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THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 87 1844.] AGRICULTURAL CHEMISTRY. | By Professor Brands, F.R.S., &c. Leoture III.—Delivered at the Royal Institution, Felt, 10, 1844. (Specially reported for this Journal.) If the agricultural chemist had been asked, a few years ago, what were the essential ingredients of the soil, he would most assuredly have said that the earths and the organic matter present were all that were important; but that the principal part of the nourishment was due to the organic matter, and that the saline ingredients were of very little use. Now. however, he would have quite a different story to tell, and it is principally to Liebig that we are indebted for a more correct view of the subject; for it is now proved beyond doubt, that although the salts present in the soil may form a small per centage of the whole, yet they must not be considered as accidental, but as being perfectly indispensable to the plant, which, according to its nature, takes up one or other into its circulation, and without which it could not exist. By the salts must be understood all the substances consisting of a base united to an acid. The principal bases are potash, soda, lime, and magnesia; these are always present in fertile soils. The acids with which they are generally in combination arc the carbonic, sulphuric, and phosphoric acids, and frequently silica, so that when the chemist talks of flint, he some times speaks of it as an acid, which it really is; for although not sour t» the taste, being insoluble, it combines with bases, forming neutral and frequently soluble salts, which is a better proof of being an acid than the action on the tongue. When plants are burnt so as to destroy their organic part, their saline constituents alone are left, forming the ashes of plants, and the quan tity of ash varies greatly with different plants and with different parts of the same plant, as will be evident by inspecting the following table Quantity of Ash in 1000 parts of Hay . . . . 90 Potatoe . . . 40 Birch . . . . 3 Red Clover . 77 Turnips . . . 70 Oak . . . . 2 Wheat . . . . 12 ,, leaves . 130 Kim leaves . . 120 ,, straw . . GO Elm . . . , , 20 Willow leaves . 82 Oats .... . 40 Willow . . . 5 Beech leaves . 42 straw . 50 Beech . . . 4 Birch leaves . 50 An investigation »f the properties of the principal salts in the soil and their components will make this part of our subject more intelligible. And first of their bases. These are metallic oxides, the metals of which were first obtained in a separate state by Sir H. Davy. They are named, respectively, of potash, potassium, of soda, sodium, of lime, calcium, of magnesia, magnesium, of baryta, barium, &c. Bnt potassium, which, is, perhaps, the most easily obtained, may be taken as the type of the class. It is a white metal, like silver, lighter than water, which is also the case with sodium. When thrown into water it runs over the surface, decomposing it with great rapidity,libe* ratingitshydrogen, which ignites from the heat evolved, and combining with the oxygen, forms potash, which is instantly dissolved. The alkaline pro- jierty of the solution, may he rendered evident by its action on vegetable co lours, turning yellow to brown, and frequently red to blue. If acids be added they will combine with it, forming neutral salts, which may be obtained by evaporation. The other alkaline metals go through the same process, al though none so energetically as potassium ; though sodium approaches very nearly to it in this respect. The proportions in which they combine are, 40 parts potassium to 8 oxygen, producing 48 potash, or 24 „ sodium to 8 „ 32 soda. From these figures it will be evident that wherever soda can be used as a substitute for potash, 321b. would do the work of 481b. of potash. As it is very important to the agriculturist to ascertain whether a soil con tains salts of potash or of soda, the distinguishing tests must he borne in mind. In order to get them in a proper state for testing, boiling water is poured on to a portion of the soil, and then the whole poured on to a filter ; the water running through carries away all the soluble portions, if this be then evaporated, the resulting salt will frequently indicate, by its shape, solubility, and behaviour in air, w hich base it contains. They are generally in combination with sulphuric acid, and if it be the sulphate of potash pre sent, it will be found to be very slightly soluble, and remaining unchanged by exposure i whereas if it be the sulphate of soda, it will be very soluble, and by exposure to air, become covered with a white powder, or efflorescence, as it is termed. This arises from its giving up to the air some of the water which it had combined with when crystallizing, and so falling into a white powder. The tests most commonly used in the laboratory, are tartaric acid and chloride of platinum. When the former is added to a solution containing soda, no precipitate is produced; but if to one containing potash, a very copious crystalline precipitate is produced of bi-tartrate of potash, or as it is commonly called, cream of tartar. When there is very little potash present, t forms very slowly, but it may be hastened by rubbing the sides of the vessel with a glass rod, when the crystals are deposited on the parts where the rod has rubbed, as though a little tickling coaxed the solution to deposit its crystals m re rapidly. With the chloride of platinum, soda gives no pre cipitate, but potash yields abundantly a yellowish brown deposit, consisting of the double chloride of platinum and potassium. Some