THE CHEMISTRY OF VEGETATION.

Press, Volume XLIV, Issue 6924, 2 December 1887, Page 3

THE CHEMISTRY OF VEGETATION.

m ' The following extracts, of great importance as regards the culture of plants, are taken from Sir Henry Roscoe's recent Presidential Address to the British Association at Manchester:— " • . The phenomena ol vegetation, no less than those of the animal world, have, during the last fifty years, been placed by the chemist on an entirely new basis. Although before the publication of Liebig's celebrated report On chemistry and its appplication to agriculture, presented to the British Association in 1840. much had been done, many fundamental facts had been established, still Liebig's report marks an era in the progress of this branch of our science.

As a proof of this I may remind you of the attack which he made on, and the complete victory which he gained over, the humus theory. Vegetable physiologists up to 1840 contiaued to hold to 1 the opinion that humus, or decayed vegetable matter, was thc only source of the carbon of vegetation. Liebig came to the conclusion that it was absolutely impossible that tlie carbon deposited as .vegetable tissue over a given area, as, for instance, over an area of forest land, could be derived from humus, which is itself the result of the decay of vegetable matter. He asserted that the whole of the carbon of vegetation is obtained from the atmospheric carbonic acid, which, though only present in the small relative proportion of four parts in 10,000 of air, is contained in such absolutely large quantity, that if all the vegetation on the earth's surface were burnt, the proportion of carbonic acid which would thus be thrown into the air would not be sufficient to double the present amount. That this conclusion of Liebig's is correct needed experimental proof, but such proof could only be given by long-continued and laborious experiment, and this serves to show that chemical research is not now confined to laboratory experiments lasting perhaps a few minutes, but that it has invaded the domain of agriculture as well as of physiology, and reckons the periods of her observations in the field, not by minutes, but by years. THE KOTHA-IS-ED EXPERIMENTS. It is to our English agricultural chemists, Lawes and Gilbert, that w&owe the complete experimental proof required. And It is true that this experiment was a long and tedious one, for it has taken fortyfour years to give the -efinite reply. At Rothamsted a plot was set apart for the growth of wheat. For forty-four successive years that field has grown wheat without addition of any carbonised manure, so that the only possible source from which the plant could obtain the carbon for its growth is the atmospheric carbonic acid. Now, the quantity of carbon which on an average was removed in the form of wheat and straw from a plot manured only with mineral matter was 10001b, whilst on another plot, for which a nitrogenous manure was employed, 15001b more carbon was annually removed, or 2_oolb of carbon are reiyoved by this crop annually without tho addition of any carbonaceous manure, so that Liebig's provision has received a complete experimental verification. THE ABSORPTION OF ***-T_tOGE-_ Liebig's views on this point have not been substantiated. He Unagin-d that the whole of the nitrogen required by the plant "was derived from at-sospberic ammonia, whereas Lawes and- Gilbert have shown "by experiments of ft similar nature to those just described, and extending we* anearly equal length of time, that this sbur-e is wholly insufficient to account for the nitrogen removed in tbo crop and have come to the conclusion that the nit-ogen must have been obtained either from a store o£ nitrogenous material in the soil or by absorption of free nitrogen from the air. These two apparently contradictory alternatives may perhaps be reconciled by the recent observations of Warington and Berthelot, which have thrown light upon thechangoa which the so-called nitrogenous op-fa! of the soil undergoes, as weU »» *a*gon its chemical nature, for % h_e bx*"-e*. lias shown that under certatta «__-__£io__* the soil has the power ol absorbing the nitrogen of the air, foj-oing-OWpounds which can subsequently b_.assimilated by the plant.