There are 16 nutrients plants need to thrive and they are divided into macro and micronutrients. Macronutrients are needed in larger quantities than micronutrients, but all 16 elements are needed for proper plant growth. The macronutrients are Nitrogen, Phosphorus, Potassium, Magnesium, Calcium and Sulfur. The micronutrients are Iron, Boron, Manganese, Zinc, Chlorine, Copper, and Molybdenum. Carbon, Hydrogen and Oxygen are also needed, but these are easily supplied by nature.

Micronutrients cannot move through the soil if the pH is high. In South Florida, we have a pH of 7.8 to 8.8, which does bind up micronutrients. If your granular fertilizer says it has Zinc and Boron in it, your plants will still lack these nutrients as they will be trapped in our high pH soil. Our limestone-based soils are the reason we apply micronutrients through foliar sprays and not through the soil. Iron, a micronutrient responsible for helping to keep a plant green, is usually applied separately from foliar sprays as a drench. The type of iron applied should be chelated (EDDHA) and mixed with water and then poured over the roots

Micronutrients should be applied to plants in the rainy season. Micros applied in the dry season will not be absorbed properly. Micros can be applied two to three times in the rainy season depending on the needs of your plants. When you apply micros as a foliar, you should spray the tops and bottoms of the leaves of the plant thoroughly until the product drips off of the plant. Iron drenches should be applied when the soil is already moist and should be placed around the drip line of the tree. The drip line is the outermost portion of the tree where the canopy ends.

Macronutrients are usually applied in the granular form and are put out based on your plants’ needs. All fertilizer bags are required to have the ration of Nitrogen, Phosphorus, and Potassium (N-P-K) on the label. A good N-P-K ratio for tropical fruit trees is where the first number, (N) is lower than the last number (K). Nitrogen helps a tree’s leaves grow, Phosphorus is responsible for overall plant health, and Potassium helps flowers and fruit develop.

A typical fertilizer labeled for fruit trees is 8-3-9. This is not a bad ratio, but if the last number were higher, I think it would be a better mix. Beware of fertilizer labeled for fruit trees that have more Nitrogen than Potassium.

Mango and lychee trees do not do well with high amounts of Nitrogen, so an 8-3-9 would not work well for those crops. Something with a higher ratio of Potassium and a lower ration of Nitrogen would be better. Applications of Nitrogen or irrigation before trees are starting to flower may cause foliar growth rather than flowering and will disrupt the fruiting cycle, so be aware of that.

Granular fertilizer should be applied three to four times a year and should not be applied before a tree is getting ready to flower. Starting your granular applications in May and putting them out every other month until the dry season is usually best. When applying granular fertilizer, it is important not to clump the fertilizer and to spread it out as you apply it. The fertilizer should be applied at the drip line and beyond. If you walk around the tree and spread the fertilizer just past the dripline, you should end up with the right amount of fertilizer per tree. A three year old tree will require about a pound of granular fertilizer per application.

Micronutrient deficiencies usually show up in new leaves and macronutrient deficiencies are expressed in older leaves. That tip will help you to know what to apply to your plants. It is not usually a good idea to try and treat individual deficiencies, but rather just try to treat micros or macros. Micronutrients usually come premixed all together and applying them separately can throw off the way the nutrients are absorbed.

Keeping your plants on a regular fertilizer program can help to keep them healthy and less susceptible to insect and pathogen pests. A dedicated mulching and composting program will help to offset the need for a large amount of fertilizer applications, so keep that in mind when crafting your fertilizer program.

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