Nitrogen is colorless, odorless, and tasteless.  Air is composed chiefly of oxygen and nitrogen; if, therefore, the oxygen in a vessel filled with air can be made to unite with some other substance or can be removed, there will be a residue of nitrogen.  This can be done by floating on water a light dish containing phosphorus, then igniting the phosphorus, and placing an inverted jar over the burning substance.  The phosphorus in burning unites with the oxygen of the air and hence the gas that remains in the jar is chiefly nitrogen.  It has the characteristics mentioned above and, in addition, does not combine readily with other substances.

245.  Plant Food.  Food is the course of energy in every living thing and is essential to both animal and plant life.  Plants get their food from the lifeless matter which exists in the air and in the soil; while animals get their food from plants.  It is true that man and many other animals eat fleshy foods and depend upon them for partial sustenance, but the ultimate source of all animal food is plant life, since meat-producing animals live upon plant growth.

Plants get their food from the air, the soil, and moisture.  From the air, the leaves take carbon dioxide and water and transform them into starchy food; from the soil, the roots take water rich in mineral matters dissolved from the soil.  From the substances thus gathered, the plant lives and builds up its structure.

A food substance necessary to plant life and growth is nitrogen.  Since a vast store of nitrogen exists in the air, it would seem that plants should never lack for this food, but most plants are unable to make use of the boundless store of atmospheric nitrogen, because they do not possess the power of abstracting nitrogen from the air.  For this reason, they have to depend solely upon nitrogenous compounds which are present in the soil and are soluble in water.  The soluble nitrogenous soil compounds are absorbed by roots and are utilized by plants for food.

246.  The Poverty of the Soil.  Plant roots are constantly taking nitrogen and its compounds from the soil.  If crops which grow from the soil are removed year after year, the soil becomes poorer in nitrogen, and finally possesses too little of it to support vigorous and healthy plant life.  The nitrogen of the soil can be restored if we add to it a fertilizer containing nitrogen compounds which are soluble in water.  Decayed vegetable matter contains large quantities of nitrogen compounds, and hence if decayed vegetation is placed upon soil or is plowed into soil, it acts as a fertilizer, returning to the soil what was taken from it.  Since man and all other animals subsist upon plants, their bodies likewise contain nitrogenous substances, and hence manure and waste animal matter is valuable as a fertilizer or soil restorer.