Blame thousands of years under the ocean for South Florida's challenging growing conditions in the soil for gardeners and landscapers today.
That's because marine deposits accumulated over that period. The ocean left a mixing of coral rock with sand and shell rock – all of which makes for an unusual planting media and difficult plant care.
“The problem with this soil is that virtually all of the beneficial aerobic bacteria – which are needed to breakdown fertilizers into a form that is available to many plants – are non-existent,” says Florida horticulturist Alexis J. Smith.
The calcium carbonate from the coral rock makes the soil alkaline; by definition, a pH always above 7.0. Soil rich in calcium carbonate is called calcareous.
Smith explains that the pH is so high that most of the nutrients become locked up in a form that is unavailable to plants. The high sands do not have any plant nutritional-holding capacity, and the fertilizers wash down through the soil past the plant roots.
Common Deficiency
The most obvious nutritional deficiency due to high pH is iron deficiency, which is most common in ixora and gardenias. A pH of 8.5 is 1000 times more alkaline than what ixoras require for lush dark green growth.
Smith points out that the usual chlorotic (yellowing) condition in ixora is because of lime-induced iron deficiency. However, iron is not the only element that is bound up. Magnesium, potassium, zinc and manganese are also unavailable.
While the ixora -- like other acid-loving plants such as hibiscus, gardenia, citrus, and Allamanda -- can be an attractive landscape plant, there are a few requirements gardeners need to know to keep ixora healthy and flowering in your yard.
“All acid-loving plants will require more fertilization management than plants that are adapted to growing in alkaline soils,” says Gail Keeler, Institute of Food and Agricultural Sciences, University of Florida. (November 2003)
“Mostly this involves being aware of the pH (or acidity) of the soil you are planting in. A pH of around 5 is good for ixora; this pH is slightly lower than for most landscape plants.”
How to Create Healthy Soil
According to Smith, there are a number of other ways to deal with theses soil conditions and create an environment for healthy plantings.
1) Start with a soil analysis on the entire project. Understanding the conditions that exist will guide the nutritional management of your plants.
2) Make a selection of plants that are tolerant of existing soil conditions. Cocoplum, for example, is tolerant to high pH, whereas ixora is intolerant and will turn chloritic within a few months in a calcareous soil.
3) Use fertilizers that have chelated materials. Keep in mind there are different chelates made especially for the higher pHs.
4) Use acidifying fertilizers. When combined with organic material, they reintroduce the microorganisms and bacteria needed for the reduction and breakdown of fertilizers.
5) Finally, to prepare healthy plants for winter in South Florida, make sure to cut plants back early and fertilize in late summer or winter.
Keeler and her UF team also recommend that you avoid planting Ixora or any acid-loving plant close to your concrete foundation, and screen soil in planting areas for any concrete fragments that may cause soil pH to be too high for good plant growth.
