Blast Furnace Slag—A Soil Builder
THE TERM CONSERVATION implies so much, and is taking on so much importance, that many folks are asking whether the soil might not be built up in fertility by using some of our industrial wastes. Numerous inquirers, interested in soil organic matter, have asked about city garbage, and septic tank sludges recently. Also, inquiries have come in concerning the value of blast furnace slags, the wastes from making steel. Such slags should be considered, though their variation in composition demands that one know something about them, and about the fineness of the materials. They are the residue from putting limestone, feldspars and other minerals in with the iron, to serve as fluxes, to purify it. The carbonate part of the limestone burns off to let it become quicklime, calcium silicate, or calcium phosphate, and other combinations possible at those high-furnace temperatures.
Slags are the rock-like residues dumped out after the molten iron has been drawn off. They are of fertilizer value, therefore, as carriers of phosphorus as well as calcium, with much of the latter in silicate form. In making some steels, manganese and other “trace” elements are often present, too, in this complex silicate rock. Sometimes the hot-rock is slaked and granulated by spraying with water, or it may be cooled and crushed. Either treatment aims to make it serve as a lime substitute for soil building.
Since, like many other original rocks, it is a silicate, therefore, it weathers down slowly. Its use is, then, a case of adding “sustaining” fertility to the soil rather than as a “starter” fertilizer. By its decomposition it adds not only calcium, some phosphorus, and “trace” elements, but also some silica. This is a clay-builder for the soil which would be helpful on sandy soil with “too little body,” or not enough capacity for adsorption and exchange of nutrients to the plant roots. In the slow reaction, by this kind of lime, there may be more of a safety factor than when lime carbonate is too generously used under the belief that “if a little is good—more will be better.” Since we lime soils now to supply calcium (and magnesium) as fertility, rather than to remove the acidity, the blast furnace slag has been shown by tests to be a good way of building up the soil in calcium (and magnesium). We can, therefore, practice conservation with profit by using this waste from the steel mills to build up the soil.
The fineness of the slag, into which it is ground—as for limestone—becomes a factor in its rate of being effective in crop nourishment. Also, its lime content (calcium and magnesium) must not be too low (preferably near 45%) if its effects are to follow more promptly after its application on the soil. These may be greater because it is not so much of a reagent to reduce the soil acidity as it is an active silicate providing calcium. Combined as a silicate rather than as a carbonate, it is more of a buffer in that it does not bring on, or permit, sudden shifts in the degree of soil acidity under heavier applications of it. In terms of the microbial life of the soil, its application is no shock because of any salting effect. Yet it stimulates (a) nitrification, as the change of ammonia nitrogen to the nitrate form; (b) the speedier oxidation of the carbon compounds; (c) the fixation of nitrogen by microbes living independently of the legume plant roots; and (d) all the other biological soil processes which distinguish a live soil from a dead one.
The employment of blast furnace slag as a fertilizer for the fields, and as an aid in composting, has been a practice in the art of agriculture by some folks long before recent scientific studies, tests, and partial commercialization gave its use greater sanction as a soil treatment—using this accumulating waste from the dump heaps of steel mills. We can use such material not only for true conservation but also with assurance that it builds up the soil for more nutritious crops.