AGRICULTURAL CHEMISTRY.

Otago Witness, Issue 2104, 21 June 1894, Page 11

AGRICULTURAL CHEMISTRY.

A SERIES OF LECTURES DELIVERED BY PROFESSOR BLACK AT THE OTAGO UNIVERSITY! Vl.— Soda Salts.

Of sodium, there are only two compounds of any interest in agriculture. These are common sarfc (chloride 'of sodium, NaCl) and Chili saltpetre (nitrate of sodium, NaNO 3 ). Both sodium and chlorine are found in the ash of all cultivated plants. Common salt, which is made up of these two elements, may therefore be regarded as part of their mineral food. They may, however, get the sodium from other sources, as there is abundance of this metal in all rocks and soils. Ihe sea is the great storehouse of salt. Every gallon of sea water contains about 4oz of it It is constantly being washed out of the soil by rain water, and out of the rocks by springs, carried thence into the creeks, and away by the rivers into the sea Ihe sea water again, by the heat of the sun and the blowing of the winds, is continually being evaporated .(rising as invisible gas or vapour) into the air, there forming clouds and descending as rain to levy another contribution of salt for the great sea reservoir. The sea water is thus becoming richer in salt as time goes on. The same thing is well seen in the great inland lakes of Asia— the Caspian Sea, Lake Baikal, the Dead Sea, &c. — into which rivers flow, but out of which no water goes except by evaporation. Every gallon of water flowing into the ocean itself or into these inland seas contains an average of a grain of salt. It does not, therefore, require any stretch of the imagination to see that as salt is always coming in and none going out (the quantity of water always remaining the same) it is merely a question of time when such inland seas will become reservoirs of brine. In this way the sea is continuously sending forth its waters to levy contributions of salt from the land, which it never returns till, by the receding of its waters or by slow elevation, its own bed, with all its spoils, is raised into continents and islands. It was, of course, by this accumulating process that the salt licks of California, Mexico, Abyssinia, and Australia were formed— namely, by the long-continued inflow of water containing a little salt, the consequent saltening and subsequent drying up of the confined water. It would not, however, be quite correct to say that no salt ever leaves the sea through the action of the winds. It is true that no salt is carried away by evaporation of sea water ; but there is a mechanical action that returns some of it to the land. It is this : The drifting spray that is lifted off the crests of the waves, by high winds consists, of course, of millions of little droplets of salt water, 1 each containing a very small modicum of salt. This spray is swept away on the wind, and, drying up as it goes, each droplet of it leaves a tiny skeleton of salt, which, being porous and light, is wafted up into the air, and away inland, there to descend quietly in calm weather, or to be washed down by the rain. In this way quite enough salt is supplied to land within, say, 100 miles of the sea. It is well enough known that cattle pasturing near th& sea, where this atmospheric salt supplies all their . needs, do not "require to have recourse much to rock salt laid for them ; while those in the interior of the large continents, beyond the reach of the air-carried salt, will travel many miles to the salt licks which Nature has there provided. The function of salt as a manure is not well understood. It is questionable whether as a direct fertiliser it has any value at all. Indeed, in large doses (scwt to Bcwt per acre) it is known to check vegetation, and thus prevent grain crops in rich soil from running to straw and getting laid in wet seasons. For the same reason it is used in large doses for killing weeds ; but, of course, it does not stop at weeds, but kills also any plants in the young and ' tender stage. For killing weeds it should therefore be applied when there is nothing else of any value to kill. It has also a bad name for diminishing the proportion of starch in potatoes, thereby making them waxy instead of mealy. One service, however, it does render, especially when applied to new land just being broken up ; the clod or sod of such land contains a good deal of lime, potash and magnesia in an insolule state, being combined with silica, and the sodium of the salt liberates these fertilisers and presents them to" the plant in a soluble condition. Salt may, therefore, be said, in this restricted sense, to give potash to plants Kxcept for the purpose of hastening the breaking up of new cloddy land and destroying noxious weeds, I do not think there is any part of New Zealand so far from the sea as to require the addi ion of salt to the soil. A very different account has to be given of the function of nitrate of soda or " Chili saltpetre," NaN0 3 . This (as in the case of all nitrates) is one of the forms in which that king of fertilisers, nitrogen, is given to the soil. It is found in immense quantities as a saline deposit spread over a wide area in the tropical rainless districts of North Chili, Peru, and Bolivia, on tho western seaboard of i?ouih America.

