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The Pamphlet Collection of Sir Robert Stout: Volume 73

Magnesia in Soil, Plants, and Manures

Magnesia in Soil, Plants, and Manures.

Magnesia is an essential, though not an abundant, constituent of all the cultivated plants, and therefore is a necessary ingredient of fertile soils.

The proportion of this compound in the more common crops may be tabulated as follows:—
Kind of Crop. Quantity of Crop. Bushels. Amount of Ash. Lb. Amount of Magnesia. Lb.
Wheat (grain) 50 51 5
Oats (grain) 60 103 7
Barley (grain) 50 78 6
Potatoes (tubers) 10 268 21
Turnips (roots) 20 310 6
Mangels (roots) 20 448 22
Clover hay 3 470 17
Peas or beans 1 57

In 800 gallons of milk (the yield of a good cow during the year) the ash is about 68lb., of which 2lb. is magnesia; and the ash of maize and of rice contains about 11 per cent, of magnesia. Magnesia is thus seen to be a "specific" for potatoes, mangels, and pastures, and all "special" manures for these crops should always contain it in some soluble condition.

In the case of the cereals—wheat, oats, barley, &c.—in is in the grain chiefly that the magnesia is found, whilst in root-crops—turnips, mangels, &c.—the leaves (weight for weight) contain more of it than the roots.

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The soil of Otago and Southland, from its origin in the débris of the mica-schists of our mountains, in the basalts of Dunedin and Orepuki, the limestone of Oamaru, Waihola, and Southland (all of which contain a very appreciable proportion of magnesia), has plenty of this mineral ingredient in one form or another. It is, however, one thing to have magnesia in the insoluble state in which it exists in the silicates of our mica-schists, and another thing to have it in a state in which rain-water and the moisture of the soil can dissolve it for the use of plants.

The soluble compounds of magnesia which are found in manure mixtures are the sulphate and the chloride, the former of which is well known as Epsom salts, and both of which are abundantly present in all the Stassfurt mineral manures. Of these, kainit, although valuable mainly for its 24 per cent, of potash salts, contains about an equal proportion of magnesium salts. Carnallite also, another of the Stassfurt products, contains usually about 34 per cent, of the chloride of magnesia; and poly-halite the same proportion of the sulphate of that metal. One hundredweight of any of these Stassfurt manures per acre will therefore supply sufficient magnesia for a large crop of wheat, oats, barley, or turnips, while 2½cwt. or 3cwt. will be required per acre for mangels, potatoes, or clover hay. By the slow breaking-down of stony materials in the soil—the disintegration of the micas, the crumbling of the basalts and the lime-stones—there is always a fresh accession of magnesia liberated and converted into carbonate, which dissolves in the soil-waters; and thus crops get constant contributions of this ingredient in a natural way. The magnesia of limestone (Oamaru and Waihola limestone contains a good supply of it) is in the form of carbonate, soluble easily in carbonic-acid water; and the lecturer did not think there is any need for applying magnesia mixtures to soils thus derived. There is, indeed, quite enough magnesia in these limestones to supply all the requirements of land which has an occasional application of lime—slaked lime—from either of these formations. There is in England a long-standing prejudice against the direct application of magnesium salts, unmixed with any other manure, to the soil. Some recent experiments give some grounds for this prejudice, as it has been found that a solution of such salts applied in large quantities rapidly impairs the growth of plants, and even kills them, by some unexplained action on their roots. It is therefore deemed advisable, when the soil is deficient in magnesia, to supply the defect by a mixture of one or other of the Stassfurt minerals with guano or superphosphate of lime. A sprinkling of any of the Stassfurt salts (kainit, &c.) would, on account of its magnesia, be a good means of arresting and fixing both the phosphorus and nitrogen of stable runnings and other putrefying nitrogenous liquids about a farmyard.

Such liquids produce ammonia, which, without some means of fixing it, evaporates away into the air. The magnesia now steps in and unites with any phosphoric acid present, and also with the ammonia thus produced to form an insoluble compound of a very fertilising nature, containing, as it does, both phosphorus and nitrogen.