Connect the delivery tube with a bottle full of water standing inverted on the shelf of a pneumatic trough. Pour water into the funnel until it is full, and keep it full during the rest of the experiment. Allow the apparatus thus arranged to stand for some hours. Air will collect in the bottle over the pneumatic trough. Where did it come from? When the soil in the bottle has become entirely saturated with water, roughly compare the amount of air collected with the volume of the bottle containing the soil. What part of the soil"s volume is the air?
The smaller the soil particles are, the more surface they present to water, the more they are dissolved, the more food the plant hairs can reach, and the more fertile is the soil, other things being equal. We have also seen by experiment that soil contains air as well as water. Air is needed if plants are to flourish, and it is necessary that it be changed frequently, just as it is necessary to change the air in a room if people are to flourish. The soil must be ventilated. Plant roots must have air to breathe.
44.Fertile Soils. -Rock disinte- gration does not furnish all the complex materials needed for the growth of ag- ricultural plants. Only the lower orders of plants, such as lichens, can grow on soil as at first formed. A fertile soil is the product of ages of plant and animal life, labor and decay.
Plants send their filaments and roots among the rock particles, pryingMOLEHILLS.
Showing how animals dig up the soil and make it porous.
open their crevices and pushing the pieces apart so that the agents of disintegration can more readily attack them. By their decay plants provide the humus so necessary for making soil fertility.
Animals like moles and gophers
plow their holes through the soil, mixing up the particles and making the soil porous, so that the water can readily get in to aid in breaking up and decomposing the soil particles. These holes also provide openings through which plant roots and soilANTHILL.
This soil has been brought from below and piled up by the ants.
organisms can obtain the oxygen and dissolved food they need. Antseach year move vast quantities of fine material to the surface, and insome places change the surface soil in a few years.
Angleworms, the most important animal soil builders, channel the soil with their burrows, thus providing readymade openings for the growing roots, and by increasing the porosity of the soil aid in its ventilation and drainage. They swallow the soil as they make their burrows, in order to get the decaying vegetable matter for food, and they grind it fine as it passes through their bodies. Every year they bring to the surface great quantities of this finely ground soil mixed with the undigested vegetable matter. Darwin estimated that theangleworms in English soil deposited one fifth of an inch of these castings each year over some parts of the surface. This is the finest kind of fertilizer. It is a common saying that the more angleworms the better the soil.
Besides harboring these visible plants and animals the soil teems with germ life. Some of these germs increase the fertility of the soil and some decrease it. It has been estimated that there are 50,000 germs of various kinds in a gram of fertile soil. It is these that cause the decay of theMUD CRACKS.
Showing the way clay cracks when it dries.
vegetable and animal matter in the soil. In the course of this decay various acids and gases are formed which help to decompose the rock particles and other compounds which are needed for the food of the plants.
The most important of these germs to agriculture are the nitrogen- fixing bacteria. Plants must have nitrogen if they are to grow, but they are unable to take it from the air where it exists in greatest abundance. For the use of the plants it must be chemically combined with other substances and these compounds must be soluble in water. Saltpeter is a compound of this kind and is often used to fertilize plants. But soluble compounds of nitrogen are not abundant, and these would be soon removed from many soils unless in some way replaced.
LUMPY SOIL.
The result of cultivating at the wrong time.
This is most often done by adding manures to the soil. In these there are nitrogen compounds which the bacteria of the soil work over and get into shape so that the plants can use them. Some bacteria are even able to take the nitrogen from the soil-air and combine it so that it can be used by the plants. If this varied and teeming life of the soil is to thrive, certain conditions must be maintained, and it is the skill of the agriculturist in maintaining and increasing these favorable conditions which determines his success or failure.
45.Agricultural Soils. -As has already been shown, soils differgreatly in fineness, mineral composition and water-holding capacity. They also differ greatly in the amount of de-cayed vegetable material or humus in them.
The humus is a most important soil ingredi- ent. It helps in holding water, it furnishes plant food and it keeps the soil from getting too compact.
In sandy soils there is usually little humus, the water soon drains out of them and plants become parched. Such soils warm up quickly in the spring and dry out rapidly after longwet spells. When humus and plant food in the form of manure are added they areADOBE SOIL.
A heavy clay soil, very fertile, but hard to cultivate.
especially adapted for growing early crops and crops that do not require a great deal of moisture, such as grapes. The "Fresno Sand" of California and the sandy coast plains of the eastern United States are soils of this kind.