose its heat by radiating it into the air. A clay soil will lose more heat by radiation than a sandy soil because the clay is more compact.
CONDITIONS WHICH INFLUENCE SOIL TEMPERATURE
It will be noticed that the dry soils are warmer than the wet ones.
Why is this? Scientists tell us that it takes a great deal more heat to warm water than it does to warm other substances. Therefore when soil is wet it takes much more heat to warm it than if it were dry.
It will be seen that of the dry soils the humus is the warmest. Why?
=Experiment.=--Take two thermometers, wrap the bulb of one with a piece of black or dark colored cloth and the bulb of the other with a piece of white cloth, then place them where the sun will shine on the cloth covered bulbs. The mercury in both thermometers will be seen to rise, but in the thermometer with the dark cloth about the bulb it will rise faster and higher than in the other. This shows that the dark cloth absorbs heat faster than the white cloth. In the same manner a dark soil will absorb heat faster than a light colored soil; therefore it will be warmer if dry.
Why was the dry clay warmer than the dry sand?
Because its darker color helped it to absorb heat more rapidly than the sand, and, as the particles were smaller and more compact, heat was carried into it more rapidly by conduction.
Why were the wet humus and clay cooler than the wet sand?
As they were darker in color and the clay was more compact than the sand, they must have absorbed more heat, but they also held more water, and, therefore, lost more heat by evaporation.
[Ill.u.s.tration: FIG. 32.
Charts showing average temperature of a set of dry and wet soils during a period of five days. _H_, humus; _C_, clay; _S_, sand.]
[Ill.u.s.tration: FIG. 33.
To show the value of organic matter. 1 contains clay subsoil; 2, clay subsoil and fertilizer; 3, clay subsoil and organic matter. All planted at the same time.]
Of the dry soils, then, the humus averaged warmest, because, on account of its dark color, it absorbed heat more readily than the others. The dry clay was warmer than the sand on account of its color and compact texture. Of the wet soils the sand was the warmest, because, on account of its holding less moisture, less heat was required to raise its temperature and there was less cooling by evaporation, while the other soils, although they absorbed more heat than the sand, lost more on account of greater evaporation, due to their holding more moisture. Why are sandy soils called warm soils and clay soils said to be cold?
How may we check losses of heat from the soil?
If we make a mulch on the surface of the soil evaporation will be checked and therefore loss of heat by evaporation will be checked also. The mulch will also check the conduction of heat from the lower soil to the surface and therefore check loss of heat by radiation from the surface.
VALUE OF ORGANIC MATTER
Figure 33 ill.u.s.trates a simple way to show the value of organic matter in the soil. The boxes are about twelve inches square and ten inches deep. They were filled with a clay subsoil taken from the second foot below the surface of the field. To the second box was added sufficient commercial fertilizer to supply the plants with all necessary plant food. To the third box was added some peat or decayed leaves, in amount about ten per cent. of the clay subsoil. The corn was then planted and the boxes were all given the same care. The better growth of the corn in the third box was due to the fact that the organic matter not only furnished food for the corn but during its decay prepared mineral plant food that was locked up in the clay, and also brought about better conditions of air and moisture by improving the texture of the soil. The plants in the second box had sufficient plant food, but did not make better growth because poor texture prevented proper conditions of air and moisture. "And that"s another witness"
for organic matter. Decaying organic matter or humus is really the life of the soil and it is greatly needed in most of the farm soils of the eastern part of the country. It closes the pores of sandy soils and opens the clay, thus helping the sand to soak up and hold more moisture and lessening excessive ventilation, and at the same time helping the roots to take a firmer hold. It helps the clay to absorb rain, helps it to pump water faster, helps it to hold water longer in dry weather, increases ventilation, favors root penetration and increases heat absorption. We can increase the amount of organic matter in the soil by plowing in stable manure, leaves and other organic refuse of the farm, or we can plow under crops of clover, gra.s.s, grain or other crops grown for that purpose.
CHAPTER VIII
PLANT FOOD IN THE SOIL
We learned in previous paragraphs that the roots of plants take food from the soil, and that a condition necessary for the root to do its work for the plant was the presence of available plant food in sufficient quant.i.ties.
What is plant food? For answer let us go to the plant and ask it what it is made of.
=Experiment.=--Take some newly ripened cotton or cotton wadding, a tree branch, a cornstalk, and some straw or gra.s.s. Pull the cotton apart, then twist some of it and pull apart; in turn break the branch, the cornstalk and the straw. The cotton does not pull apart readily nor do the others break easily; this is because they all contain long, tough fibres. These fibres are called woody fibre or cellulose. The cotton fibre is nearly pure cellulose.
