1S44.] 



THE CIVIL ENGINEKK AINU AKUHlTECrS JOURNAL. 



87 



AGRICULTURAL CHEMISTRY. 

 I By Professor Brande, F.R.S., &c. 



Lwturp III.— Detivfrcii at the Itoi/n! Inst'Uutim, Feb. 10, 1844. 

 (Sfecialli/ reported for this Journal.) 



If the agricultural chemist bad been asked. » few years aRo, what ttere the 

 essential ingredients of the soil, he would most assuredly have said that the 

 earths and the organic matter present were all tliat were important ; but 

 that the principal part of the nourishment was due to the organic matter, 

 and that the saline ingredients were of very liUle use. Now. however, he 

 would have quite a different story to tell, and it is principally to Liebig that 

 we are indebted lor a more correct view of the subject ; fur it is now proved 

 beyond doubt, that although the salts present in the soil may form a .small 

 per cenfage of the whole, yet they must not be considered as accidental, but 

 as being perfectly indispensable to the plant, which, according to its nature, 

 lakes up one or other into its circulation, and without which it could not 

 exist. By the saltt must be understood all the substances consisting of a 

 base unlli'd to an acid. The principal bases are potash, soda, lime, and 

 maonesia; these are always present in fertile soils. The .acids with which 

 they ar? generally in combination are the carbonic, sulpbiiric, and phos|phoric 

 acids, and frequently silica, so that when the chemist talks of Hint, 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 fretpienily 

 soluble salts, which is a better proof of being an acid than the action on the 

 tongue. Wlicn plants are burnt so as to destroy their organic pari, ibeir 

 saline constituents alorte are left, forming the ashes of plants, and the (|uan- 

 tity of ash varies greatly with diflerent plants and with diflerent parts of (he 

 same plant, ai 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 Elm leaves . . 120 



straw . . 60 Elm .... 20 Willow leaves . 82 



Oa(s 40 Willow ... 5 Beech leaves . 42 



straw . . 50 Beech ... 4 Birch leaves . 50 



An investigati(m tf the properties of the principal salts in the soil and 

 their components will make this part of our subject more mtclligible. And 

 first of their bases. These are metallic oxides, the metals of which were first 

 obtained in a separate state by Sir H. Davy. I'hey are named, respectively, of 

 potash, polassiuni, of soda, sodium, of lime, calcium, of magnesia, magnesium, 

 of baryta, barium, &c. Bnt potassium, which, i.s, 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- 

 ratingits hydrogen, which ignites from the heat evolved, and combining with 

 the oiygen, forms potash, which is instantly dissidved. The alkaline pro- 

 perty 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 tliey comljine 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 uscil 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 be borne in 

 mind. In order to get them in a proper state for testing, boiling water is 

 poured on to a portion of the .«oil, and then ihe whole poured on to ;i filter ; 

 the water running through carries away all the soluMe portions. If this be 

 then evaporated, the resulting salt will frequently indicate, by iis shape, 

 solubility, and behaviour in air, which 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 unulianged 

 by exposure i whereas if it be the sulphate of soila. it will be very soluble, 

 and by exposure to air, become covered with a while powder, or efllorescence, 

 as it is termed. This arises from il.s 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, arc tartaric acid 

 and chloride of platinum. \\'hen 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 abimdanlly a yellow ish brown deposit, consisting 

 of the double chloride of platinum and polassium. 



.Some plants absorb but little alkali from Ihe 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 tins is the grape, which appro- 

 priates to itself abundance of potash, which it deposits from its juice in fer- 

 menting, as salt o( 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, o« ing 

 to the presence of binox.rlate of jxitash ; 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 Ihe bases of the inorganic acids. 

 Silicic acid or silica has already been touched upon. Sulphur, the base of 

 ■sulphuric acid, familiar to every one as brimstone, is found in nature both 

 free and in combination ; free, in abundance in Sicily, and in combination, 

 plentiful in our own islands. With inm 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 clid's 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 ajar 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 ditsolved by the water, 

 forming a weak solution of oil of vitriol. Now this is remarkable for its fixily. 

 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 18 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 Ihe most sterile of land. But as will be shown hereafter, some 

 plants possess the properly 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, Ihe essential oil of the mustard, and the 

 radish, in which there is a consideralile qiiantttv of sulphur. But decaying 

 vegetables will do the same, the sulphur in this case combining with the 

 hyilrogen which ia being given off. and forming the oll'ensive gas. sulphuretted 

 hydrogen, familiar to all who have smell a loul gun barrel, or a rotten egg. 

 It is to thisdecomposlion is due thenausious smell of water in which vegeta- 

 bles have been boiled, and is continually taking place at the mouths of rirers, 

 which empty into the sea vast quantities of rotting vegetable matter, which 

 there meets with the sulphates in the sea water, and Ihe decomposition take* 

 place. Ships anchored in such situations have their copper corroded oil in 

 one-half the usual time, and to Ihe same cause is also attributeil the un- 

 heallhiness of certain African rivers. But altliough 5ul|ihuretlel hydrogen 

 is known to be very destructive of lifi' w lu-n 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 woidd seem to c.uifirm this, for, from ilslieingso 

 much used as a test, ho 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 influeiiees present in Ihe air of these shore?, 

 and nothing seems more probable than that it is due to certain dtcompusing 

 organic particles, acting on the blood in the manner of a ferment. The best 

 lest for its presence, either in solution, or in the air, is a solution of sugar of 

 le.ad. which it bl.ackens 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 Fl. 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 glulen of wheal, 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 [ilants. I'Vom this it is seen that the 

 alkaline sulphates are frequently doubly useful in the soil, as being the 

 source of alkali and also of sulphur. Their presence in solution is readily 

 ascertained Ijy baryta dissolved in nitric or muriatic acids, which forms the 

 very insoluble while sulphate of baryta, not redissolved by nitric arid. 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. 



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