So where does all this energy come from? We all know the answer - mostly oil, some natural gas and electricity (mostly from coal). The term sustainable means lasting well into the future. How long will the oil last?

In the March issue of Scientific American two oil industry analysts estimate that the current proved reserves of conventional oil could last 43 years. That is the good news. The bad news is that world oil production should peak and then begin its slow decline sooner than most people think, within 10 years. This is because the last barrel of oil extracted from a well is much harder to get than the first barrel. After the peak is reached, oil will become harder to pump and therefore become more expensive.

Proved world-wide reserves of natural gas could last 100 years at current consumption rates. But as oil starts to decline, natural gas use will increase so it will be used up faster. As with oil, gas prices will rise when it becomes harder to extract.

Estimates of how long these fuels will last are always changing. But the fact remains that fossil fuels are non-renewable and therefore not a sustainable energy supply.

Another factor in determining sustainability is it ask what harm is being done to the environment and health of the people by using this resource. Fossil fuels certainly fail this test. The extraction, processing and burning of these fuels is the largest single cause of environmental degradation - oil spills, toxic waste, urban smog, acid rain and ultimately global climate change.

Seven billion tons of carbon dioxide are emitted a year. The world's top climate scientists predict global temperatures will rise 2-6 degrees F in the next century causing sea levels to rise 1-3 feet, disrupting growing seasons, causing worse storms, heat waves, floods and spread to disease.

Where will energy come from and how will it be used in a sustainable agriculture system? The first step is energy efficiency. Reduced tillage is one way. As above figures show, chisel plowing used almost half the fuel of plowing. Using less fertilizer and pesticides also reduces energy use. Organic fertilizers generally take less energy to produce than their chemical equivalents.

Local marketing where ever possible would greatly reduce the huge amount of energy now used to transport agricultural products.

Rotational grazing to produce livestock offers a large energy savings over conventional methods. Just compare the amount of energy to run an electric fence to the amount to produce and transport all the hay and grain to feed those animals in confinement.

In addition to more energy efficient practices, other sources of energy must replace fossil fuel use.

An over-looked source of energy in agriculture to human power. Fossil fuel energy has replaced human energy in every facet of modern life, including farming. Smaller, more intensive farms employ more people and use less energy.

On my farm I have about one acre in vegetable gardens. A little less than a quarter of that is raised beds which I work entirely by hand - no tiller. The rest I work with a tractor using conventional tillage. Last year that quarter acre produced 42% of my gross sales. Human labor may work on small intensive farms but not for crops such as grain and forage that require large acreage.

A century ago animals powered agriculture. Using animal power looks sustainable from the criteria of how long will it last and what environmental damage is done. But there is also the element of extent - how widely can we use this practice and still produce enough food to feed a growing population? Whether using rotational grazing or conventional methods, feeding draft animals takes a lot of land. (I have 2 horses to feed on my farm.)

If animal power were to replace a significant part of the tractor power, how much land would be used to feed those animals? Would there be enough to feed the almost 6 billion and growing human population? I don't have any answer here, but am sure there is a limit to the extent draft animals could be used.

What alternative sources of energy are there? Biogas is one. Methane can be made from manure, and the residue left over is still good fertilizer. It can also be made from garbage. All our landfills should be producing methane.

Ethanol is another. In Brazil ethanol is made from sugar cane and in the U.S. it is made from corn. But these are annual crops raised on prime farmland so it puts energy production into direct competition with food production. Also, counting all the energy to grow the crop and produce the ethanol, there is more energy put in than you get out.

A better system would be to make ethanol from wastes or perennial crops grown on marginal land. The practicality of this has not yet been demonstrated, but it holds promise.

The ultimate source of sustainable energy for agriculture, as well many other uses, is solar generated hydrogen. But who knows when it will be available, or even it if could be practically used in agriculture?

So is the way I farm sustainable? No. Why not? Lots of good excusses - too busy trying to make a living, too set in my ways. I am raising the question what is sustainable just to get people to start thinking about it. But it is much easier to ask a question than put in practice the answer.