Geology for Organic Growers

While organic growers support soil biology in preference to soil chemistry, there is an awareness of the need for trace elements as provided by either seaweed spray, rock minerals or zeolites.  Basic elements of geology are discussed with a view to assisting organic growers understand the nutrients in their soils and identify what useful elements or mineral supplements are required. Being scientists, geologists are big on classifying various groups of rocks and minerals. The three main groups of rocks are igneous, sedimentary and metamorphic rocks.  Igneous rocks solidify from hot magma that exists below the earth’s crust.  Volcanic rocks such as basalt, rhyolite and tuff are formed on rapid cooling of magma that comes out of volcanoes. Granites are coarse grained igneous rocks that cool slowly deep underground.

Sedimentary rocks are formed from transported rock and minerals that have been deposited and subject to some form of consolidation and cementation.  Coarse rounded materials become conglomerate.  Sand articles are cemented to form sandstone and clays are consolidated to form shale and mudstone.  Coal forms from the collection and consolidated of organic material. Limestone is formed from dissolution of calcium carbonate or from aggregation of shell or coralline material (or both).

Metamorphic rocks are rocks that have been chemically and / or physically changed by either pressure and / or heat.   They may have been originally igneous, sedimentary or metamorphic rocks.

From a gardening perspective, the rock that lies below the garden soil has two effects.  The first is to provide a base for the soil.  This base may vary from porous to impermeable.  Porous rocks drain well and provide for aeration of the soil. Water soluble nutriments are leached out more quickly.  Impermeable rocks trap water and impede air flow through the soil.  They reduce nutrient leaching.   The second property is to slowly release various elements as they are slowly degraded by physical, chemical or biological degradation.  This process may provide plants with a wide range of elements needed for microbe and plant growth.   What gets released depends on the chemistry of the original rocks.  We now need to look more closely at the mineral structure of many rocks.

Whether fine grained or coarse grained, (basalts verses granites), igneous rocks fall in a spectrum that ranges from basic rocks that are rich in calcium and iron (and other trace elements) to acid rocks that are rich in silica, sodium and potassium.  Many of these elements are mentioned in the organic growing literature.  Potassium is important for growth of stems and fruit while calcium and iron are important for the development of leaves.  These elements and rock types then relate to prime horticultural areas.  Market gardens grow particularly on basalt (basic) derived soils at Toowoomba, Lockyer valley, Mount Tamborine and Maleny.  Prime fruit growing areas are located on acidic granite soils as around Stanthorpe. However, few of us are blessed with such naturally rich soils.  Limestones and many sandstones are rich in calcium but need iron, potassium and magnesium as well as various trace elements.

Clays (from shale and mudstone) give some calcium and retain other minerals when they have been added.  They are also useful in retaining moisture.  Some clays expand dramatically when they become saturated and form deep cracks when they dry-out and shrink.  My place in Brisbane’s inner west is underlain by phyllite, hornfels and quartzite that are metamorphic rocks derived from shales and sandstones.  Heat and pressure has caused some chemical and physical reforming of minerals so that the few useful elements are locked in the rocks and very, very slowly released.  Being very hard strata, it weathers and erodes very slowly and is thus one of the more hilly parts of Brisbane.  The result is that I need to work harder than most to build a rich soil and slow the natural process of leaching that strips the useful elements from my soil.

For those who wish to locate their property on a geological map, I have access to numerous geological maps and I can assist with interpretation.  Plans are also available through the Geological Survey at the Department of Mines and Energy. 

To obtain maps:


Authored by: 
Bruce Ham
Sourced from: 
STFC newsletter Apr May 2008
Date sourced: 
Apr 2008