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 quantities.
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 grass. 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, butternuts, 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 mass called gluten. This is the same as the
wheat gum and is called an albuminoid because it contains nitrogen and
is like albumen, 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 albumen.
[Illustration: FIG. 34.
Soy-bean roots. Showing nodules of tubercles, the homes of
nitrogen-fixing bacteria.]
[Illustration: 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 passes 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, albuminoids, 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 |
+---------------------+----------------+
Albuminoids | Oxygen | Water |
| Hydrogen | |
+---------------------+----------------+
| _Nitrogen_ | |
| Sulphur | |
| Phosphorus | |
-------------------+---------------------| Soil +
| _Phosphorus_ | |
| _Potassium_ | |
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
albuminoids.
Sulphur, a yellow solid.
Phosphorus, a yellowish white solid.
Potassium, a silver white solid.
Calcium, a yellowish solid. Found in limestone.
Magnesium, a silver white solid.
Iron, a silver gray solid.
Of these elements the nitrogen, sulphur, phosphorus, potassium,
calcium, magnesium, and iron must not only exist in the soil but must
also be there in such form that the plant can use them. The plant does
not use them in their simple elementary form but in various compounds.
These compounds must be soluble in water or in weak acids.
Of these seven elements of plant food the nitrogen, phosphorus, and
potassium and calcium are of particular importance to the farmer,
because they do not always exist in the soil in sufficient available
quantities to produce profitable crops. Professor Roberts, of Cornell
University, tells us that an average acre of soil eight inches deep
contains three thousand pounds of nitrogen. The nitrogen exists
largely in the humus of the soil and it is only as the humus decays
that the nitrogen is made available. Here is another reason for
keeping the soil well supplied with organic matter. The decay of this
organic matter is hastened by working the soil; therefore good tillage
helps to supply the plant with nitrogen.
If the nitrogen becomes available when there is no crop on the soil it
will be washed out by rains and so lost. Therefore the soil,
especially if it is sandy, should be covered with a crop the year
through. Many lands lose large amounts of plant food by being left
bare through the fall and winter, especially in those parts of the
country where the land does not freeze. The phosphorus, potassium and
calcium also exist in most soils in considerable quantities, but often
are not available; thorough tillage and the addition of organic matter
will help to make them available, and new supplies may be added in the
form of fertilizers. Calcium is found in nearly all soils in
sufficient quantities for most crops, but sometimes there is not
enough of it for such crops as clover, cowpea, alfalfa, etc. It is
also used to improve soil texture. The entire subject of commercial
fertilizers is based almost entirely on the fact of the lack of these
four elements in the soil in sufficient available quantities to grow
profitable crops. The plant gets its phosphorus from phosphoric acid,
its potassium from potash, and its calcium from lime.
There is a class of plants which have the power of taking free
nitrogen from the air. These are the leguminous plants; such as
clover, beans, cowpeas, alfalfa, soy bean, etc. They do it through the
acid of microscopic organisms called bacteria which live in nodules or
tubercles on the roots of these plants (Figs. 34-35). Collect roots of
these plants and find the nodules on them. The bacteria take nitrogen
from the air which penetrates the soil and give it over to the plants.
Here is another reason for good soil ventilation.
This last fact brings us to another very important property of soils.
Soils have existing in them many very small plants called bacteria.
They are so very small that it would take several hundred of them to
reach across the edge of this sheet of paper. We cannot see them with
the naked eye but only with the most powerful microscopes. Some of
these minute plants are great friends to the farmer, for it is largely
through their work that food is made available for the higher plants.
Some of them break down the organic matter and help prepare the
nitrogen for the larger plants. Others help the leguminous plants to
feed on the nitrogen of the air. To do their work they need warmth,
moisture, air, and some mineral food; these conditions we bring about
by improving the texture of the soil by means of thorough tillage and
the use of organic matter.