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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
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.
Organic Advisory Service
Organic Retailers & Growers Association of Australia