plants, on the other hand, can manufacture their own food from the inorganic materials of the soil and atmosphere.

Sunlight Necessary to Carbon Fixation.-The decomposition of carbonic acid by the plant, and the assimilation of the carbon take place only during the daytime. A certain amount of energy is necessary to break apart the carbon and oxygen of carbonic acid, and this energy is furnished by the sunlight. The stronger the light the faster the fixation of carbon. This explains the commonly observed fact that most plants grow more vigorously in full sunlight than in shade or diffused light. The plant has not the power to use carbonic acid in the absence of light, so that this process ceases during the night. It is well known that seeds will germinate in the dark, and produce a feeble spindling growth of pale foliage, but that the plants so produced soon cease to develop. Such plants grow until they exhaust the food stored in the seed, but have no power to use the food in the air and soil, and analysis shows that the plant contains less dry matter than was present in the seed. In the presence of light, however, the plant absorbs the carbonic acid of the air by means of its leaves, and causes the carbon to combine with the water and mineral matter taken in through its roots to form carbohydrates, proteids and the other complex compounds of which the plant is composed.

Carbonic Acid the Sole Source of Carbon.—It has been thoroughly demonstrated that the green plants derive their carbon solely from the carbonic acid of the atmosphere, and are not dependent in any way

upon the carbonaceous matter in the soil. In fact, that they are incapable of using carbon except in the? form of carbonic acid gas. The quantity of carbonic acid in the air seems so insignificant that it might be feared that the supply might some day be exhausted. The bulk of the atmosphere is so enormous, however, that the total amount of carbonic acid is very large. Johnson calculated that the atmosphere when taken to its entire height contains not less than 3,400,000,000,000 tons of carbonic acid. This amounts to about 28 tons over every acre of the earth's surface, and as only one-fourth of the earth's surface is land he estimated that the carbonic acid in the air is sufficient, without renewal, for a hundred years of growth. As a matter of fact, the supply of this gas is being constantly renewed, and at such a rate that the proportion found in the air remains about constant. When wood or coal or any other substance containing carbon is burned, carbonic acid is formed and passes into the atmosphere. This gas is produced during all kinds of decay and fermentation. Animals live directly or indirectly on plants, and breathe out the carbon they consume in the form of carbonic acid gas. It will thus be seen that the carbon is continually in circulation, being combined by the plant into complex compounds only to be broken down, and returned again to the air through these various agencies. In all probability that now present in the air has been many times built up into organic matter, only to be again set free by decomposition.

Numerous experiments have proved that the supply of carbon in the air is ample for the largest crops. To ⚫be sure, in certain pot experiments a larger yield was

obtained by increasing the carbonic acid in the air, but under field conditions the yield is limited by other factors, and never by the supply of carbon.

Carbon Costs the Farmer Nothing. The point of practical importance brought out by this study of the fixation of carbon is that the carbon is furnished free of cost. In other words the carbon compounds produced in the crop result in no impoverishment of the soil. Hence, there is no need of supplying strictly carbonaceous manure to the field, as the crop does not use the carbon in the soil. It will be shown later that such manures may be indirectly beneficial to the plant, however.

CHAPTER III

NITROGEN AS A PLANT FOOD

Nitrogen the Most Costly Plant Food.-A reference to the table given in Chapter I shows that only about 11⁄2 per cent of the dry matter of the corn plant consists of nitrogen. Some plants contain more nitrogen than this, but the amount rarely equals 3 per cent of the dry matter, or six-tenths of one per cent of the green plant. In spite of the small quantity of nitrogen in the crop it is the most important of all plant foods from the practical point of view. In fact the solution of the problem of the maintenance of fertility depends upon an economical method of conserving and renewing the nitrogen supply of the soil. This does not imply that nitrogen is more necessary to vegetation than are the other constituents, but that it is the most expensive element to be furnished by means of fertilizers, and is also, unfortunately, the element most easily lost and wasted.

The Nitrogen of Most Plants Comes from the Soil. -Most of the crops raised by the farmer are entirely dependent upon the soil for their supply of nitrogen. The greater part of the nitrogen present in the soil is locked up in the insoluble organic matter, and in this form is not available to plants. Some of the nitrogen exists in simple compounds called nitrates, which consist of nitric acid combined with one of the mineral elements of the soil. The majority of farm crops can

use only that part of the nitrogen in the soil that is present as nitrates, so that so far as the nitrogen is concerned, the fertility of the land depends upon its nitrate content. The nitrate present in the soil at any one time is exceedingly small, but under proper conditions the supply may be renewed with sufficient rapidity to meet the needs of the plant.

Source of the Nitrogen of the Soil.-A small part of the nitrogen in the soil is derived directly from the atmosphere. Minute traces of ammonia (a compound of nitrogen and hydrogen) are always found in air, and during electrical storms small quantities of the nitrogen and oxygen in the atmosphere are combined to form nitric acid. These substances are dissolved in the rain water during showers and are carried into the soil. The quantity received by the soil from this source is very small, amounting only to from 3 to 8 pounds an acre a year, the maximum amount being less than one-tenth of that required by a crop of corn. Nearly all of the nitrogen in the soil is present in the more or less decayed organic matter left behind by the plants that it has previously produced. Plants build up the nitrogen into complex protein compounds, and, under ordinary conditions, when they die these substances in connection with the other constituents of the plant become a part of the soil. As long as the nitrogen remains in this form it is of no value to the new generation of plants, for the organic matter must first be decomposed, and the nitrogen changed into the form of nitrates.

Nitrification. The soil must not be regarded as an inert mass of mineral matter and refuse of former