ORGANIC and amino acids. CONTAINING NITROGEN. COMPOUNDS( mss chlorophyl and coloring matter.
Carbohydrates, including cellulose, starch and sugars, or fats or proteins make up most of the dry matter of plants. Except for cer tain terpenes which contain only carbon and hydrogen the organic compounds all contain carbon, hydrogen and oxygen, and some also contain nitrogen. Sulphur occurs mainly in one of the amino acids which are the constituents of proteins. Phosphorus occurs in conjugated proteins and also as inorganic and organic phosphates. Potassium and calcium are present as salts of inorganic and of organic acids. Iron, magnesium and manganese occur in or ganic combination, magnesium being present in the green pigment chlorophyl. Sodium, chlorine and silicon are also found in most plants, but they do not seem to be essential to plant life.
The Air.— The growth of plants is greatly influenced by temperature and rainfall, both of which arg controlled by the atmosphere. The nitrogen of the air is one of the elements essential for plant and animal life, but it can be utilized only when it is in combination with other elements. Except for a small amount which is combined with oxygen during lightning discharges and brought to the earth by the rain, and that which is now artificially fixed by chemical processes and added to the soil in commercial fertilizers, all nitrogen that is used by plants has been fixed or combined with other elements by micro-organisms which are present in fertile soils and especially in the nodules on the roots of leguminous plants, or by certain very low forms of plant life. The nitrogen is then taken from the soil by the roots of the plant in the form of nitrates. The oxygen of the air oxidizes organic matter in the life processes to furnish energy and also oxidizes waste matter during decomposition. The main products of complete oxidation are carbon dioxide and water. The source of all the carbon in organic compounds in plants is, with few exceptions, the carbon dioxide of the air. In the presence of chlorophyl, the green pigment of plants, and under the influence of light, carbon dioxide and water, are converted into organic compounds which occur in plants. During this process a volume of oxygen equal to the volume of the carbon dioxide is returned to the aid.
The Soil.— Plants obtain from the soil, in addition to nitrogen which comes originally from the air, water and inorganic salts con taining phosphorus, sulphur, potassium, calcium, magnesium, iron and manganese. Soil is formed by the disintegration of the rocks which make up the earth's crust, mostly by the me chanical and chemical action of water and air, and the elements necessary for plant food are made available by chemical decomposition of the mineral compounds of which the rocks are composed. The soil then consists of decom posed rocks, mainly in the form of sand, clay and limestone with small amounts of unchanged minerals and of the salts necessary for the growth of plants. In addition to these, an important constituent of fertile soil is humus, which is decaying organic matter, either of plant or animal origin. Humus benefits the soil both on account of its physical and its chemical properties. It contains the elements required by plants, which are made available for plant food during decomposition, and it is able to retain ammonia and some other bases, thus preventing their loss from the soil by leaching. It has a lower density, a higher specific heat and a greater capacity for holding water than the other constituents of soil, thus making a moist warm soil in which the air can circulate. These conditions are most satis factory not only for the growth of plants but also for the action of nitrifying bacteria which oxidize different forms of nitrogen into nitrates which can be utilized by plants. When the air is excluded from the soil either because the soil is too compact or because it is not well drained, the conditions are more favorable for the action of denitrifying bacteria which reduce nitrogen compounds to free nitrogen which is then lost as a plant food. An important prop erty of fertile soils is the ability to retain plant food by forming insoluble compounds which are not washed out by water but which are soluble in the plant juices. In addition to the retention of ammonia by humus, which is men tioned above, phosphates, potash and ammonia are retained by clay, the phosphates in com bination with oxides of iron and aluminum, the bases in combination with complex silicates.