[Scpg] good Australian article on "The Living Soil" by Robyn Francis

[LBUZZELL at aol.com]

THE LIVING SOIL

No-dig gardeners and no-tillage farmers realise that  healthy plants and
good yields can be obtained on a sustained basis from  undisturbed soils,
nevertheless for many, lingering doubts remain. Doesn't  soil need to be
periodically aerated to stimulate microbial activity and  liberate
nutrients to plants?

Soil scientist Alan Smith has been the  principal research scientist for
the New South Wales Department of  Agriculture. IPJ put the question to
him, his reply, and the article which  follows contained some real
surprises!

"I can understand your  confusion when trying to interpret the claims and
counterclaims made  regarding the value of 'aerating' soil. Obviously, I
am a non-believer in the  value of aeration, at least in Australian
conditions. One thing that we must  always be wary about are treatments
that may give initial, short-term gains  but lead to long-term problems. I
believe the 'aeration' theory is such a  treatment. There is no doubt 
that ploughing soil does initially increase  aeration and does result in
intimate contact between the mineral soil and any  organic residues. This
stimulates microbial activity and nutrients  immobilised in the organic
reserves are liberated rapidly into the soil.  However, unless plants are
growing in the soil to take immediate advantage of  these mobilised
nutrients, they are leached or rapidly fixed in unavailable  forms.
'Aerating' soil, of course, usually results in the removal of  plant
material and so there are no plants (or only a few) left to  take
advantage of the released nutrients. If this practice is continued  
season after season then it is obvious that a loss of nutrients results.  
The 'aeration theory' really developed in the northern hemisphere where  
the extended cold winters prevent microbial decomposition of organic  
residues in soil. In spring it is advantageous to stimulate the  
decomposition rate so that plants can obtain nutrients during a  
relatively short growing season. In Australia, conditions are generally  
favourable right throughout winter for at least some organic matter  
breakdown. Thus, our conditions are very different. It is also worth  
considering just what problems arise when this 'aeration' is attempted  
in tropical soils where conditions are even more favourable for the  
breakdown of organic matter, Yes, we all recognise that under those  
conditions it is a recipe for disaster."

MICROBIAL INTERACTIONS   IN SOIL AND HEALTHY PLANT GROWTH
Microbial interactions in soil play a key  role in the biological control
of plant diseases, the turnover of organic  matter, and the recycling of
essential plant nutrients. An understanding of  the mechanisms involved
may lead to more efficient methods of growing plants,  whether they be
food crops in agriculture or plants in gardens.

Before  these interactions can be discussed, however, it is essential to
reaffirm the  unique position that plants have in any ecosystem. They are
the only living  organisms that can directly utilise the energy of the 
sun and in the process  they transform this energy into forms available 
to other living things. The  green pigment, chlorophyll, in their leaves
traps the light energy from the  sun and an interaction occurs in leaves
with carbon dioxide gas from the  atmosphere to produce carbon compounds
then available as energy sources to  other living things, including man,
other animals, insects and  micro-organisms when they consume plants or
plant remains.

Although  plants have this unique ability to trap the energy of the sun
and transform  it into the chemical energy they need to grow, metabolise
and reproduce, they  also require other materials that they are unable to
produce directly. For  example, they require various elements, including
nitrogen, phosphorus,  sulphur, calcium, magnesium, potassium and trace
elements. The soil is the  reservoir of these elements, but to obtain
adequate supplies plants must  alter the environment around their roots 
to mobilise these nutrients. One  important way the plant achieves this 
is by stimulating the activity of  micro-organisms in soil around their 
roots and the microbes then enhance  nutrient mobilisation.The plant 
stimulates microbial activity in soil by  supplying chemical energy in 
the form of root exudates and litter. Thus, an  intimate relationship 
exists between the plants and soil microbes.  Unfortunately, in many of 
the conventional methods used in agriculture this  relationship is 
impaired, resulting in problems of nutrient supply to the  plant and an 
increase in the incidence of disease.

