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While these facts indicate that the soil can never be completely exhausted, it is exhausted for all practical purposes when the crop produced ceases to be profitable. The first question that naturally suggests itself is, Why does the productive power of the soil diminish?
The Plant Removes Something from the Soil.It is evident that the virgin land must have contained large quantities of some substance or substances that were necessary to vigorous plant growth and that these materials were removed from the soil when the crop was harvested. It is not possible to explain the rapid decrease in fertility on any other basis; for it can not be ascribed to any changes in climatic conditions. The change in the physical condition of the soil has been suggested as a possible explanation for its decreased productive power, but even this is not an adequate explanation. It is apparent also, that plants vary in their power to extract these substances from the soil; for it is well known that a soil may be unfertile for one class of plants and still produce a luxuriant growth of another. To ascertain what these materials are that the plant removes from the soil, it is necessary to analyze the plant and then to determine the sources of the ingredients found there. For the purpose of this study the corn, or maize, plant is chosen, as it is perhaps the most important of all plants to the American farmer. Before presenting the analysis it is advisable to devote a moment to a few preliminary considerations.
Elements and Compounds.—Chemistry teaches that all matter is composed of simple substances called elements. Between 70 and 80 of them are known. They are called elements because they are the simplest sub
stances known, and can not by any means yet discovered, be separated into simpler or different substances. Iron, gold, silver and sulphur are examples of elements. Two others, both gases (i. e. oxygen and nitrogen), make up the bulk of the air.
Most materials with which we are familiar are complex and are combinations of two or more elements. Such bodies are called compounds. While the number of elements is small there are many thousands of compounds. This is due to the fact that the same elements can combine in many different ways, each combination forming a different compound. Alcohol, sugar, starch, fats and acetic acid, for example, are substances very unlike in their properties and yet all consist of the three elements carbon, hydrogen and oxygen, but these elements are present in different proportions. Plants are composed of a large number of compounds, and an ideal analysis would first separate the plant into its compounds and then these compounds into the elements of which they are composed. Approximately such an analysis can be made.
Chemical Composition of the Corn Plant.—If a quantity of green corn is allowed to wilt in the sun it loses a large percentage of its weight by the evaporation of the water which it contains. If the remainder is now heated in an oven at 212° F. it again decreases in weight, but finally reaches a point where the weight does not change, because all the water is driven off. Water is composed of the two elements hydrogen and oxygen. What remains after expelling the water is called the dry matter of the plant. The dry matter burns on being ignited and a very small amount of mineral matter remains, which is called ash. The part that burned, and completely disappeared is known as organic matter. The organic matter is composed of four classes of compounds known as fat, crude fiber, carbohydrates and protein. The first three of these compounds are made up of the elements carbon, oxygen and hydrogen; and the protein contains in addition to
Corn or maize is one of the most important crops for the American farmer.
It removes large quantities of plant food from the soil, however. The analysis of the corn plant is given on page 11.
these the element nitrogen. The ash contains the elements potassium, phosphorus, calcium, magnesium, iron, sulphur, chlorine, sodium and silicon. The following table shows the ingredients found in 1,000 pounds of the matured corn plant, i. e. when the plant is in condition to be cut for shocking.
From what has been said it will be seen that of the elements known only thirteen are found in plants; for what is true of the corn plant holds true of all other plants. It will be shown that of these thirteen, three are probably not necessary to plant growth, leaving only ten elements that are essential. The table shows
that three elements (i. e. hydrogen, oxygen and carbon) make up 9872 per cent of the entire composition of the plant, the remaining elements constituting only 1/2 per cent.
(NOTE.-All of the elements mentioned above as occurring in the ash, with the exception of chlorine, are combined with oxygen. In the table the names under "ash” represent these combinations, i. e. potash is composed of potassium and oxygen; phosphoric acid is phosphorus and oxygen; lime is calcium and oxygen, etc.)
Importance of Water to the Plant.—One of the most striking points brought out by the chemical analysis is the large proportion of water that enters into the composition of the plant. A reference to the table shows that nearly 800 of the 1,000 pounds of the matured corn plant consist of water in a form that can be driven off at a heat not above the boiling point. In the organic matter is found 12.7 pounds of hydrogen and 88.9 pounds of oxygen which practically all came originally from water, making a total of nearly 900 pounds derived from this source. These figures represent but a small part of the water actually required by the crop. Water is being continually given off into the air by the plant-leaves. This exhaled, or "transpired,” water is in the form of a vapor and is invisible, but that it actually exists can be proved by a simple experiment.
Invert a wide mouthed bottle or fruit jar over a small plant, and after a short time the inner surface of the bottle will be found to be covered with moisture. The earth around the plant should first be covered with a piece of oil cloth or oiled paper to make sure that the water does not come from the soil. If the underside of a leaf is examined with a magnifying glass or microscope it will be found that the surface is not entire,