So you have found out that the rock on you farm is granite. What does that mean for how your soil has formed? What is granite anyway?
Granite is an igneous rock that formed from of a pool of molten rock deep in the Earth’s crust. The molten pool was buried under many miles of rock layers which acted like insulation and allowed the molten rock to cool slowly. Because of the slow cooling, the minerals formed large crystals which ended up all interlocking to form the rock. Over millions and millions of years the rocks layers above the granite have been uplifted and eroded away so that the granite has been exposed at the surface. So when you stand on granite on your farm you are standing on considerable and dramatic Earth history!
Some granite bodies are HUGE. They can be tens of miles across.
Granites contain a mix of the following minerals.
Quartz – a glassy mineral.
Feldspar – a group of minerals that can be white or pink and break (fracture) in flat surfaces.
Mica – a group of mineral that can be silvery white or black and flake off on very thin sheets, like pages of a book.
Other dark minerals – small amounts of the minerals like Hornblende. Sometimes these minerals are rich in iron and may contain sulfur.
The overall color of the granite is because of the amount of these minerals. On geological maps some of these terms can be used to describe to type of granite. A pink granite has a lot of pink felspar, called Orthoclase. A white granite has a lot of white feldspar, called Plagioclase. Some granites have black mica (Biotite) and others have silver mica (Muscovite).
Weathering and outcrops
Granite normally has a distinctive way that it weathers (reacts with the air and water). On a large scale it tends to form rounded boulders which are called tors. Sometimes these appear to be stacked on top of each other – but in fact they have not been ‘placed’ but have weathered in place in that pattern.
On a smaller scale, the mineral grains first are broken down by water freezing and expanding in cracks to become a course sand of rough mineral fragments called grus. Grus can normally be found around the base of tors, and in some places it is mined and used as road base.
The broken down granite grus minerals can then react chemically with the air and rain and change to other minerals.
|Original Mineral||New Mineral||Released Elements|
|Felspars||Clays||Potassium Sodium Calcium|
* Notice how quartz does not break down into other minerals. It is a very stable mineral in almost every environment as so often it remains as the only mineral in sands in rivers etc.
This is then the basis of the soil that develops on granite. It has sand sized grains of quartz with clays and often some of the remnant feldspar and mica fragments. It normally produces an acidic soil (low pH) that is quite poor in nutrients. That means that you need to add loads of organic matter to increase its nutrient base and that many people add lime to counteract the low pH.
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