Soda Salts.

Of sodium, there are only two compounds of any interest in agriculture. These are common salt (chloride of sodium, NaCl) and Chili saltpetre (nitrate of sodium, NaNO₃).

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. The 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. The 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 rivets 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 lakes and licks of California, Mexico, Abyssinia, and Australia were formed—namely, by the long-continued inflow of water containing a little salt with no outflow, and 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, 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 page 27 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 the sea, where the 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 (5cwt. to 8cwt. 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 insoluble 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.

Except for the purpose of hastening the breaking-up of new cloddy land, and destroying noxious weeds, there is probably no part of New Zealand so far from the sea as to require the addition of salt to the soil.

A very different account has to be given of the function of nitrate of soda, or "Chili saltpetre," NaNO₃.

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 Bolvia, on the western seaboard of South 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 presence of soda salts, the nitrogen furnishing the nitric acid, and the salts the sodium, of which the nitrate of sodium consists. Such a transformation could not, of course, take place except in a hot, dry climate and on a rich, organic, and saline deposit, under conditions which we can now only guess. The temperature of such a tropical region would 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 crystallizing out the impurities by boiling down. In general appearance nitrate of soda is not unlike common salt, kainit, and other cheap and less 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 been 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, while the American continent consumes about the same quantity.

In 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 46½ per cent, or 46* units of potash, and for its potash alone, without considering the nitrogen, is 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, Märcker, &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 100lb. of nitrate of soda, 78lb. of sulphate of ammonia, 63lb. of muriate of ammonia, as each of these contains 16½lb. 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 soils 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 limestone soil it should always be used instead of ammoniacal 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 page 29 barley, &c.); (2) rape; (3) beets and potatoes; (4) meadow grasses; (5) leguminous plants, such as clover, peas, beans, lucerne, &c.; (6) linseed.

These experiments were tried on one acre of land in each case. The wheat land is described as clay-land of good quality; 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.

The lecturer did 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 27.5lb. of nitrogen; the nitrate of potash, 31.1lb.; the sulphate of ammonia, 23.7lb.; and the muriate of ammonia, 29.3lb. It may have been the market price of these manures that guided him—putting on the same market value of each—but it would, he thought, have been more satisfactory had he used in the manures tried against each other the same 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 lcwt. each of sulphate and muriate of ammonia against his 1½cwt. of nitrate of soda, he should have applied 200lb. of nitrate of potash, 130lb. of sulphate of ammonia, and 105lb. of muriate of ammonia, for these are the quantities that contain the same amount of nitrogen as 1½cwt. (168lb.) of nitrate of soda.