So you have found out that the rock on you farm is limestone or marble. What does that mean for how your soil has formed? What is limestone/marble anyway?
Limestone is a white to grey rock made up from the mineral calcite or its sister mineral aragonite. Both of these minerals have the same chemical compositions – calcium carbonate. In some cases some of the calcium in the mineral is replaced with magnesium and the mineral is known as dolomite. The real issue here is that all these minerals are carbonates. Carbonate minerals easily react with acids, including the very weak acids found in rainwater and soil water. Limestones however, often include some very minor amounts of other minerals, such as clays and iron rich minerals. These may make up only 1% of the rock in pure limestones, but can be 40% or more in limey sandstones (a sort of hybrid between pure limestone and sandstone). These minor minerals become important when soil is formed.
Where did all these carbonates come from?
The vast majority of limestones have formed from the accumulation of the dead remains of ocean animals, like corals. Many animals that live in the ocean make their living quarters out of calcium carbonate that they extract from the water. Corals reefs, for example, are huge deposits of dead coral and calcium carbonate deposited by algae living within the coral structure. The living coral that we all see are colorful polyps on the outermost layer of the reef. In some places the limestone deposits are not made from corals, but from the tiny shells of a plankton called foraminifera (or forams for short).
In some other places in the ocean calcium carbonate precipitates out of sea water to form sand sized white pearl-like balls called ooliths. Some of these places are the banks off the Florida keys and the Bahamas which are made up of these oolithic limestone sands. These sands can later turn onto a rock called oolithic limestone.
So most of the limestone we find on our farms have once been part of deposits found on the ocean floor, turned into rock and uplifted into land over millions of years.
What about marble?
Marble is the rock type that is formed when limestone is squashed and/or cooked due to mountain building processes. Or put another way, marble is the metamorphic rock that forms from limestone. It’s chemical composition is also calcium carbonate – it is just more crystalline than limestone.
Weathering of limestone and marble
In some places the limestone is so pure that the rain and soil water weather away vast amounts of limestone materials to form sinkholes and caves – all these features are collectively know as a karst landscape.
Limestone/Marble and soils
When a limestone and marble weathers, most of the carbonate materials is dissolved away by rain and soil water and what soil is left behind is made up of the minor minerals. In many places the soils developed in limestone areas are very red (and are called terra rosa soils) because the minor minerals are iron based and they ‘rust’ to form red clays that make up the soil. In other places where the country rock is a ‘dirty ‘limestone (like a limey sandstone or limey shale) the soil is more like that found on sandstone or shale than the limestone.
One feature of these soils is that they have higher pH levels (7 and above) because any acids in the soil react with the carbonate minerals. I have recently been told that some folks who live on limestone-based soils have been told to spread decomposed granite (which is acidic) to raise the pH of their soil.
Limestone and other soils
Because of the wonderful property that carbonate minerals have to react and basically ‘remove’ acid from the soil, limestone has been used, in a crushed form, as a way to change the acidity of soil. Dolomite – the magnesium rich limestone mineral- has been used to both the raise the pH and add magnesium to the soil.
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