The latest  research indicates that during the life of the plant up to 25
per cent of the  chemical energy in the form of carbon compounds that is
manufactured in the  leaves is lost by the plant into the soil directly
adjacent to the root. This  material is lost either as root exudates or 
as dead, sloughed plant cells.  On a first examination this seems to be a
highly inefficient, wasteful  mechanism. The plant goes to considerable
trouble to trap the energy of the  sun and convert it to chemical energy,
but then loses almost a quarter of the  energy into the soil! One view is
that nothing in nature is perfect and  'leaky' roots are inevitable. I
certainly do not subscribe to this view. I  firmly believe that if some
living system is apparently wasting a quarter of  the energy that is goes
to the trouble to manufacture, then this loss must  ultimately benefit 
the organisms directly. IF this is not the case, then  evolution should 
have resulted in the selection of plants that lost less of  their energy.

How does this loss of carbon compounds into  the soil  benefit the plant?
Most importantly, these compounds are energy sources for  the soil
micro-organisms which proliferate in the rhizosphere, i.e. the soil  zone
directly adjacent to the plant root. These micro-organisms multiply  so
rapidly that they deplete the soil of oxygen at numerous microsites  in
the rhizosphere. Thus, oxygen-free anaerobic microsites are formed.  The
formation of these anaerobic microsites plays an important role  in
ensuring the health and vigor of plants.

ETHYLENE PRODUCTION IN  SOIL
Our research shows that ethylene, a simple gaseous compound, is  produced
in these anaerobic microsites. Furthermore, this ethylene is a  critical
regulator of the activity of soil micro-organisms and, as such,  affects
the rate of turnover of organic matter, the recycling of plant  nutrients
and the incidence of soil-borne plant diseases. Concentrations  of
ethylene in the soil atmosphere rarely exceed 1 to 2 parts per  million.
Ethylene does not act by killing soil micro-organisms, but simply  by
temporarily inactivating them - when concentrations of ethylene in  coil
fall, microbial activity recommences.

Soil ethylene is produced  in what we call the OXYGEN-ETHYLENE CYCLE.
Initially, the soil  micro-organisms proliferate on the plant root
exudates and deplete the soil  of oxygen at microsites. Ethylene is them
produced in these microsites and  diffuses out, inactivating without
killing the soil micro-organisms. When  this occurs the demand for oxygen
diminishes and oxygen diffuses back into  the microsites. This stops or
greatly reduces ethylene production, which  enables the soil
micro-organisms to recommence activity. Favourable  conditions are then
recreated for ethylene production and the cycle is  continuously repeated.

In undisturbed soils, such as found under forest  and grasslands, 
ethylene can be continually detected in the soil atmosphere,  indication 
that the oxygen-ethylene cycle is operation efficiently.  Conversely, in 
most agricultural soils, ethylene concentrations are  extremely low or
non-existent. This is to be expected if ethylene plays an  important role
in regulation microbial activity in soil. It is well  established that in
undisturbed ecosystems where there is a slow, balanced  turnover of
organic matter, efficient recycling of plant nutrients and  soil-borne
plant diseases are unimportant. When these ecosystems are  disturbed for
agricultural of forestry usage the situation changes  dramatically, There
is an alarming decline in the amount of soil organic  matter, 
deficiencies of plant nutrients become commonplace and the incidence  of 
plant disease increases dramatically. We attempt to overcome these  
problems by additions of inorganic fertilisers and by the use of  
pesticides, which increase our production costs considerably. It is also  
generally true that the longer we farm soil, more and more of these  
inputs are necessary to maintain our yields.

We argue that the trend  could be reversed, at least partially, if we
could create favourable  conditions for ethylene production in these
disturbed soils. We now know that  one of the major reasons why 
disturbed, agricultural soils fail to produce  ethylene is because our 
techniques cause a change in the form of nitrogen in  soil. In 
undisturbed soils, such as under forests or grasslands, virtually  all 
the nitrogen present is in the ammonium form with just a trace of  
nitrate nitrogen present. When these ecosystems are disturbed for  
agricultural usage, virtually all the soil nitrogen occurs in the  
nitrate form. This change in form of nitrogen occurs because the  
disturbance associated with agricultural operations stimulates activity  
of a specific group of bacteria which convert ammonium nitrogen to  
nitrate nitrogen. Plants and micro-organisms can use either form of  
nitrogen, but our research has conclusively shown that ethylene  
production in soil in inhibited whenever the nitrate form is present at  
more than trace amounts. Ammonium nitrogen has no such inhibitory effect  
on ethylene production.