Its origin there is unknown. Probably it is the result of microbe action on the nitrogenous matter of dead fish, seaweeds, and other marine animals and plants in the preseu r e of sea salt, the nitrogen furnishing the nitric acid and the salt the sodium, of which the nitrate of sodium consists. Such a transformat ion could not, of course, take place except in a hot, dry climate and on a rich, organic, and saline deposit such as would result from the isolation of a body of sea water and its subsequent drying up. The temperature of such a tropical region wo -Id just suit the microbe, and the rainless character of the climate would prevent the after removal of the nitrate by solution. , In these nitrate beds the saltpetre is by no means pure, being associated with from 35 to 70 per cent, of useless salts, chiefly sulphate and chloride of soda. It has therefore to be purified before it is sent into the market. This is done by dissolving it in hot water and crystallising out the impurities by boiling down. In general appearance nitrate of soda is not unlike common salt, kainit, and other cheap and ess valuable salts. It would therefore be desirable that the farmer, when he is taking any quantity of it, should get an analysis The accepted standard contains at least 95 per cent. of the pure nitrate. Anything less than this either has not been properly purified from the crude deposit, or has beeu adulterated. The most common adulterant is common salt, of which up to 65 per cent, has been found in English samples. There are nearly half a million tons of this salt exported to Europe every year, whilst the American continent consumes about the same quantity. As already said, it is for its nitrogen that Hie fertiliser is valuable. Its formula is NaNO 3 , and it contains 14 parts qf nitrogen combined 23 parts of sodium and 48 parts of oxygen.

In other words, 851b of it contain 141b of nitrogen, 231b of sodium, and 461b of oxygen. It contains therefore a shade less than 16£ per cent, of nitrogen. Compared with other nitrogenous manures (ammoniacal salts, &c.) it comes out as follows :—: —

It the case of common saltpetre, however, the value of the potassium, being very considerable, must not be overlooked in the comparison. Neglecting the sulphur, the other manures named do not contain anything of value except the nitrogen. Excluding nitrate of potash, therefore, the value of these manures is nearly proportional to the percentage of nitrogen (shown in this table) which they contain. In the case of nitrate of potash, it has to be considered that this salt contains 46£ per cent or 46£ units of potash, and worth about 5s per unit per ton. The potash of a ton of nitrate of potash, therefore, is worth about £11 10s besides the value of its nitrogen ; and the other nitrogen manures named above have nothing to show as an equivalent for this. All the leading authorities on agriculture in England, Europe, and America (Lawes and Gilbert, Voelcker, Griffiths, Storer, Stutzer, Marcker, &c.) unite in proclaiming the superiority of nitrate of soda over sulphate of ammonia and muriate of ammonia, all taken in quantities containing the same amount of nitrogen. Such quantities would be 1001b of nitrate of soda, 781b of sulphate of ammonia, 631b of muriate of ammonia, as each of these contains 16£lb of nitrogen. The agricultural authorities named have published as the results of field experiments in favour of nitrate of soda that (1) in dry weather and on dry land it is better than sulphate of ammonia for all crops ; (2) that it is a specially good manure for barley and potatoes ; (3) that it reduces the organic matter of raw soil more rapidly than any of the ammoniacal salts, thereby enriching the land with organic nitrogen in an available form ; (4) that it takes particularly kindly in connection with guanos, bonedust, and other insoluble phosphates ; (5) that it does not tend to exhaust the soil by forcing out of it too much mineral matter, inasmuch as the stimulated growth is at the expense, not of potash, phosphoric acid, and lime, but of nitrogen, which itself supplies to the soil and procures by decomposing dead roots and other useless plant debris. To these good points should be added (1) that for supplying nitrogen on lime-

stone soil it should always be used instead of am- ' moniacal manures, since limestone expels ammonia from its compounds ; (2) that nitrate of soda should not be used in conjunction with superphosphates, since in hot weather the free acid of these would be liable to expel nitric acid from it, thus removing part of its valuable nitrogen.

Dr Stutzer, writing on ' ' Nitrate of Soda as a Manure," and quoted by Dr Griffiths, summarises the results of hundreds of experiments performed by different scientific agriculturists in England and on the Continent by saying that the crops most benefited by it are (1) the cereals (wheat, oats, barley, &c 1 ; (2) rape ; (3) beets and potatoes ; (4) meadow grasses ; (5) leguminous plants— such as clover, peas, beans, lucerne, &c. ; (6) linseed Dr Griffiths, in his " Treatise on Manures," gives the following results of field experiments to show (1) The merits of nitrate of soda as against the ammoniacal salts ; and (2) the advantage of applying the nitrate in instalments as a top dressing instead of all at one time.

These experiments were tried on one acre of land in each case. The wheat land is described as clay land of good qua'ity ; the potato land as sandy loam ; and the clover land is not described.

In looking at these results one is struck with (1) the comparative failure of nitrate of potash all through ; (2) the marked success of the nitrate of soda against all competitors ; (3) the very pronounced advantage of applying the nitrate of soda in instalments instead of putting it on, as is usually the practice, at once with the seed.

