Iron as a Manure.

It had long been universally believed that iron in its lower states of combination (fcrrous salts) is injurious to plants; while in its more highly oxidized compounds (ferric oxide and ferric salts) it was supposed to be, in moderate quantities at least, comparatively harmless. Recently it has been discovered by practical experiments that the green granules page 44 that constitute the green colouring-matter (chlorophyll) of leaves, and that, under the action of light, play so great a part in the elaboration of plant-material from the carbonic-acid gas and the atmosphere, cannot grow without both iron and potash, no matter what abundance there may be of soda, lime, phosphorus, magnesia, and every other kind of mineral food, with all the best conditions of light, warmth, moisture, carbonic acid, and every other requisite of plant-growth.

These are small quantities, but they play an essential part in the economy of the plant—a part that, as stated above, cannot be performed by any substitute. From an inspection of the table given above it is seen at a glance what kinds of vegetable food are best suited for supplying iron to the human system when there is supposed to be any deficiency of that element in the blood.

Although the fact of the presence of iron in plants has been long known, and its absolute necessity for plant-growth recognised, it has been reserved for Dr. Griffiths to prove by field experiments during the last ten years or so that in many cases one at least of its salts is a valuable manure.

The salt he experimented with is the sulphate of iron (sold in the shop as copperas or green vitriol). Its symbol is FeSO₄, 7H₂O, containing in 100 parts 21 parts of iron, 37 parts of sulphuric acid, and 42 parts oil water, all chemically combined into the green crystals, which have an astringent taste and are very soluble in water.

Up till the time when, about ten years ago, Dr. Griffiths made his experiments these green crystals were supposed to be injurious to vegetation. They contain the iron in the lower state of combination as a ferrous salt, which had long ago got a bad name; and there is still considerable hesitation in accepting the favourable results of the practical field-experiments of Griffiths and of numerous practical farmers and agricultural experts both in England and on the Continent on this subject.

The crops experimented with included wheat, beans, potatoes, turnips, mangels, beets, meadow-hay, tobacco, and cabbages; and on all of them a dressing of sulphate of iron, it is claimed, had a most marked beneficial effect, always in point of quantity of product, and in many cases also on its quality.

The 1886 plots had both received the same quantity of the same farmyard manure. To get more information on the subject, and to determine whether the iron of the manure actually entered into the tissues of the plants themselves, Dr. Griffiths burnt to ashes a considerable number of the plants grown on both plots A and B, and analysed the ash.

This revealed the facts: first, that the ash of the entire bean-plants grown on A contained four times as much iron as that of the plants grown on B; second, that there was more phosphorus in the A plants than in those of B; and, third, that the ashes of the beans (seeds) themselves were of quite the same composition in A and B. It was therefore in the straw that the iron had accumulated, and with it the phosphoric acid.

With turnip experiments similar results were obtained—namely, a larger crop with a dressing of ½cwt. of copperas per acre than without it; and also, as in the case of beans, more iron and phosphoric acid in the ash of the plants—both roots and leaves. With meadow hay the difference was more striking. Two plots (A and B, one acre each, as before, and similar in every way) were manured with the same quantity of farmyard manure, and then sown each with 16lb. of the same grass-seed mixture. When the grass had made its appearance well above ground, A received a top-dressing of ½cwt. of the iron manure, while B got none. The effect was soon very apparent in the more vigorous growth and deeper green colour of plot A; and, on harvesting, A yielded 3 tons 2cwt. against the 1½ tons produced by B. The quality of the hay of A was also better than that of B, the former containing 64 per cent, of food-materials (albumen, sugar, starch, and fat), against the 59 per cent, found in B.

There was here also found to be an increase of iron and phosphoric acid in the manured crop. It is claimed for this copperas manure, also, that it attacks injuriously and destroys moss, with which old pastures are often infested.

The explanation of this destructive action is that, as the mosses absorb a comparatively large quantity of liquid in proportion to their weight, and as the iron-sulphate is rapidly dissolved in the rain and soil water, the troublesome mosses take in with their liquor a great deal of the iron-salt page 46 —more indeed than is good for them—the consequence being, when the proportion of iron exceeds 10 per cent, of their ash or mineral matter; that it kills them. It was by analyses of the ash of moss plants thus treated to a solution of copperas, and thereby killed, that this destructive action was fully traced to the iron.

There are many old grass paddocks in Otago and Southland infested with this moss plague that would offer a capital field for experiments in this direction.

With mangels similar results were obtained—32 tons on A against 26 tons on B—and, as with the hay crop, the proportion of food-matter was also higher in A.

The increase of iron in the ash of potatoes and other food-plants grown with iron manure suggests a mode of supplying a larger proportion of that metal to the human system, if such increase be at any time required for our perfect health. It is at least quite possible that food grown on new lands, in which the iron of the soil is not yet, for want of the proper cultivation, in a soluble state, may, in some cases, be deficient in iron for food purposes. And here we have a suggestion for supplying in the most natural way conceivable such deficiency if it exists.

With wheat the increase of grain grown with sulphate of iron was very slight—28 bushels against 27 in 1884, and 32½ bushels against 30 in 1885; but it is claimed for the iron manure that the wheat grown with it completely resists the attack of the wheat mildew—"rust." There was also observed an increase of iron in the ash of A.

The lecturer dwelt at some length on this part of his subject owing to the novelty of the information conveyed by the results of these experiments, recorded in Griffiths's "Treatise on Manures." He had, however, no information on the subject beyond what is contained in that handbook, which he strongly recommended to his students and all who are interested in agricultural pursuits.