BIOFUELS LAND USE TOTALS:
Ten percent of U.S. cropland acreage is devoted towards ethanol/biofuels production. This is some of the very best, richest soil in the world, requiring expensive corporate inputs backed by taxpayer subsidies, and, requiring water, including large amounts of fossil aquifer water. This amounts to 38.1 million acres, according to Bloomberg.
Land use for ethanol and biodiesel includes well over one-third of the corn cropland, and about ten (or as high as 13) percent of soybean cropland.
AGRICULTURAL COMMODITY LAND USE DEVOTED TO EXPORT:
Twenty-two percent of this cropland is devoted to growing wheat, and other grain and feed to be exported. Presumably, that export total does not include the cropland used to grow and raise livestock and ethanol for export. (This year, again, ethanol exports are shattering previous year's levels.) I assume Bloomberg places acres such as almond trees into the "food we eat" category, some of which also gets exported.
Generally, but not always, the idle and fallow land is less arable.
TOTAL PERCENT U.S. LAND USE FOR CROPLAND:
In summary, land use in the U.S. for cropland is one-fifth of the total, or 391.5 million acres, much of that across the Great Plains Midwest which was covered by prairie grasses up until one-hundred-fifty years ago.