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Carl mentioned variable moisture in a particular area causing changes in ground conductivity. This is certainly true for the first few inches of surface soil, but there is less effect to ground conductivity due to precipitation than might be supposed. This is because the RF penetrates quite deep into the soil, and the water from brief rainfall may not penetrate very deep.
Please don’t “salt the earth” in your back yard to get better ground conductivity, so that nothing will grow near the antenna. Use metal, like ground radials, chicken fencing, and the like. Salt provides electrolytic conductivity, and electrolytic conductivity is thousands of times less than metallic conductivity. A metallic ground screen near the antenna will improve antenna efficiency.
The conductivity of the earth a long distance from the antenna affects the propagation of your signal, but there is nothing you can do about the conductivity of the soil beyond your yard.