In a gold rush, the firms that supply the gold diggers with tools — not the gold diggers themselves — make the highest and steadiest profits. That’s a platitude, but it’s also usually true. And it’s now playing out in the boom in corn-based ethanol.

Don’t waste much time envying corn farmers. Sure, they’ve seen the price of their product double over the past year and a half or so. But they’ve also seen their costs inch up. Fertilizer, land rents (much of the farmland in the midwest is rented), pesticides, and seeds — all have risen since the corn rally. Before long, much-heralded “record farm income” in the corn belt will likely evaporate under those pressures.

As for ethanol producers — the ones buying up all that corn and spinning it into auto fuel — even they’ve seen their profits drop, despite heavy government support. They flooded the market with so much ethanol, so fast, that they overwhelmed it, leading to a glut. Helpfully, though, the federal government solved that problem, for a few years at least, with the 2007 Energy Act and its lofty ethanol mandate.

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Corn farmers and even ethanol producers are pikers compared to the input suppliers — the firms that peddle the special seeds and chemicals required for industrial-scale agriculture. And the granddaddy of all those firms, the genetically modified seed and herbicide giant Monsanto, just delivered what’s known on Wall Street as a “blowout” quarterly profit report.

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In the three-month period that ended Nov. 30, Monsanto reeled in profit of $256 million. That’s nearly three times the amount it made in the same period of a year earlier, and well more than Wall Street analysts had expected. Monsanto shares, which more than doubled in value over the course of 2007, leapt more than 9 percent in Thursday afternoon trading on the news.

How did Monsanto pull off this neat trick? By selling boatloads of herbicide and genetically modified corn, Reuters reports:

Sales of corn seed and traits during the quarter jumped to $467 million from $360 million a year ago, while sales of its Roundup and other glyphosate-based herbicides climbed to $1.0 billion from $649 million.

The company told Wall Street to expect more of the same in 2008, boosting earnings expectations significantly.

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South America — particularly corn-happy Argentina and soy-mad Brazil — has been a main driver of Monsanto’s profitability, sucking in mass quantities of the company’s flagship herbicide RoundUp, to complement RoundUp-Ready corn and soy, AP reports.

The company expects to bring in a cool billion in gross profit from RoundUp alone in 2008, according to AP.