plants absorb but little alkali from the soil, whilst others take an immense quantity. Amongst the latter is the common wormwood, which impoverishes a soil of its alkali in a very short time. Indeed, so well known is that, that it has, for years past, been collected and burnt, and its ash,, known as salts of wormwood, applied to many purposes on account.of the quantity of alkali it contains. Similar to this is the grape, which appro priates to itself abundance of potash, which it deposits from its juice in fer menting, as salt of tartar. The alkalis are seldom found combined with car bonic acid, for although they are so in the ashes of plants, it arises from the decomposition by heat of other organic acids, they being converted into car bonic acid. In the wood sorrel, for instance, the juice is intensely sour, owing to the presence of binoxalate of potash ; but after being burned, the oxalic acid is all decomposed into carbonic acid, the whole of the salt having be come carbonate of potash. But it will be Interesting here to notice the bases of the inorganic acids. Silicic acid or silica has already been touched upon. Sulphur, the base of sulphuric acid, familiar to every one ae brimstone, is found in nature both free and in combination ; free, in abundance in Sicily, and in combination, plentiful in our own islands. With iron it is exceedingly common as iron pyrites or sulphuret of iron-, recognized in coal by its bright yellow colour, and washed out of our chalk cliffs in rounded masses of almost every size, which are commonly looked upon as thunderbolts. When sul phur combines with oxygen, it forms sulphuric acid, which takes place spontaneously when iron pyrites is exposed to air and moisture. This acid may be formed artificially on a small scale by immersing a lighted mixture of sulphur and saltpetre (nitrate of potash) into a jar of oxygen gas standing over water; the sulphur then burns with a beautiful blue flame, combines with the oxygen, and forms sulphuric acid, which is dissolved by the water, forming a weak solution of oil of vitriol. Now this is remarkable for its fixity, so that it may be placed in a proper vessel over the fire, and the water boiled away, leaving the sulphuric acid. This is the method commonly employed in the manufactories for strengthening it. The acid consists of 16 parts of sulphur, 24 of oxygen and 9 of water, forming 49 parts of the strongest oil of vitriol. This acid is very rarely found free in the soil, as its noxious properties would make it the most sterile of land. But as will be shown hereafter, some plants possess the property not only of separating the acid from its alkili, but even of separating from it the sulphur, which it employs to form new combinations, as for instance, the essential oil of the mustard, and the radish, in which there is a considerable quantity of sulphur. But decaying vegetables will do the same, the sulphur in this case combining with the hydrogen w hich is being given off, and forming the offensive gas, sulphuretted hydrogen, familiar to all whp have smelt a lout gun barrel, or a rotten egg. It is to this decom postion is due the nauseous smell of water in which vegeta bles have been boiled, and is continually taking place at the mouths of rivers, which empty into the sea vast quantities of rotting vegetable matter, which there meets with the sulphates in the sea water, and the decomposition takes place. Ships anchored in such situations have their copper corroded off in one-half the usual time, and to the same cause is also attributed the un healthiness of certain African rivers. But although sulphuretted hydrogen is known to be very destructive of life when present in considerable quantity, it is doubtful whether it is so injurious to man when in the minute quantity which it must be in the open air, even in the worst situations ; the daily ex perience of the chemist would seem to confirm this, for, from its being so much used as a test, he is continually breathing an atmosphere sensibly im pregnated with it, and yet with impunity, as it has never been known to produce any effects analogous to the eastern fevers. To other causes, then, must be attributed the contagious influences present in the air of these shores, and nothing seems more probable than that it is due to certain decomposing organic particles, acting on the blood in the manner of a ferment. The best test for its presence, either in solution, or in the air, is a solution of sugar of lead, which it blackens even if present in a very minute quantity, producing sulphuret of lead. Though sulphuretted hydrogen is undoubtedly very per nicious to animal life, it is not so to plants, and its solution in water has been used with advantage even in horticulture, by Sir E. Solly. Indeed it is essential that many plants should be supplied with sulphur in some shape or other, as they require it to assist in forming some of their constituent parts. The gluten of wheat, for instance, could not be formed without it, and it is essential to the mustard, cabbage, turnip, water cress, and indeed to the whole of the large class of cruciferous plants. From this it is seen that the alkaline sulphates arc frequently doubly useful in the soil, as being the source of alkali and also of sulphur. Their presence in solution is readily ascertained by baryta dissolved in nitric or muriatic acids, which forms the very insoluble white sulphate of baryta, nut redissolved by nitric acid. By this means it is proved that whereas in wood ash the alkali is present as car bonate, in coal ash it is as sulphate, which is therefore a good top dressing for many crops. 9