Salt

Salt can be in many forms but usually it is considered Sodium Chloride. It is commonly from salt water either from spray from the breakers of from an old sea bed. Sometimes these old seabeds are not where you think they should be, like a hundred miles from the ocean.
Sometimes the salt is in the form of Calcium, that is usually not nearly the big deal that sodium is. Some places  there are plants growing in 10,000 parts per million Calcium. Not great looking plants and usually halophytes, but there are plants. Sodium on the other hand starts knocking out plants at maybe 250 ppm., at 1000 ppm there is not going to be much there except maybe kelp  and maybe saltbush.
I do not know of any easy way to 'fix' high salt soils. If you have high rainfall or a good clean source(low sodium) of water you can cover the ground with mulch and water heavily a few times to drive the salt down below your crop. This takes a lot of mulch and a lot of water. Sometime gypsum will help, but often only for a little while and the salt is back.
Classic signs of high sodium are halophytes or plants with black leaf margins.