You can improve the calcareous and alkaline soil, commonly found in south Florida, for growing oranges by using the right fertilizers and soil amendments. According to Purdue University, without careful attention to the soil pH and both macro- and micronutrients in the soil, orange trees in south Florida can suffer. Chlorosis will set in, making it difficult for the trees to take up needed nutrients from the soil, causing them to under-perform or die back over time. Determining your specific soil conditions is the key to a properly tailored fertilization regimen and ideal calibration of the soil content.
Test the pH of the soil surrounding your orange trees once a year. You should also test soil before you plant a new tree. The soil pH will help guide how much and what types of fertilizer and soil amendments to use to ensure healthy, productive trees.
Feed your trees with an organic or synthetic citrus tree fertilizer, sometimes called special citrus fertilizer. Apply according to the product label directions for your soil type. Feed young trees, under three years, lightly every six weeks or so to speed growth. Skip fertilizer in the fall and winter on trees under three years and feed trees year-round according to the label directions in the third year and thereafter.
Apply an ammonium thiosulfate fertilizer with a guaranteed analysis of 12-0-0-26S or 0-0-25-17S that has its sulfur in the S2O32 form. This will break down into the soil and acidify the soil, creating a more supportive environment for your orange trees in the native alkaline soil. Apply according to the product label dosing directions and in keeping with the results of your soil pH test, as mildly alkaline soils will require less product to correct the soil than very alkaline soils.
Amend alkaline soils with an iron chelate product to prevent chlorosis. Apply according to the product label directions but do not exceed 2 oz. of iron per tree, as a general guide. Use your soil pH level to determine how much and how often to apply the chelate by consulting the label on the package. Look for a chelate product labeled Fe-DTPA for soil with a pH below 7.5 and one labeled Fe-EDDHA for soils with a pH of 7.5 or higher.