Answer: The factors most affecting the size of watermelons is the variety and the availability of adequate water and nutrients. I have not heard of magnesium being a factor in either sweetness or size. It's best to have a soil test done before applying magnesium fertilizer; too much magnesium may affect the availability of calcium to plants. You can contact your Cooperative Extension Service for soil testing information; their number is (816) 792-7787. A good source of magnesium for acidic soils is dolomitic limestone. For alkaline soils, use epsom salts (magnesium sulfate)..
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