=Experiment.=--Get together some slices of white potato, sweet potato, parsnip, broken kernels of corn, wheat and oats, a piece of laundry starch and some tincture of iodine diluted to about the color of weak tea. Rub a few drops of the iodine on the cut surfaces of the potatoes, parsnip, and the broken surfaces of the grains. Notice that it turns them purple. Now drop a drop of the iodine on the laundry starch. It turns that purple also. This experiment tells us that plants contain starch.
=Experiment.=--Chew a piece of sorghum cane, sugar cane, cornstalk, beet root, turnip root, apple or cabbage. They all taste sweet and must therefore contain sugar.
Examine a number of peach and cherry trees. You will find on the trunk and branches more or less of a sticky substance called gum.
=Experiment.=--Crush on paper seeds of cotton, castor-oil bean, peanuts, Brazil nuts, hickory nuts, b.u.t.ternuts, etc. They make grease spots; they contain fat and oil.
=Experiment.=--Chew whole grains of wheat and find a gummy mucilaginous substance called wheat gum, or wet a pint of wheat flour to a stiff dough, let it stand about an hour, and then wash the starch out of it by kneading it under a stream of running water or in a pan of water, changing the water frequently. The result will be a tough, yellowish gray, elastic ma.s.s called gluten. This is the same as the wheat gum and is called an alb.u.minoid because it contains nitrogen and is like alb.u.men, a substance like the white of an egg.
If we crush or grate some potatoes or cabbage leaves to a pulp and separate the juice, then heat the clear juice, a substance will separate in a flaky form and settle to the bottom of the liquid. This is vegetable alb.u.men.
[Ill.u.s.tration: FIG. 34.
Soy-bean roots. Showing nodules of tubercles, the homes of nitrogen-fixing bacteria.]
[Ill.u.s.tration: FIG. 35.
Garden-pea roots, showing tubercles or nodules, the homes of nitrogen-fixing bacteria.]
=Experiment.=--Crush the leaves or stems of several growing plants and notice that the crushed and exposed parts are moist. In a potato or an apple we find a great deal of moisture. Plants then are partly made of water. In fact growing plants are from 65 to 95 per cent. water.
=Experiment.=--Expose a plant or part of a plant to heat; the water is driven off and there remains a dry portion. Heat the dry part to a high degree and it burns; part pa.s.ses into the air as smoke and part remains behind as ashes.
We have found then the following substances in plants: Woody fibre or cellulose, starch, sugar, gum, fats and oils, alb.u.minoids, water, ashes. Aside from these are found certain coloring matters, certain acids and other matters which give taste, flavor, and poisonous qualities to fruits and vegetables. More or less of all these substances are found in all plants. Now these are all compound substances. That is, they can all be broken down into simpler substances, and with the exception of the water and the ashes, the plants do not take them directly from the soil.
The chemists tell us that these substances are composed of certain chemical elements, some of which the plant obtains from the air, some from the soil and some from water.
The following table gives the substances found in plants, the elements of which they are composed, and the sources from which the plants obtain them:
----------------------------------------------------------+ Substances found | Elements of which | Sources from | in plants. | they are made. | which plants | | | obtain them. | -------------------+---------------------+----------------+ Cellulose or | | | woody fibre | Carbon | Air | Starch |---------------------+----------------+ Sugar | | | Gum | Oxygen | Water | Fat and Oil | Hydrogen | | -------------------+---------------------+----------------+ | Carbon | Air | +---------------------+----------------+ Alb.u.minoids | Oxygen | Water | | Hydrogen | | +---------------------+----------------+ | _Nitrogen_ | | | Sulphur | | | Phosphorus | | -------------------+---------------------| Soil + | _Phosphorus_ | | | _Pota.s.sium_ | | Ashes | _Calcium_ | | | Magnesium | | | Iron | | -------------------+---------------------+----------------+ Water | Oxygen | Soil | | Hydrogen | | -----------------------------------------+----------------+
Here is a brief description of these chemical elements.
Oxygen, a colorless gas, forms one-fifth of the air.
Hydrogen, a colorless gas, forms a part of water.
Carbon, a dark solid, forms nearly one-half of all organic matter; charcoal is one of its forms. The lead in your pencil is another example.
Nitrogen, a colorless gas, forms four-fifths of the air. Found in all alb.u.minoids.
Sulphur, a yellow solid.
Phosphorus, a yellowish white solid.
Pota.s.sium, a silver white solid.