Part 1, Section 9: Nontraditional Soil Amendments
Nontraditional Soil Amendments
SLAG
Slag is a by-product of the steel industry and consists primarily of calcium silicate minerals. Slags have nearly the same ability to neutralize soil acidity as agricultural limestone, and they have been used extensively as soil liming materials. There are two kinds of slag, air cooled and water quenched. Water-quenched slag is cooled rapidly with water, which causes the material to “pop” and generates porous, granular particles. Slags should be used just like conventional ground agricultural limestone. Application rates should be based on soil test liming recommendations and the calcium carbonate equivalency (CCE) of the slag. Most slags have a CCE in the range of 80-100 percent. As with any liming material, the fineness of the product is an important consideration. Both conventional agricultural limestone and slag react very slowly with soil acidity unless the material is finely ground. Ideally, the slag should meet the fine-sized specification for Pennsylvania agricultural liming materials. For a discussion of CCE and fineness of liming materials, see “Soil Acidity and Liming” in Section 1.2, Soil Fertility Management. Before using slag as a liming material, be sure it has been analyzed for CCE and fineness.
Some slags may contain elevated concentrations of trace metals such as iron, cadmium, chromium, copper, lead, molybdenum, nickel, and zinc. Concentrations of these metals will vary in slags from different sources. All of these metals occur naturally in soil, and many are essential plant nutrients. If concentrations in the slag are similar to soil concentrations, they present no problem. If they are present at substantially higher concentrations in the slag than in the soil, repeated application of the slag could significantly increase soil concentrations of the metal in soil. This possibly could lead to plant toxicity, increased plant uptake and transfer of metals to animals or humans, or to other environmental problems. Before using a slag, be sure to obtain several laboratory analyses of the total concentrations of these trace metals in the slag.