Nitrate nitrogen stops ethylene production  because it interferes with 
the formation of the anaerobic microsites. When  all the oxygen is 
consumed in the microsite a series of complex chemical  changes then 
occur. One of the most important changes that occurs is that  iron goes 
form the oxidised or ferric form to the reduced or ferrous form.  Iron is 
one of the major constituents of soil, making up somewhere between 2  and 
12% of its weight. In adequately aerated soil virtually all the iron  
exists as minute crystals of iron oxide and in this oxidised or ferric  
form is immobile in soil. If oxygen is completely consumed in microsites  
in soil, and reducing conditions exist, these minute crystals break down  
and iron is then transformed into the highly mobile ferrous or reduced  
form. Again our research has shown that ethylene production occurs is  
soil only when iron is in the reduced or ferrous form. In other words,  
ferrous iron is a specific trigger for ethylene production. If there is  
no oxygen in the microsites, but nitrate nitrogen is presents. then the  
complex chemical changes leading to the reduction of iron form the  
ferric to the ferrous form are inhibited. This is how nitrate nitrogen  
stops ethylene production.

How does ferrous iron trigger the release  of soil ethylene? This form of
iron reacts with a precursor of ethylene that  is already present in the
soil and a reaction occurs that results in the  release of ethylene. Our
work has established that this precursor originates  from plants and, 
more importantly, it accumulates to appreciable amounts  only in old, 
senescent plant leaves. When these old leaves fall to the  ground and 
decompose, the
precursor accumulates the soil. Then, when  conditions become favourable
for mobilisation of ferrous iron, ethylene is  produced.

We have also show that different plant species vary markedly in  the
quantities of precursor that accumulate in their old leaves. This  is
important to know when selection plant species to use as cover crops  to
increase the ability of agricultural soils to produce ethylene. A few  of
the plant species that produce high concentrations of precursor are  
rice, phalaris, chrysanthemum, avocado, bullrush and Pinus radiata. Some  
of the low producers include Dolichos, paspalum, lucerne and bracken  fern.

In retrospect it should not be too surprising that the ethylene  
precursor accumulates appreciably only in old, dead plant leaves. After  
all, in natural communities of plants old dead leaves comprise the bulk  
of the litter that falls on to soil. Also, it is equally clear that  in
agricultural situation most of the old plant leaves are removed  either
during harvest or by grazing or by burning crop residues.  Thus,
agricultural soils are usually deficient in precursor.

It is now  possible to specify the soil conditions necessary for ethylene
production -  (1) there must initially be intense aerobic microbial
activity, at least in  the rhizosphere, to ensure that oxygen-free,
anaerobic microsites form; (2)  conditions in the microsites must become
sufficiently reduced to mobilise  ferrous iron to trigger ethylene
release; (3) concentrations of nitrate  nitrogen in soil must be kept to
trace amounts, otherwise ferrous iron will  not be mobilised; (4) there
must be adequate reserves of the ethylene  precursor in soil.

MOBILISATION OF ESSENTIAL PLANT NUTRIENTS
A major  limitation to plant growth in most agricultural soils is an
inadequate supply  of essential plant nutrients. This occurs even though
there are adequate  reserves of these nutrients in soil, but they are 
held in highly insoluble  forms. Their high degree of insolubility 
prevents loss from the soil by  leaching, but since they are only 
available to the plant in the soluble  form, problems of supply rate to 
plants are created. Formation of anaerobic  microsites in the rhizosphere 
of plants, which is of such paramount  importance to ethylene production, 
can play a critical role in the  mobilisation and thus supply rate of 
these essential nutrients to  plants.

