Putting a price tag on the gross domestic product or the stock market is relatively easy. But these indicators ignore our need to sustain natural resources, which are vital. We must bring security of our planet's ecological capital into the marketplace's calculations.

We should start with agriculture, whose footprint is huge. It uses 40 percent of the world's land to make our food, feed and fiber. As agriculture goes, so goes the planet's health.

Farming and forestry prices reflect the immediate costs of labor and capital, but do not include long-run ecological costs. Paying $1.25 per pound for supermarket chicken does not cover cleaning up rivers polluted by poultry factories. That charge goes to society at large.

Rivers can clean themselves given time. But the soil loss that results from the way we farm today is essentially irreversible. This erosion washes about the thickness of a business card from our topsoil each year, an estimated 5 tons per acre, exceeding the soil's natural rate of formation. Large areas of the midwest have lost half of their productive topsoil in just 150 years of farming.

This earthy gold mine helped build our nation. Other powers—Greece, Carthage—gutted their soils and fell. We can mask this erosion by mining other ecological capital—fossil fuels used to make fertilizer, for example—but not for the long run.

Meanwhile, also washing away are pesticides and fertilizer that poison groundwater. Taken by the Mississippi River to the Gulf of Mexico, the fertilizer drives life from 8,000 square miles of the sea, costing millions of dollars in lost fishing.

Future generations will foot the bill for fouled water, ravaged soil, lost wildlife and global warming.

That is, unless something replaces our shortsighted, narrowly defined accounting so that we pay for the total cost of our food consumption.

And something is emerging: ecological economics. It's a field of study that does account for long-term costs. We can now calculate the environmental costs of different types of food production, and compare in monetary terms the long-term impact of global versus local food production. If we can devise a way to include these ecological costs in the prices we pay at the supermarket, we can show consumers the true costs of their food-buying decisions—that it makes more sense, for example, to buy local apples in season rather than similar products imported from New Zealand or China.

Any corporation, government or family that spends its economic capital eventually will go bankrupt. Our spending of ecological capital is no different.

Our human-centered approach to nature assumes earth's resources are here for our exploitation. It is essential that we recognize ourselves as only one species in a complex web of relationships, and that ultimately we are just as threatened by ecological losses as are our co-inhabitants.

We must learn to see our true vested interest. We must reform our markets so we don't keep cooking the ecological books. we must learn to pay a fair price to earth.

—Charles Francis, professor of agronomy and horticulture at the University of Nebraska-Lincoln, is a member of The Land Institute's Prairie Writers Circle, Salina, Kan.

This essay is the opinion of the writer and does not necessarily reflect the views of the Land Institute.