Michael Pollan is not a reliable guide to agricultural policy in the 1970's, particularly as he places Earl Butz in the center.  Shortly after Butz was fired as Agriculture Secretary, the President increased the loan level for price supported commodities.  Why?  Because Ford was fighting for re-election and needed farmer votes. So much for his legacy. (Pollan fails to note the devaluation of the dollar when Nixon took us off the gold standard, an action that had much more to do with prices in the 70's than any action by Butz.)

And, in case you haven't noticed, it's not simply the price of corn that's soaring, soybeans, wheat, and rice are also rising.  In today's market environment, the levels of government subsidies don't determine which crop a farmer will grow.

bharshaw: Do you really think that "government subsidies don't determine which crop a farmer will grow?"

Would you not consider the federal government's mandate for ethanol in gasoline a subsidy for the corn industry? After all, is it not true that without the government mandate, we would not have seen the American farmer's great rush to corn over the last couple of years? And there has clearly been a rush -- for example, according to the National Agricultural Statistics Service, acreage planted in corn in the U.S. increased by about 15% between 2005 and 2007. Much of that increase appears to have occurred at the expense of acreage planted in soybeans.

So does it not follow that, even in "today's market environment," government policies/subsidies can affect a farmer's choice of what to grow. And if government policies influence what farmers grow, should we not consider whether those policies make sense?

Though this post focuses on corn as an example, soybeans, wheat, and rice also get subsidies.

I beg your indulgence on the long post, but it might help.

     Yes, the government subsidy DID and to a lesser extent is driving the planting of corn prior to the price surge.  Payments to farmers for planting corn are divided into two types.  The first, called Direct payments are allocated by acreage at the onset of a farm bill (ie. every 5 years).  The farmer says, "Of my 100 acres, 50 are base corn acres."  A farmer only gets paid for the acres he plants.  If he plants all 50 of his corn acres, he gets paid a small amount per acre for these acres.  If he plants more than 50 acres, he doesn't get paid for any more acres than the 50 he declared.  
     The second type of payment is called a counter cyclical payment.  The Farm Bill was designed to deliver most of its farmer subsidy in this payment.  This payment establishes a floor under which the farmer is reimbursed, per bushel, for the price of corn he pays.  The arbitrary price set in 2002 was $2.40.  If the actual price of corn was $2.00 and the farmer produced 100 bushels per acre, the farmer would be paid $40 per acre to offset the price.  The lower the actual price of corn, the greater the subsidy.
As you're aware, the prices have been above the 2002 base price ($2.40) for some time.  This has led to the unspent largesse falling back to the USDA.
     As was noted in the initial post, these payments only apply to commodities, which prevents, say, a grain farmer from suddenly switching to onions or tomatoes.  The actual price of commodities,coupled with the availability of inputs is now primarily driving planting decisions.  The subsidies provided by the goverment as direct payments can hardly offset the spike in input cost and will do very little to sway farmers this season.
     Another thing that ought to be mentioned is that at least 30% of Farm Bill funding is made up of WIC, food stamps, Farmers Market Nutrition Program (and its senior counterpart), National School Lunch Program and other programs designed to get fresh fruits and vegetables in the hands of the nation's most needy.  That's what makes this piece of legislation so important to EVERYONE!

Without getting into the nitty-gritty, because it's relevant to my reply to the good Professor, I think it's worth quoting a line from from USDA description:

"Direct payments are not based on producers' current production choices, but instead are tied to acreage bases and yields. Because direct payments provide no incentive to increase production of any certain crop, the payments support farm income without distorting producers' current production decisions."

Because direct payments are the only ones likely to be made this year (assuming a farm bill), the farm program doesn't favor corn planting as opposed to soybean, or wheat.

Some random comments:
While current policies incentivize corn production, so did the supply management policies of the past.  I agree that the emphasis moved production over price in the 70s (for whatever reason).  But as long as farm programs have existed, corn (and other eligible commodities) have received an economic subsidy.  The amount of that subsidy is worth discussing, but it has been there since the 30s.  I get tired of the view that somehow farm programs before the 80s created some sort of farmer and consumer paradise.

There are three main types of farm programs- directs, countercyclicals, and marketing loans (LDPs).  The current target price is $2.63 (though the farmer does not get a countercyclical payment until the price falls to $2.35, because the direct payment is subtracted from the countercyclical target price).  Usually, the USDA does not get to keep the money that is not spent as a result of high prices; that money is taken out of the budget baseline for the next farm bill, though the budget forecasts are occasionally behind the times.

The ethanol tax credit is 51 cents per gallon.  A little under 3 bushels of corn make a gallon of ethanol.  Therefore one could argue that the ethanol tax credit adds $1.50 to the price of corn, but that credit is for the blender (ie oil company) so it doesn't all pass through.  The RFS standard is another discussion.

B. Harshaw says that when direct payments are the only subsidy going to broadacre crops, farmer decisions as to which of those crops gets planted are not materially affected by those subsidies. One can quibble over whether the payments are completely decoupled, but basically he is right. Otocco points out that the payments only apply to commodities, which discourages a grain farmer from suddenly switching to onions or tomatoes, and of course he is right.

Bill Chameides says, "yes, but what about the ethanol subsidy? That's very distorting", and Sideshow1979 points out that they do raise the market price for corn. It looks to me that B. Harshaw agrees.

So what's there to argue?

These are only my personal opinions.

There really isn't a whole lot of disagreement here.  I think everybody here is making slightly different points but they're all in agreement.

The broad point I would make that as long as farm programs are subsidizing certain crops only, those crops have an advantage.  And while direct payments are the only ones being paid, there is still a safety net below those that are bound to influence planting decisions now and especially if prices ever fall again, which is inevitable.

Perhaps even more broadly, after decades of farm programs subsidizing a short list of crops, in many places the infrastructure for growing any other crop has disappeared.  Pollan makes that point, and Tom Philpott has written about it as well in some great posts.