A luxuriant and healthy collection of trees is evidence that the gardener is aware of the soil requirements of the species under cultivation and has a basic understanding of soil science. Before addressing gardening soils for South Florida, some background is called for. Soil science has its origins in the 19th Century linked to attempts to improve crop productivity by replacing nutrients removed from the soil by harvesting, with the applications of appropriate formulations of fertilizer. These early efforts led to soil classification and mapping and an appreciation that soils were a valuable natural resource. The success of modern gardening is built upon scientific soil management. Tree species around the world have evolved under soil conditions in their native areas. In some instances, trees are tolerant of a broad range of soil types, in other cases a tree may require very specific conditions to flourish. Soils are made up of sand, sild and clay, derived from the decomposition of rocks of various types. Soils are characterized according to four properties: physical, chemical, biological and organic content. Physical properties refer to the texture, particle size, structure, density and drainage. Erosion is added to this list if it is a factor. Chemical properties pertain to the pH, nutrient availability and salinity. Biological properties relates to microbial and faunal activity within the soil, such as mycorrhizae and earthworms. The organic element of a soil includes the residues of plants and animals at various stages of decomposition to release their mineral components. Soil assessments are typically organized around these four subjects. What has been stated so far is one-dimensional and refers only to the soil at the surface, but soils have another dimension, a profile of soil horizons moving downward from the surface which is another important aspect of any soil. Soil types are made up of three or four distinctive horizons of varying thickness from the ground surface to parent rock. Beginning at the surface, the O horizon consists of non-decomposed organic matter; the A horizon is the surface soil and is made up of organic matter and mineral soil along with soil fauna; the B horizon is the subsoil, composed of partially decomposed rock material altered by physical and chemical processes; the parent rock is the final horizon, the raw material for future soil. Tree roots are predominantly found in the A horizon, and extending into the B horizon.
In South Florida, the typical soils are derived from limestone rock. These soils tend to be porous such that the A horizon and into the B horizon do not retain water or nutrients very well. They have a high pH (about 8) with a water table generally just a few inches below the surface. Areas of poor drainage are natural swamplands that when reclaimed have a surface layer of humus (muck). In Some areas exist of rocky outcrops or pockets of sand. Successful gardening in typical limestone soils requires either the cultivation of trees which tolerate high pH soils, e.g. native trees, or by amending the soil to make it more neutral and acceptable for exotic trees. Because of soil porosity, trees may require supplementary watering, along with fertilizer application to provide the nutrients the soil cannot retain. Commercial fertilizer containing nitrogen and potassium are recommended; because of water pollution issues phosphorous is excluded from South Florida fertilizer. Most trees grow well in light clay-loam soils, with good drainage. By knowing the soil requirements of the trees in your garden and having the soil tested to determine the ideal amendments, natural soil conditions can be overcome.