How to Plant in Marl Soil

Planting in marl soil is challenging and time consuming, but it may be the only way to have a garden. Taking the steps to plant properly in marl will save a lot of trouble in the future.

What is marl?

Marl soil is one of the two most predominant soils in South Florida. Marl looks something like cement mixed with shell, and is almost as hard. While it may seem impossible that anything could grow in this soil, that isn't so. With the proper amendments, most plants can grow quite well in the marl of South Florida.

South Florida sits on a base of limestone. Marl is a base soil that is mostly limestone, mixed with sand. Limestone + sand = concrete. Marl sets up very hard, like concrete, but it is not impermeable to water, making it the soil of choice for building up low lying lots.

Contractors usually only put a shallow layer of 3-6" of topsoil down to get the sod to grow, therefore, houses built after 1989 probably have mostly marl soil underneath the topsoil.

How to plant in marl soil

To plant in marl soil, you will need:

• A strong shovel
• A water hose
• Dishwashing liquid (not Dawn)
• Composted Manure
• Organic Matter
• Epsom Salts

When dealing with planting in marl, water is a necessity. 

To plant vegetables in marl, it is necessary to dig either a hole 18 inches in diameter or a trench 18 inches wide. Both of these should be at least 12 inches deep.

To plant ornamentals in marl, it's necessary to dig a hole 3 times the diameter of the container, that's a 24" diameter hole for a 1 gallon pot, and a 30" diameter hole for a 3 gallon pot. The depth of the hole should be 3" more than the height of the rootball.

This will allow the roots to have room to spread. If this is not done, the plant will begin to decline and die within a year when the roots hit the marl, because young plant roots cannot penetrate and draw nutrients from marl.

Digging in marl is a process

First, squirt dishwashing liquid on the ground, and sprinkle it with water until it starts soaking in. The dishwashing liquid acts as a surfactant, breaking the surface tension of the soil. Then it will be possible to dig a small hole, fill it with water, and let the water sink in. It will now be possible to dig out to where the water soaked into the marl. Why is this so? Because water breaks the chemical bonds between the limestone and the sand in the marl.

This is sort of a "rinse and repeat" operation. Every time a larger hole is dug, fill it with water, let it sink in, and repeat the process until the proper size hole is completed. Time consuming? Yes, but if the yard has been built up with marl, this is the only choice, other than having someone with a backhoe dig it.

Filling the hole

The marl will now go back into the hole, along with at least 50% organic matter added. This is done by layering. First, put three inches of composted manure into the bottom of the hole to feed the plant during the first four weeks when it is getting established. NEVER put non-organic granular fertilizer into a hole when planting, as it will burn the roots.

Then place the plant and layer the marl and organic matter in 2-inch layers. After every two layers, fill the hole with water to keep it from developing air pockets, and to also make sure the organic matter gets completely saturated. Stop when it is filled within 1" of the top of the hole. When planting ornamentals, the top of the root ball should be level with the top of the hole.

When the hole is filled to within 1 inch of the top of the hole, sprinkle epsom salts around the top of the hole (1/4 cup for 1 gallon or smaller, 1 cup for 3 gallon or larger) and water in. This helps the roots establish themselves better. Mulch the plant to retain moisture, and the planting is done.
If planted properly in marl soil, your plants will thrive in their new environment for years to come.