This mechanism revolves around the importance of iron in soil. As  
already discussed, under normal conditions in soil most of the iron  
occurs as minute crystals of iron oxide. These crystals have a large  
surface area and are highly charged. As a result plant nutrients such as  
phosphate, sulphate and trace elements are tightly bound to the surfaces  
of these crystals. In this form they are virtually unavailable to  
plants. If, however, anaerobic microsites develop, these crystals break  
down and the bound nutrients are released for uptake by the plant.

At  the same time high concentrations of ferrous (reduced and mobile 
form) iron  are released into the soil solution in the microsite. The 
other essential  plant nutrients, including calcium, potassium, magnesium 
and ammonium, are  held on the surfaces of clay and organic matter. When
concentrations of  ferrous iron increase so much, these nutrients are
displaced by the ferrous  iron into the soil solution, where they too are
now available for uptake by  plant roots. Since anaerobic microsites are
most likely to form in the  rhizosphere of plants, the nutrients are
mobilised exactly where they are  required by the plant. An additional
advantage of this mechanism is that if  the released nutrients are not
utilised by plant roots they cannot be leached  in the soil. As soon as
they migrate to the edge of the anaerobic microsite,  reoxidation of the
iron occurs with recrystallisation of iron oxide. These  crystals then
rebind the nutrients and prevent their loss by  leaching.

The soil conditions necessary for this mechanism to operate are  
identical with those required for ethylene production. Thus in  
agricultural soils, where ethylene production is inhibited or impaired,  
this mechanism of nutrient mobilisation is also restricted. Again, under  
these conditions, the elevated concentrations of nitrate nitrogen that  
occur in agricultural soils are a major inhibitor of efficient  nutrient
mobilisation.

Successful management of soils to increase the  likeihood of anaerobic
microsite formation, which will help ensure a balanced  oxygen-ethylene
cycle and enhance mobilisation of essential plant nutrients,  will demand
alterations to some of the established practices in agriculture.  For
example, techniques aimed at increasing aeration and the oxidation  
states of soil, which give short-term increases in plant growth but  
rapidly create lone-term problems of nutrient depletion and increased  
plant disease incidence, will require modification. Treatments which  
stimulate rates of nitrification (transformation of ammonium nitrogen to  
nitrate nitrogen), such as excessive use of nitrogenous fertilisers,  
overuse of legume dominant pastures, or excessive removal of plants by  
overgrazing or forestry operations, require re-examination.

Some  practical guidelines for successful management of soils include:-
(1) It is  essential that organic residues be returned continually to the
soil. Organic  residues contain essential plant nutrients for recycling,
stimulate microbial  activity in soil, supply ethylene precursor, and
restrict the rate of  nitrification in soil. It is best to use mature
plants as a source of organic  amendments and it is better to return the
residues to the soil surface rather  than incorporate the into the soil.

(2) Techniques of minimum tillage  should be utilised wherever practical.
This ensures that plants are growing  in soil virtually continually, that
there is a minimum of disturbance to the  soil and increases the amount 
of organic matter that is returned to the  soil. Again, nitrification is
restricted when these techniques are  used.

(3) Whenever soil is amended with nitrogenous fertiliser it is best  to
apply the nitrogen in the ammonium form and to apply it in several  small
applications rather than one or two heavy dressings. This again  
restricts the chance of nitrification.

(4) In some situations it will  be advisable to add chemical inhibitors 
of nitrification (e.g. N-Serve or  Terrazole) to soil with the 
nitrogenous amendments to further ensure that  nitrification is restricted.

This article first appeared in 'Australian  Plants' Vol. 9 No. 73, 1977,
then in issue #7 of the International  Permaculture Journal in March 1981.

Djanbung Gardens Permaculture  Education Centre
home to:
Permaculture Education
ERDA Institute  Trust
Nimbin Eco-Village Project Office
Robyn Francis - permaculture  designer & educator
PO Box 379, Nimbin NSW 2480 Australia
Ph 02-6689  1755 Fax 02-6689 1225
permed at nor.com.au   www.earthwise.org.au


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