Soils Media

by Jeff Wasielewski 

South Florida soil consists of rock, sand, marl and muck. The rock is known as Miami limestone, which is an alkaline calcium carbonate. It is not coral rock as some believe. Miami limestone is high in pH (7.8-8.1), does not retain water or nutrients well and makes growing many plants a challenge. Our type of limestone is very young geologically and is found only one other place in the world (Bahamas). It was formed when the shallow seas covering much of South Florida receded exposing the ocean floor to the sub-tropical sun. Miami limestone is a general term for the two types of limestone found in South Florida: oolitic limestone is found near the east coast and bryozoan limestone is found further west. Oolitic limestone was formed when small particles of sand and shells were coated with calcium carbonate making tiny particles, called ooids, which cemented together when the seas receeded.  Oolitic limestone is found on the eastern edge of South Florida in what is known as the Miami Rock Ridge. This ridge varies from four to twelve feet in elevation and extends from the city of Miami, south to the Deering Estate, where it curves westward towards the Everglades. The ridge comes to an end in the Everglades at Long Pine Key. The ridge’s high elevation caused early settlers to build their homes upon this ridge; consequently, at present time the ridge is densely covered in homes and urban sprawl.  The natural vegetation that was found on the ridge was hardwood hammocks and pinelands. Pictured: Pines on a limestone ridge.

Pockets of sand are also sometimes found on the ridge. Before South Florida was riddled with canals, there were seasonally wet finger glades intersecting the limestone ridge. The soil found in these glades is known as marl, a grey, clay-like substance formed by the erosion of the surrounding limestone. Marl is high in pH and holds water well. To the west of the Miami Rock Ridge, there is a slight drop in elevation which historically caused shifts in plant habitats. The soil found in this area is a combination of sand, muck and limestone. Sand is found in large quantities around the Redland as well as other areas in South Florida. Muck is half-decomposed organic matter and is often sold as potting-soil or top-soil. Its use as a potting soil is not recommended. Muck has very poor aeration and drainage. It is very hard to get it dry once it is wet and very hard to get it wet once it is dry. Pictured: Exposed marl.

The limestone found west of the Miami Rock Ridge, is known as bryozoan limestone. Bryozoan limestone was formed when the lowered seas exposed sea creatures called bryozoans which also hardened under the glare of the sun. This area of South Florida is prone to flooding and homes are typically built on several inches of builder’s fill. This is mostly crushed limestone and therefore has all the qualities of limestone such as a high pH, good drainage, and poor nutrient holding capacity.

Rock, sand, muck, marl and builder’s fill are not a recipe for good soil and yet looking around South Florida one sees a lush tropical paradise. Has the soil been amended by better components? No, the key to growing plants in South Florida is the right plant in the right location: one must choose plants that are adapted to growing in our soil. It is not recommended to add superior media components to South Florida soil. Media components are best used in container growing. Pictured: Builder’s fill.


Media Components

A good media mix will have four major functions: it will hold nutrients; it will hold water; it will provide oxygen to the roots (drainage); and it will support the plant.  No single component has all four of these qualities; therefore, a combination of different media components is necessary to obtain a useful mix.

Here are some of the components that are available:
Non-augmented Potting Soil has poor drainage and aeration but does hold on to nutrients well. This is a type of native soil called muck.

Perlite is heat expanded volcanic rock. Excellent in providing drainage and aeration, but it does not hold nutrients or support the plant. Perlite is a must in any media mix due to its drainage and aeration qualities. Vermiculite is similar to perlite but tends to compact and should be avoided.

Peat is partly decomposed organic matter. This has very good nutrient and water holding capacity and is slightly acidic. Its nutrient holding ability makes it a desirable component of most mixes.

Coir is shredded coconut fiber which has very similar qualities to peat and can replace peat in a prepared mix. Coir should be soaked in water overnight before use.

Sand is good for adding weight to a mix. Particle size is crucial. Builder’s sand has large grains that will not compact and will provide adequate drainage and should be considered in most mixes.

Pumice, Chicken Grit and Aliflor are all used to add aeration and drainage to a mix. These components do not hold water or nutrients well and are usually used in combination with peat. These components may be more expensive than the others mentioned.

A good potting soil combines several of the components listed above to achieve the four major functions of a proper soil mix. A good general mix would combine the water and nutrient hold capacity of peat (45%) with the weight of sand (30%) and the aeration and drainage of perlite (25%). Your mix can be tailored to meet your needs. An acid loving plant may need more peat while a plant that needs excellent drainage might require adding some pumice or chicken grit.