Soil containing organic materials holds water much more readily than inorganic soil, as a glance at the accompanying figure shows. If we fill each of the vessels with a given weight (say 100 grams each) of gravel, sand, barren soil, rich loam, leaf mold, and 25 grams of dry, pulverized leaves, then pour equal amounts of water (100 c.c.) on each and measure all that runs through, the water that has been retained will represent the water supply that plants could draw on from such soil.
The Root Hairs take more than Water out of the Soil.—If a root containing a fringe of root hairs is washed carefully, it will be found to have little particles of soil still clinging to it. Examined under the microscope, these particles of soil seem to be cemented to the sticky surface of the root hair. The soil contains, besides a number of chemical compounds of various mineral substances,—lime, potash, iron, silica, and many others,—a considerable amount of organic material. Acids of various kinds are present in the soil. These acids so act upon certain of the mineral substances that they become dissolved in the water which is absorbed by the root hairs. Root hairs also give off small amounts of acid. An interesting experiment may be shown (see Figure on page [80]) to prove this. A solution of phenolphthalein loses its color when an acid is added to it. If a growing pea be placed in a tube containing some of this solution the latter will quickly change from a rose pink to a colorless solution.
Effect of root hairs on phenolphthalein solution. The change of color indicates the presence of acid.
A Plant needs Mineral Matter to Make Living Matter.—Living matter (protoplasm), besides containing the chemical elements carbon, hydrogen, oxygen, and nitrogen, contains a very minute proportion of various elements which make up the basis of certain minerals. These are calcium (lime), sulphur, iron, potassium, magnesium, phosphorus, sodium, and chlorine.
That plants will not grow well without certain of these mineral substances can be proved by the growth of seedlings in a so-called nutrient solution.[11] Such a solution contains all the mineral matter that a plant uses for food. If certain ingredients are left out of this solution, the plants placed in it will not live.
Nitrogen in a Usable Form necessary for Growth of Plants.—A chemical element needed by the plant to make protoplasm is nitrogen. The air can be proven by experiment to be made up of about four fifths nitrogen, but this element cannot be taken from either soil, water, or air in a pure state, but is usually obtained from the organic matter in the soil, where it exists with other substances in the form of nitrates. Ammonia and other organic compounds which contain nitrogen are changed by two groups of little plants called bacteria, first into nitrites and then nitrates.[12]
Diagram to show how the nitrogen-fixing bacteria prepare nitrogen for use by plants; t, tubercles.
Relation of Bacteria to Free Nitrogen.—It has been known since the time of the Romans that the growth of clover, peas, beans, and other legumes in soil causes it to become more favorable for growth of other plants. The reason for this has been discovered in late years. On the roots of the plants mentioned are found little swellings or nodules; in the nodules exist millions of bacteria, which take nitrogen from the atmosphere and fix it so that it can be used by the plant; that is, they assist in forming nitrates for the plants to use. Only these bacteria, of all the living plants, have the power to take the free nitrogen from the air and make it over into a form that can be used by the roots. As all the compounds of nitrogen are used over and over again, first by plants, then as food for animals, eventually returning to the soil again, or in part being turned into free nitrogen, it is evident that any new supply of usable nitrogen must come by means of these nitrogen-fixing bacteria.