Organic Gardener's Composting by Steve Solomon

_Basic slag_ is an industrial waste from smelting iron. Ore is refined by heating it with limestone and dolomite. The impurities combine with calcium and magnesium, rise to the surface of the molten metal, and are skimmed off. Basic slag contains quite a bit of calcium plus a variety of useful plant nutrients not usually found in limestone. Its exact composition varies greatly depending on the type of ore used.

Slag is pulverized and sold in sacks as a substitute for agricultural lime. The intense biological activity of a compost pile releases more of slag's other mineral content and converts its nutrients to organic substances that become rapidly available once the compost is incorporated into soil. Other forms of powdered mineralized rock can be similarly added to a compost pile to accelerate nutrient release.

Rodale Press, publisher of _Organic Gardening_ magazine is located in Pennsylvania where steel mills abound. Having more experience with slag, Rodale advises the user to be alert to the fact that some contain little in the way of useful nutrients and/or may contain excessive amounts of sulfur. Large quantities of sulfur can acidify soil. Read the analysis on the label. Agriculturally useful slag has an average composition of 40 percent calcium and 5 percent magnesium. It must also be very finely ground to be effective. See also: _Lime_ and _Rock dust._