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  • From: "Tradingpost" <tradingpost@lobo.net>
  • To: livingontheland@lists.ibiblio.org
  • Subject: [Livingontheland] Greensand & other rock dusts
  • Date: Wed, 25 Nov 2009 23:11:41 -0700


Greensand & other rock dusts
by Chris Alenson
http://www.pmac.net/rockdust.htm
SANET Post August 1997

The debate about rock or mineral dusts and their potential for use in
agricultural is an interesting one. Most rock dusts whether they might be
granite, basalt, diorite, etc consist of a range of minerals, often silicates
which are not particularly soluble. Basalt for example with its range of
minerals contains upwards of 50 or more elements many of which are valuable
sources of nutrition for plants if they can be mobilized.

To think only of solubility as a factor in providing plants with nutrition
perhaps is acting as a disservice in dispensing with possible sources of
mineral nutrition for plants. Through time rocks weather due to physical,
chemical and biological factors. Elements are released and either go into the
soil solution where they might be taken up by plant roots or leached through
these weathering factors and lost for ever into streams and into the sea.
This is particularly so for the elements calcium, magnesium and potassium.
The other mechanism for storage of these released elements is in the exchange
complex of the soil where the elements might be attached to soil colloids
such as clays and humus. They are held against the weathering forces but can
be accessed by plants through the principle of cation exchange.

The question may now be asked how can these seemingly insoluble minerals
release their elements such that plants might benefit from them. Many papers
have been published that attest to the release of plant nutrients from
minerals and rock dusts by organic acids released from decomposing organic
matter ( humic acids) and from soil micro organisms. Acids such as malic,
citric, acetic are but some of the weak acids produced by these micro
organisms that have been demonstrated to be able to release the so -called
insoluble elements.

As growers the question then remains, how can my management practices assist
with these processes. The addition of rock dusts with compost or with the
incorporation of a green manure/cover crop provides the opportunity for
increased biological activity and organic acids to make some of these
elements available for plant nutrition. Just as lime has to be incorporated
with the soil for best effect so the addition of rock dusts requires this
incorporation and more. For example the addition of rock phosphate with
compost produces a product that is equal in trials to the addition of single
superphosphate.

Don't discard the opportunity to rebuild your soils with rock dusts if it can
be done cost effectively. If the source is local and transport costs are low
then give it a go. Trials we have carried out with farmers have definitely
demonstrated the benefit. Know your rock type, don't choose mono minerallic
rocks such as quartz thinking they will contain a broad range of minerals.
Ask a geologist or your quarry manager what the rock type is and access
material which has the consistency of talcum powder. The finer the powder the
greater the surface area that can be attacked by soil acids.

We have always seen the addition of mineral bearing dusts as supplementing
the soil for now and the future and not a replacement for sound soil
management practice based on soil analysis, correct liming, stocking rates,
rotations, etc.

I welcome further debate on this issue.

Chris Alenson
Technical Adviser
Organic Advisory Service
Organic Retailers & Growers Association of Australia




  • [Livingontheland] Greensand & other rock dusts, Tradingpost, 11/26/2009

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