WHAT FARMERS OUGHT TO KNOW.

Temuka Leader, Issue 1135, 5 February 1884, Page 2

WHAT FARMERS OUGHT TO KNOW.

AGRICULTURAL CHEMISTRY. ( Written specially for this Paper.) i. Agricultural Chemistry is one of the most entertaining and practical of sciences. It is one that the tiller of long cultivated soils should have a thorough knowledge of ; it is one that is useful in the highest degree to the farmer of new lands. It is an error to think that the study of this science leads but to a knowledge of the application of manures. It teaches many things besides —matters that should be of great interest to the colonial husdandman, though he may not need to trouble himself with manures for a good while to come. One of the great stumbling blocks in the way of the study of this highly interesting subject is the scientific manner in which the majority of the treatises on it are written. And here we may mention that in this and in the following few articles we have followed the arrangement of a treatise on the subject by Alfred Sibson, F.C.S., since it appears the most natural and lucid. To understand some of the books on the science, a knowledge of Chemistry itself, and of kindred sciences is very often necessary. We purpose to deal popularly with a few of the great leading facts and principles of Agricultural Chemistry, such as the formation of soil, the changes that go on during the growth of the plant, the reaction between the atmosphere and itself that takes place during the plant’s life, the chief division of the constituents of plants, the exhaustion of the soil, the methods of re-fertilising it and how they are to be applied. To take but a cursory glance at these important points, some knowledge of the elements and compounds that exert the greatest influence in the various reactions and changes is absolutely necessary. The consideration of these substances ought to be very interesting, as it lays open many of the phenomena which to the uninitiated seems unfathomable, and a knowledge of their properties is quite indispensable to understand the subject further on.

We shall first remark a few of the properties of the atmosphere, and after that pass on to its constituents. ATMOSPHERE. The atmosphere that surrounds our globe extends to a distance of between forty and fifty miles above us. It is transparent, invisible, colourless, inodorous, elastic, and destitute of taste. It is mostly composed of two gases, oxygen and nitrogen. Nitrogen occupies about four-fifths, while oxygen, with small traces of water in the state of vapour, ammonia, nitric acid and carbonic acid occupy the remaining fifth part. By measure the air has the following percentage composition ; —79 parts nitrogen and 21 parts oxygen, carbonic acid, nitric acid, ammonia and aqueous vapour. Air exerts a pressure of about 151bs on the square inch, but since it presses equally on all sides we do not perceive the burden we are constantly carrying. Air like all gases, and mixtures of gases, has a tendency to penetrate and fill every hole and corner, thus “ abhorring a vaccjum.” In consideration of its weight and expansive energy we are not surprised to find it penetrating the soil, and in so doing exerting a great influence on the soil and growth of plants. The atmosphere is said to be a mechanical mixture and not a chemical compound, and here we must pause to note the distinction.

A mechanical mixture is a mere mingling of two or more materials which do not unite or combine. The individual characters of the substances mixed, remains. A mechanical mixture only can be made of sand and sugar, they cannot be made to unite and form a substance differing from either of the constituents. Gunpowder is a good example of a mixture, it is an intimate blending of charcoal, saltpetre and sulphur. These separatematerials are ground up together and go through various mills in order to ensure their complete mingling. To the naked eye gunpowder appears a single compound, but by the microscope and chemistry it is readily proved that the constituents have not combined. In a chemical compound, however, we have the atoms of the different substances uniting in such a manner that they cannot by ordinary mechanical means be broken up, and so that the union produces anew substance quite d : stinct in its properties from the elements constituting it. Thus the individual characters of the components of the compound are entirely lost in the new substance that is the product of the union. Thus water is a chemical combination of two gases, oxygen and hydrogen, the union of two gases in this case producing a liquid. Again, common salt is known to chemists by the name of chloride of sodium, that is, a compound composed of a gas chlorine and metal sodium; each of these elements when existing by itself is a poison, but when the two unite they form a material, salt, one of the most necessary and beneficial substances in the food of man. Air, it must carefully be remembered, is a meie mixture of the gases, oxygen, nitrogen,etc., and when the air exerts any chemical influence, the oxygen, ammonia, nitric acid and other constituents act separately, and each on its own account. We now pass on to rapidly glance at the gases constituting the atmosphere, and first of all we shall consider the life giver, OXYGEJT. This gas oxygen is one of the most abundant elements on the face of the earth. It forms, as we have seen, onefifth of the atmosphere, it exists also, in combination with a great many metals, in rocks, stones, soil, etc.; of water, it forms eight parts in every nine, threefourths of the weight of animals is composed of oxygen, and of vegetables it forms a very important part. When the gas is combined with a metal the resulting product is called an oxide of the metal. In many cases the name of the oxide is easily formed by a slight change in the name of the metal. The most common chaugo is that of the forming the end “ nm ” of the name of the metal into <l «.” Thus metal magnesium forms the oxide of magnesium, magnesia ; again metal aluminum : corresponding

oxide, alumina; metal potassium : corresponding oxide, potassa, changed still further to potash ; metal sodium : oxide, soda.

When a chemical union takes place between two substances, heat is invariably produced, and the amount of heat produced is always in direct proportion to the rate of chemical action taking place. The union of oxygen vvitli various substances accounts for the phenomena of heat and combustion. In a fire th union of this gas with the constituents of the coal causes the sensation of heat and light. The gas from the air combines principally with the carbon and hydrogen in the coal to form two important substances, respectively carbonic acid and water. It is the elements rushing into chemical combination that makes the fire. When iron is rusting slow combustion is going on—that is a slowly progressing chemical union of oxygen and iron. A great rise in heat during rusting can be noticed. When some of the Atlantic cables were lying up for use, oxidation (the uniting of oxygen and the metals) took place rapidly, and an extraordinary rise in temperature in the iron was registered. The evolution of heat by chemical means is carried out on a grand scale in man and the animals. Living things are constantly inhaling air, the oxygen of which, in the lungs, effect a union with carbon and hydrogen, the products being, as we have seen before, carbonic acid and water. These products the system is relieved of by exhalation. It is by this means chiefly that the heat of the body is kept up —by the chemical combination of oxygen with certain elements in the lungs. Thus, deprive the animal of the atmosphere or oxygen, it at once becomes cold and dies.

We have thus seen what a vast part oxygen plays in the economy of nature, how it unites with a very great many substances to forms a large body of useful compounds called oxides ; how without it we could have no fires, and how we ourselves would die, if deprived of it for any length of time. After considering oxygen, we shall pass on to consider nitrogen and one or two other things still more briefly, and thus the parts dealing more specially with soils, plants, food, etc., may be readily understood. {To he Continued.)