Mineral resources are also growing scarcer. As noted earlier, the richest, most available ores are used first, followed by ores of decreasing quality. We previously used hematite ore from the Mesabi rang in Minnesota, which is about 60% pure iron. That ore is now exhausted, and we must use taconite ore, at about 25% pure iron. The situation with other ores is similar. At least metal can be recycled, and other materials are adequate substitutes. If we consider topsoil as a mineral resource, the situation looks more serious. Rates of topsoil depletion in the U.S. are currently 100 times the rate of formation. Globally, experts estimate that 40% of agricultural land is seriously degraded, and this number is as high as 75% in some areas. Currently, the most widely used substitute for declining soil fertility is petroleum-based fertilizers.