829. The four organogens. It has already appeared in the preceding chapters that plants consist chiefly of four simple organic elements, viz.: carbon, oxygen, hydrogen, nitrogen. The first exists in a larger proportion, the last in a smaller than either of the others. Unitedly these four elements constitute about 94 per cent. of all vegetable matter.

830. Carbon (essentially charcoal) enters so largely into the composition of plants that it retains generally the exact form and texture of the wood after the other elements have been expelled by heat. On this element chiefly depends the solidity and strength. Its proportion is from 40 to 60 per cent. Nitrogen, although equally essential, is less abundant in the tissues, and exists largely only in certain vegetable products. as gluten, albumen, casein, theine.

831. Oxygen and hydrogen exist in plants combined with other elements, and also combined with each other forming water, especially in all fresh green vegetable matter. The water is expelled by drying, and the following table shows, in a few cases, the proportion for each 100 lbs.

Peas lose of water ...............

8 lbs.

Wheat....................

14 1bs.

Rye and oats .....................

15 lbs.

Wheat straw .....................

26 lbs.

Potatoes about ...................

75 lbs.

Apples and pears ...............

83 lbs.

Red beet ..........................

85 lbs.

Strawberries and gooseberries.

90 lbs.

Turnips .......................

93 lbs.

Watermelons .........................

95 lbs.

832. Earthy elements. Besides these four universal elements, many other substances, earthy and mineral, are found in quantities greater or less, in different species. Thus forest-trees and most inland plants contain potassa; marine plants, soda, iodine; the grasses, silex, phosphate of lime; rhubarb and sorrel, oxalate of lime; leguminous plants, carbonate of lime; the Cruciferae, sulphur, etc.

833. The proportion of earthy matter is small and may be estimated from the ashes. As drying expels the water, so burning expels all other organic elements, and the inorganic earthy, whatever they be, remain in the form of ash. The following table from Bousingault is instructive on this point

Wheat

Oats

Yellow Peas.

Clover Seed.

Hay.

Turnips.

Potatoes.

Grain.

Straw.

Grain.

Straw.

46.1

48.4

50.7

50.1

46.5

49.4

45.8

42.9

44.0

Hydrogen...

6.8

5.3

6.4

5.4

6.1

5.8

5.0

5.6

5.8

Oxygen

43.4

38.95

36.7

39.0

40.1

35.0

38.7

42.2

44.7

Nitrogen....

2.3

.35

2.2

.4

4.2

7.0

1.5

1.7

1.5

Ash........

2.4

7.

4.

5.1

3.1

2.8

9.0

7.6

4.0

100

100

100

100

100

100

100

100

100

834. Agricultural chemistry. Since all these elements are found in plants, we infer them to be essential ingredients in the food which they require for healthy vegetation; and an inquiry into the sources from which they may be supplied constitutes the chief object of Agricultural Chemistry.

835. The food of plants is air, earth, and water. It is evident that plants do not create a particle of matter, and therefore do not originate in themselves any of the elements which compose them. Consequently they must obtain them from sources without. Carbon is derived from the carbonic acid contained in the atmosphere, and from the decaying vegetable matter of the soil. Oxygen is derived from the water and from the carbonic acid of the atmosphere; hydrogen from water and ammonia, and nitrogen from ammonia and nitric acid, drawn either from the atmosphere or the soil.

836. The atmosphere contains about 1/2500 part of carbonic acid, diffused throughout its whole extent; and as this gas contains 27 per cent. of carbon, it is demonstrable that the whole atmosphere contains more than 600 billions (600,000,000,000) of tons of solid carbon, derived from the sources already mentioned (§ 835),- an amount fully adequate to the demands of the vegetable kingdom.

837. Soil consists of two classes of materials, viz.: mineral, and organic. The former, called earths, consists of disintegrated and pulverized rocks, including all the various mineral substances which are found to enter into the composition of plants, as potassa, soda, silica, lime, etc, all of which are more or less soluble in water. The organic materials consist of the remains of former tribes of plants and animals mingled with the earths; and which, having access to the air, are decomposed, evolving carbonic acid and ammonia both to the air and the water.

838. Water is composed of oxygen and hydrogen (H O) in the proportion of 8 to 1 by weight, or one atom of each to each. Having pervaded the atmosphere in the state of vapor and rain, and percolated through the soil, it holds in solution carbonic and nitric acids, ammonia, and many of the various minerals above mentioned.

839. Ammonia consists of nitrogen and hydrogen combined in the proportion of one atom of the former to three of the latter (N H3). It arises from decaying animal and vegetable matter, as above stated, and is diffused through the atmosphere in the proportion of about 1 part in 10,000.

840. Nitric acid is also believed to yield nitrogen to plants. It consists of one atom of nitrogen to five of oxygen (NO3). During thunder-storms it is generated in the air by lightning and brought down by rain. When combined with the bases, as potassa, soda, etc., it forms nitrates - substances known to be efficient fertilizers in soils.

841. Air plants. Thus it appears that water, carbonic acid and ammonia (or nitric acid) may yield to plants their four essential organic elements. And since all of them are contained in the atmosphere, some plants are capable of subsisting on air alone (long moss, lichens); but most species are dependent on water, earth, and air, and demand a copious supply.

842. The external circumstances, therefore, first requisite to healthy vegetation are, - 1, free access to an atmosphere which is often agitated by winds; 2, a proper supply of rain or river water; 3, a soil possessing the peculiar minerals required by the species to be grown upon it, together with a certain proportion of vegetable mold.

843. The supply. The first of these is everywhere abundantly supplied by nature, and asks no aid from man. The second and third are often deficient, and are to be supplied by the labors of agriculture. By irrigation streams of water are turned from their natural channels to add to the scanty moisture of fields parched with drought; by drainage the inundated bog is converted into a luxuriant lawn.

844. The object of tillage is to pulverize and lighten the too compact soil; and thus expose every part to the oxygen of the air in order to hasten its decomposition. Subsoiling, or deep ploughing, is an operation whereby that stratum of earth which lies just below the ordinary soil is moved and subjected to atmospheric influence. The subsoil with less organic matter, contains often soluble fertilizing earths which may thus be rendered available for the use of plants.