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I read something or other somewhere in passing that compost was supposed to ameliorate salts in the ground but I have no idea at what capacity.
For my money, I think if you follow the 10 percent rule on the peanut shells you should run in to no troubles. I've found peanut shells somewhat slow to breakdown.
composting may deal with some of the salts, but just to be on the safe side, i tend to soak (witha few changes of water) salted peanut shells before they go in the pile.
and slow-to-break-down, you say? sounds like unsalted peanut shells would be a nice mulch material, maybe?
Salted peanut shells will attract deer to your compost pile if they are in your area. My deer play king of the kill on my compost pile. Not too much of a problem unless you have a thing for neat high piles.
Peanut shells might make a very good mulch/ammendment to your tomato patch (if deer are not a problem). Low levels of sodium in the soil improves the flavor of tomatoes (scientific study, not old wives tale), and the high lignin content of the shells would slow decay such that nitrogen robbing (robbery?) would probably not be a problem, and the cupped nature of the shells would keep the soil from compacting.