I do not know why Dr Griffiths did not give to each competing acre the same quantity of nitrogen in these trials. The nitrate of soda contains 27 51b of nitrogen, the nitrate of potash 31 -Jib, the sulphate of ammonia 23 71b, and the muriate of ammonia 29 - 3. It may have been the market price of these manures that guided him — putting on the same market value of each— but it would, I think, have been more

satisfactory had he used in the manures tried against each other the salhc fixed quantity of nitrogen, which was really the subject of his experiments. To rectify this, instead of putting on 2cwt of nitrate of potash and lewt each of sulphate and muriate of ammonia against his l^cwt of nitrate of soda, he should have applied 1981b of nitrate of potash, 1301b of sulphate of ammonia, and 1051b of muriate of ammonia, for these are the quantities that contain the same amount of nitrogen as l^cwt (1681b) of nitrate of soda.

Potash. — A high value is justly put on the manurial salts of potassium These rank on a slightly higher level, in this respect, than even soluble phosphate of lime, and are exceeded only by the nitrates and the salts of ammonia. Potash is contained in all soils, ranging from two parts in 1000 to two parts in 100. Ihe rocks of our mountain ranges contain it in still more abundance. Some of the felspars contain as much as 16 per cent of this compound, the micas from 1 to 3 or 4- per cent., and the basalts and trachytes of Dunedin, Otago Peninsula, Banks Peninsula, and of Lyttelton about 3 j>or cent It is, of course, contained in the ashes of all the cultivated plants, and is therefore required for their food. • 'Ihe proportion of potash in the ash suggests the kinds of plants that need ie most and will guide the farmer in its application At the same time L should not be* overlooked that the amount of it already in the soil varies widely, so that it may oftenhappeu that money is spent on potash, kainit, &c , which is put on land that already contains an ample supply of that ingredient. Only an analysis of the soil will show in the case of potash, as well as everything else, what ingredients are abundantly there already, and what require 1o be supplied to make up for its deficiencies and strengthen the weakest link in the chain The following may bo taken as representing the amount of ash and of potash in the more common crops— wheat, oats, barley, potatoes, turnips, mangels, clover hay, peas, and beans.

For milk it would work out this way : Suppose one cow yields SOOgal of milk per annum, weighing, say, a little ovev 80001b. This would give 681b of ash, including 161b of potash. These results show at a glance that root crops

— turnips, mangels, and potatoes (including, though erroneously, potatoes among roots for the present)— require a great deal of potash, and would rapidly remove that constituent from the soil. Potash should therefore figure well in special manures for these crops. Next to root crops we have the leguminous crops — peas, clovers, beans, lucerne, lentils, &c. ; and after them the cereals — wheat, oats, barley, maize, and other grain crops, all of which still require potash, and remove it from the soil. In dairying also we send away potash as well as phosphorus and lime in the milk and the cheese. The very highest English authorities, Lawes and Gilbert, give potash as the specialty for leguminous plants, phosphates for turnips, and active nitrogen (nitrates and ammonia salts) for grain.

The sources of potash for manures may be summarised rs follows: — (1) Wood ashes, (2) common saltpetre or nitrate of potash, (3) muriate of potash, (4) sulphate of potash, (5) kainit, carnallite, and sylvin, all derived from the Stassfurt deposits in Prussia, and (6) carbonate of potash, derived from various sources

Concerning each of these a few words of explanation will be required.

Wood ashes. — Not only do plants vary widely as to the quantity of ash they yield, but even in the same plant there is quite as great a difference in the amount as well as quality of the ash got from various parts of it. The leaves, roots, and branches, for example, yield, weight for weight, at least 10 times as much ash as the heartwood. # The ash of young plants, also of shrubs - and, indeed, of all small plants - and of the leaves, roots, bark, and branches of trees, is richer in potash than that of hard, largo timber. The amount of ash, taking plants in the average, may be put as probably 1£ per cent. — that is, 100 tons of mixed timber of all sizes will yield on burning one ton and a-half of ashes. Wood ashes contain from 5 to 15 per cent , or an average of, say, about 8£ per cent, of pure potash. The ashes of 100 torts of timber, therefore, may yield about 2£ cwt of that fertiliser. From the origin of sawdust I would not expect quite so much potash as this. Of course, when bush country is burned, or a considerable proportion of the branches and crops of the trees are allowed to decay on the ground, there is a very valuable contribution of potash, and sometimes of phosphates and lime, made to the surface soil, brought from below, by the far-reaching roots. It is in the form of carbonate of potash (K 2 CO 3 ) that this valuable nutrient is found in the ashes. It is quite soluble in water, and therefore instantly available for plant food This high degree of solubility, however, is often the cause of its speedy removal from the hard

sloping surface of the land by rain water washing it away into the creeks and off to the sea. It would be very desirable, therefore, to grubhoe, or break up somehow the burnt land at once and lay it down in grass, so as to get the first benefit of the ferti iser and get it into the soil. Besides supplying directly po'ash food for plants wood ashes have a valuable chemical action on ihe soil itself, by wliich they liberate lime and magnesia and even potash from clay, and thus render these constituents available to the growing plants The carbonate of potash in the ashes also neutralises the very objectionable low acids in damp soils (lumic, geic, ulmic acid), and thus warms and sweetens cold, sour lands after a wet winter and a late spring

