About one year after a new garden tree has been planted, fertilizer application may begin, to achieve vigorous growth, and bountiful flowering and fruiting. Already established older trees in a garden also benefit from regular fertilizer applications. Basically, the nutritional needs of a tree relate to the individual tree species, and to the known deficiencies of the soil, which in South Florida are calcareous and excessively alkaline.
The most challenging situation in South Florida is to plant a tree which naturally prefers acidic soils. In such an instance, modifying the soil pH is the most important action to take for tree health. Most trees grow well in soils with a pH of between 6 and 7.5 (neutral soil has a pH of 7). If soil analysis reveals a pH above 7.5, it is recommended that it be lowered, making it more acidic; a safe and inexpensive chemical to use is granular sulfur, with at least 3 months between applications. Mulching the ground surface around the tree with sphagnum peat is also an effective alternative. Adjusting the soil pH may be as important as fertilizer application in achieving healthy vigorous tree growth.
Plant fertilizer content is a mixture of (N) nitrogen, (P) phosphorous and (K) potassium, the three major nutrients required for plant growth. Commercial fertilizers are labelled with a set of three numbers (e.g. 10-10-10, meaning each element, N P and K, is represented by 10% of the total weight of the bag). In Florida, phosphorous has been removed from most fertilizers because it may contribute to algal blooms in the ocean. A gardener has the choice of three basic forms of fertilizer.
One, slow-release granular fertilizer. This method is easy and economical. Fertilizer grains are spread around the base of the tree at about the outer limit of the canopy, in the area of feeder roots. At each rainfall or watering event, the roots take up a bit of the fertilizer. Granular fertilizer can be applied three times per year in spring, summer and fall.
Two, controlled-release fertilizer is sprinkled closer to the trunk than slow- release. It will not burn the tree. It can be used in small planting spaces and within groundcover plantings. The fertilizer is activated by a combination of water and warm temperatures. This form of fertilizer is ideally applied three or four times a year, limiting applications to the warmer months.
Three, water-soluble liquid fertilizer. The advantage of liquid fertilizer is its rapid uptake by the tree. This fertilizer is often used to encourage flowering and more vigorous leaf production. Prior to application of liquid fertilizer, the tree first should be given a good watering, drenching the base and waiting about 1 hour. Then the liquid fertilizer is sprayed on the ground. It may also be sprayed directly on the leaves. To be most effective, application should be carried out about once per month during the warm season.
A general fertilizer recommendation, applicable to South Florida, for the more than 200 species of trees or shrubs in the TreeWorld inventory is a product with a content of 7-0-7 NPK.