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Concerns? I wouldn't say
concerns, exactly. To a farmer, manure is money. If you over-manure a field, you:
- Don't get the profit you'd get if you fertilized the field properly, so it's like burning hundred-dollar bills.
- Might (depending on how soluble the excess nutrient are) have the nutrients run off into nearby streams, which is like choking fish with hundred-dollar bills.
- Might (depending on the nutrient) eventually end up with soil with so much excess fertilization that nothing will grow, which is like using hundred-dollar bills as barrier cloth to keep your crops from germinating.
Which is not to say that lots of farmers don't do it wrong. If you have a few commercial broiler sheds, it takes a lot of acreage to turn all that litter into hundred-dollar bills. As the size of commodity poultry operations keeps going up, yesterday's well-balanced operation is now long on manure, and since commercial poultry farms tend to be clustered in the same area (near the processing facilities), you can even run out of other people's farms to spread the manure on. America's intensive poultry-growing regions used to be in areas of marginal or sub-marginal farmland, but these can't keep up with the manure, and the Midwest seems to be taking a bigger share now.
With backyard flocks, over-manuring can be a serious problem. Not only does the chicken run become barren due to an overload of manuring (plus the hens scratching the turf to death), but parasites do pretty well in this environment. Geoffrey Sykes promoted the "Henyard System" in the Fifties, with the idea of using a LOT of straw or wood chips in the yard, adding a LOT more if you ever saw mud, and removing all of it once a year, hopefully removing most of the nutrients and parasites, too. A small chicken run can absorb a surprising amount of litter this way, but not enough to overload a whole suburban lot, I wouldn't think.
Robert