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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