The soil around the rootball of a newly transplanted palms should be kept moderately moist for the first 4 to 6 months after planting. At that point is can be gradually weaned off of supplemental watering. How many times you water a week is not the important factor, but rather how moist the soil is staying. Watering very lightly several times or more thoroughly once may yield the same general soil moisture levels. That said you should not need to water 3-4 time a week to maintain adequate soil moisture.
You are correct that magnesium is often needed but so is potassium, and at time micronutrients depending on how poor and sandy the soil is. According to the Florida Extension Service, "'Palm Special' fertilizers are available with chelated micronutrients that contain a ratio of N:P:K:Mg of 2:1:3:1 for sandy, nutrient-poor soils. To ensure a steady supply of nutrients, multiple applications are recommended at a rate of 1.5 pounds per 100 square feet of canopy 4 times per year, or alternatively at 1 pound per 100 square feet of canopy 6 times per year. N, K, and Mg should be in controlled- release form to provide nutrients over a 3-month period. When possible, dry, granular fertilizers should be broadcast under the palm canopy, not up against the trunk and not in bands around the trunk." When your new transplant has been in the ground about 3-4 months sprinkle a light surface application of the "Palm Special" fertilizer around the plant near the edges of the root ball and water it in well.
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