Healthy soil and sustainable growing

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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