ROCK DUST/MINERAL FERTILISER
Rock minerals help restore a broad spectrum of minerals to the soil. They are best applied in a pulverized form called 'rock dust', a waste product obtained from rock crushing processes (thus the name 'crusher dust').
Rock minerals help to reduce soil compaction. They also provide an environment that supports the proliferation of soil microbes. As the increased soil life slowly accesses the beneficial minerals, plants also benefit.
The major minerals available from different rock dusts are potassium (K), calcium (C), phosphorus (P), and magnesium (Mg). Rock dusts also provide trace elements such as silicon (Si), iron (Fe), and zinc (Zn) to name a few. Nitrogen will generally not be obtained from rock minerals.
For a comprehensive overview of the benefits of rock dust, visit Remineralize The Earth.
Types of Rock Dusts
The source rock will determine what minerals are supplied by the rock dust.
Igneous Metamorphic Rock
Basalt, Greenstone, or Hornfels rock dusts are types of igneous metamorphic rock. They provide calcium, magnesium and phosphorous to the soil. They also provide many other valuable trace elements such as iron and silicon. Note that Greenstone generally provides much higher levels of phosphorous than Basalt or Hornfels rock.
Granite
Crushed granite and naturally decomposed granite (aka deco) provides potassium, calcium, and magnesium. They also provide many other valuable trace elements such as silicon and iron. Reference the 'granite' Wikipedia entry for a worldwide average of the chemical composition of granite. Note that deco often contains fine clay particles, so it is best worked into the soil (as with all rock minerals).
How to use Rock Dust
Particle Size
While fine particles of rock minerals are best for the soil life as they can be broken down faster, it can make it hard to remember where and how much you have used. Rock dust with some very small pebbles through it can make things much easier for the gardener. This is because the pebbles remain intact and visible long after being applied to the soil.
How Much to Use
Light application: Three ( 3 ) tonnes per acre, worked into the top four ( 4 ) inches of soil.
Normal application: Ten (10) tonnes per acre, worked into the eight ( 8 ) inches of the top soil. This is a long term application. Since one large application eliminates the need for further applications at a later stage, this means substantial savings and higher yields.
Remedial application: Twenty (20) tonnes per acre to remediate impoverished, dry soil.
As a general guide, you should be able to see the larger particles of the rock minerals throughout the soil when the soil is less than a couple of metres away. However from a distance the rock minerals should not be visible. If they are visible, then you might have used too much, which might cause your soil to have reduced water holding capacity. This can be corrected over time by working more organic matter into your soil.
[Source: Brisbane Organic Growers Inc.] |
