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.