Steve Diver
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>From Alenson.Chris.CJ@bhp.com.au Sat Sep 20 12:59:06 1997
>Date: Wed, 27 Aug 1997 11:23:18 +1000
>From: "Alenson, Chris CJ" <Alenson.Chris.CJ@bhp.com.au>
>To: "'sanet-mg@ces.ncsu.edu'" <sanet-mg@ces.ncsu.edu>
>Subject: Greensand & other rock dusts
>
>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
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