Saltpetre or nitrate of potash (KNO 3 ). — This manure, valuable both for its potash and nitrogen, is found in large quantities as an efflorescence in the soil in India, Egypt, Arabia, and other hot countries, especially in the neighbourhood of the old cities and populous villages Its origin is due, again, to the action of the microscopic microbes for ever working in the dark on the huge mounds of household refuse, ashes, dead nitrogenous animal and vegetable substances, and excrementitious matters that had been accumulating in such places for probably thousands of years Their action is to convert the nitrogen of such substances into nitric acid (HNO 3 ). This acid then attacks the carbonate of potash (K 2 CO 3 ) which it finds in the ashes, and at once converts it into nitrate of potash (KNO 3 ) by the equation :

K 2 CO s +2HNO 3 =2KNO 3 +CO a +H 3 0, which means that 1331b of carbonate of potash acted on by 1261b of nitric acid will yield 2021b of saltpetre, 441b of carbonic acid gas, and 181b of water. When the next wet season sets in the saltpetre thus formed will be dissolvedby the rainwater and carried away into the soil as far as the water percolates At the end of the rainy season, then, the soil will, over a a considerable area, bo charged with saltpetre in solution ; and when the dry weather sets in and the soil, having dried to some depth, becomes full of small cracks and tubes and pores and openings, the underground water (charged as explained above with saltpetre in solution) will rise through these tubes and openings by capillary action, and, on reaching the surface, will dry up by the heat of the sun, leaving a small contribution of saltpetre where it disappeared.

There is a whole caste of native Indians whose sole occupation it is to search out patches of rich soil impregnated with these white crystals of saltpetre. They dig out the fat earth so charged, rub it up with hot water in wooden tubs to dissolve this salt, using the same water over and over again till it has as

much saltpetre as it can hold in solution. The sediment is then allowed to settle ; the clear liquid is decanted off and boiled down, or evaporated in shallow vessels by the heat of the sun, till, on cooling, the saltpetre in it crystallises out. If intended for manure, it is now ready for the market ; but for making gunpowder and nitric acid it has to be purified by re-solution and re-crystallisation repeated a number of times.

In France, Belgium, and Sweden, and in some parts of Germany, since the time of the first Napoleon, when England cut him off from getting supplies of that ingredient of gunpowder from India, saltpetre has been made artificially in what are called the nitre plantations of these countries These are simply mounds of rich loam mixed with offal and other dead nitrogenous animal matter, wood ashes, and lime. These mounds are protected from rain (which would dissolve out the nitrates) by a loose, open roof or shed, which, however, allows the air free access. They are watered from time to time with stable runnings and similar putrefying nitrogenous liquids, and turned over occasionally to expose all parts to the action of the air and the liquids.

These mounds, it is now known, become the homes of countless millions of the nitric acid microbe (which is the real nitre-maker) working in the dark, feeding on the nitrogen and converting it into nitric acid, which then unites with the lime and the potash present, converting them into soluble nitrates These heaps are then treated for the recovery of the saltpetre just as the saltpetre Indian treats his white and grey patches of nitre soil already described.

The subject of next lecture will be " Potash (continued) and Lime."

o 111 Bush. Tons. Tons. Nitrate of soda, l£cwt, in three instalments of Icwt each ... Nitrate of soda, ljcwt (once) ... Niitratc of potasli, 2c\vt (once) Sulphate of ammonia, Icwt (once) Muriate of ammonia, Icwt (once) Farmyard manure, 24 tons (once) No manure 50 42 21 34 10 ? n ? 2 21 28 6 l'l IS* 11 61 2

1001b of Manure Taken. Titrate of potash, common saltpetre Titrate of soda, Chili saltpetre ... iulphate of ammonia luriate of ammonia Titrate of ammonia lbof Nitrogen, ... 139 ... 16-5 ... 212 ... 26-2 ... 35

£md of Crop. Wheat ... )ats Jarley ... Potatoes... turnips ... llangels... Dlover hay ?eas Quantity of Crop. Bushels. ... 50 ... 60 ... 50 Tons. ... 10 ... 20 ... 20 ... 3 ... 1 Amount of Ash. lb. 51 103 78 268 310 448 470 57 Potash in Ash. lb. 17 17 16 140 155 200 94 31