Agricultural Chemistry.

Poverty Bay Standard, Volume IX, Issue 948, 1 June 1881, Page 2 (Supplement)

Agricultural Chemistry.

[By

Mr. R. W. E. Mclvor].

(Continued). Then there is a substance called chlorine, which you find in the soil combined with another element in the form of salt. These nine substances constitute together the ash of plants. Although all plants contain these substances, they do not all contain them in the same proportions, and, indeed, the different parts of some plants are characterised by containing these substances in different proportions. For instance, If I take a quantity of wheat straw and burn it, and estimate the proportion in 100 parts of the ash, I find it to be a little over 5 per cent., and if I analyse the ash of grain, I find it runs to 45 per cent. In some root crops we find the tops to be poor in lime, but if w T e analyse the potatoetop we find it particularly rich in lime, and poor in pot-ash, so that it has been said that while the tube of the potatoe is a pot-ash plant, the potatoetop is a lime plant. Without going further into this subject, I wish you to bear in mind that different plants and different classes of plants contain the constituent elements in dfferent proportions. All plants contain these nine mixtures I have named, but different classos are derived by the plant from the water that enters by the roots. The charcoal elements of water compose, I should say in round numbers, at least 90 per cent, of dried woods of plants, 45 per cent, of charcoal, and 45 per cent, of warer. There is 10 per cent, to be accounted for, and of that possibly 5 per cent, is nitrogen, or the flesh-forming constituent, and 5 per cent, will be represented by ash. Now we come to the question: Where do plants get these substances from ? First of a 1! there >is charcoal. The w r hole of the charcoal in our forests and in our farm crops is derived, not as was formerly believed, from the soil, but from the air. It is taken up in this way:— There exists in the atmosphere to the extene of some 4 parts in every 10,000 parts, a gas called carbonic acid, which is a similar gas to that which escapes from aerated waters and wines. This carbonic acid, when in a separate state, is a deadly poison, it will extinguish any burning body immersed in it, and any living thing placed in even per cent, of it will instantly perish. This poisonous gas is poured out from the lungs of animals in enormous quantities. Indeed one man will expel from his lungs in 48 hours an amount of this poisonous gas containing carbon that would be represented by a piece of charcoal about the size of a man’s fist, so that you can readily understand that there must be immense quantities of this sent into the air, and, therefore, there must be immense quantities of carbon. If you put a piece of chalk in a quantity of water and blow in it, you will find it very soon become quite milky. The cause of this is that quicklime is soluble in water to a limited extent, and chalk which consists of this lime I am speaking of, is insoluble in water. Lime has a great tendency to absorb carbonic acid when it comes in contact with it, and when you blow your breath through the lime-water, the carbonic acid in your breath unites with the other substance, and produces the effect I have explained. Well, as I said, not only is carbonic acid thrown out in immense quantities from the lungs, hut it is also a particular product of coal, vegetable and carbon, it is produced from vegetable substances, thrown up from craters, and hundreds of thousands of tons of carbonic acid are thrown into the air from one source and another every hour of our lives. If nature did not provide some means whereby this composition could be removed as rapidly as it is put into the atmosphere the probability is that our atmosphere would become so charged with carbon that animal life would be impossible. But under the influence of the sunlight, the green colouring matter of plants, their leaves and other green plants absorb this poisonous matter, separate it, put it into its constituents, and let loose the gas w'hich is united with the carbon, hold the carbon to build up their structure, and in this way the plants cleanse the air. Though some of you may say 4 parts of carbonic in every ten thousand parts is very small for plants to depend upon, I can assure you it is a great deal. There is floating up from the leaves and surface, at least, in the form of intangible air, a very large quantity of this element which is rejected. So much then for the charcoal of plants. lam going to Beet-root Sugar ■directly. Well carbon in plants is

derived from the atmosphere, and the removal of this carbon in crops has nothing to do with the exhaustion of our soils. The elements of carbon, which constitute so large a portion of the plants, come from the water, which is received through the roots. It was formerly believed that plants could take in water by their leaves, but recent scientific experiments have have demonstrated that plants do not take in water by their leaves, but from the roots. Having told you that charcoal comes from the air, and that oxygen, and hydrogen, the elements of w r ater, come from the air, you can easily see that starch, sugar, and vegetable substances of an analogous nature, being composed of these substances, are not built up by anything that comes from the soil, and, therefore, if you are taking off so many tons of sugar from your farm, you are actually sending away in the form of sugar, not what you have taken from the soil, but from the air. You may send away 100 tons of sugar, but nothing goes from the soil, and provided you return the waste of your crop, you can go on growing sugar for a thousand years without iu the slightest degree influencing the fertility of your soil. So much for the source from which the elements that compose soil are derived. I will now endeavor to explain what is The Nature of Agricultural Chemistry. The amount of nitrogen overlying every acre of land has been estimated at something like 32,000 tons, and nothing can be more evident than that plants would take up a small proportion of the elements they require from this immense store-house. It has been found that plants cannot avail themselves of nitrogen in the air, but take up this flesh-forming constituent from the soil, where it occurs as nitrous ammonia or saltpetre. Many elaborate experiments have been made in all parts of the world, and it is now generally admitted that plants obtain this nitrogenous constituent, or their combustible part from the soil, and not as Liebeg supposed, from the atmosphere. It is unnecessary for me to tell you that while the ash constituents are derived from the soil, that your plants will depend for the ashforming and flesh-forming constituents on the soil, or that in order to grow a scrop from the soil it must contain the constituents which together contain the plant food. Well, now I will say a word or two with regard to The General Character of Soils. Soils consist of three different classes of substances, first, substances which, without in themselves giving a plant food, serve to maintain the organical conditions necessary to growth of the plant. These organical conditions are retention of water, of moisture and warmth, and keeping the root in its place. This class of substances consists of sand, decomposed vegetable matter, and lime when it exists in great quantities. These serve purely mechanical purposes, and they constitute in round numbers about 95 per cent, of average soil, that is to say 95 per cent of average soil consists or substances which are of no use as food substances but are necessary for growth. The second class consists of unavailable or locked-up plant food, unavailable as it exists in the soil, but becomes available by fallowing. This exists in certain minerals, such as felt spar, hornblend, mica and others, the names of which you are not familiar with, so I will abstain from mentioning them. The third class are soluble, or available plant food constituents, and these constitute, even in the richest soil, not more than in round numbers—l will go so far as to say—one half per cent. Now I have mentioned that in order that a plant may grow it must have a supply of all the substances required to build up its structure, so that if in the soil there is absent one or other of the constituents required by the plant, then you cannot possibly obtain a crop, or even grass, until by some means or other that (deficient substance is supplied. For instance if you had a soil containing an abundance of soluable food except one, let it be phosphoric acid, you might be ever so careful about selecting seed or sowing, but could not possibly get a root crop, but the moment you put on phosphoric acid you would get an indication that your land would yield you a crop, so that this is proof positive that in order to obtain a crop from your land it must have within itself an abundant supply of all the substances that the vegetable world demands from the soil. (To be Continued.)