From the Archives
of the Rare Fruit Council of Australia, inc.
by Lou Gonano
Mulching Fruit Trees
Mulching is a naturally occurring process whereby nutrients are
recycled to the trees roots whilst simultaneously protecting the
nutrient manufacturing occurring around the roots. It takes place in
the natural cycle of growth and decay of all plants where there is no
harvesting - rainforests are a good example - and also where some
pioneer plants prepare the soil for later species in the evolutionary
cycle. This process can be mimicked in the orchard, as we will see
later. This brief article focuses on the practical aspects of
artificial mulching under fruit trees in the subtropics.
In the
subtropics, mulching and mulch breakdown occurs much more rapidly than
in temperate climates; the higher temperatures (which accelerate
chemical reactions) and the heavy rainfalls see to that. Rain readily
leaches nutrients and organic matter from the soil, and the consumption
of nutrients so fast, that the percentage of organic matter in the soil
is relatively low and most of the biomatter of a tree system becomes
stored in the tree itself. Subtropical fruit trees have especially high
mulching requirements because they are not deciduous (generally), they
have shallow sensitive feeder root systems, and organic matter is being
continually harvested.
Mulches provide tremendous benefits;
through decomposition by fungi, bacteria, worms and other organisms,
they slowly release valuable nutrients to the soil, they insulate the
soil and roots against extremes of moisture and temperature and thus
encourage the health of the often invisible biological processors
feeding the tree. They also conserve moisture, reduce evaporation and
help to control weeds. Mulches also need to possess other properties in
order to provide practical benefits to the orchardist. These include
the adequate transmission of moisture down to the roots, a pH reaction
appropriate to the acidity level required by the tree, an absence of
weeds or chemicals which may be toxic in the fruit or to the soil life,
and they should not break down too quickly. Of course, cost and ease of
handling are prime consideration in the success of any mulching
operation. From practical experience, I have no doubts that mulching in
the orchard is a big plus to the bottom line, both short and long term.
I
can sympathise with the next obvious question: what to use and how to
get so much of it. I have come across a number of approaches
practicable for the commercial or hobby orchardist. Basically, you can
grow your own or import it; i.e. buy it off somebody else. Because of
the bulk, transport costs are high, handling is usually much more
onerous, supply is out of your control, and there is the very important
problem of the potential introduction of soil pathogens and new weeds.
Banana
growers etc. beware. One advantage is that mineral deficiencies in your
area can be overcome by introducing a customised mulch, but I believe
that this fine-tuning is better undertaken using special applications
such as crusher dust, dolomite, etc. or proprietary products. Buying
mulch material is an admission that you are farming too intensively;
you are importing organic matter to replace the depletion in the soil.
The
cycle of nutrient exchange can be organised to take place between one
area of the property where it is grown and the orchard proper. Better
still, although not always practical, is to grow the mulch in the
orchard, using interplanting between rows or between trees in a row.
This is easiest when the orchard is still young. It can even be grown
under the tree as a living mulch (not as beneficial from a nutrient
point of view). All these are used at Fonterra where 10 acres of
yet-unimproved, mowable pasture are set aside to effectively service a
3 to 4-acre orchard. Sorghum is planted alone or in a row next to a row
of trees and harvested using a slasher which throws the stalks and
leaves into a bin behind the tractor. If it is too long, it is simply
cut down and run over by the mulcher to chop and pick it up.
Sorghum
is nutritious, has lots of organic bulk, is cheap and fast-growing and
does not mat or become impermeable. Grown in rows right next to the
fruit tree, there are multiple benefits; the fertilising of the sorghum
prepares the soil for later when the fruit tree roots extend further
out, the roots decay and add organic matter for the soil life to
consume, and the mulching process itself is very efficient. Comfrey,
pigeon pea, vetch, oats, papaws and bananas are also grown as mulch.
Comfrey is a 'miner' bringing up nutrients from deeper in the soils and
through decay, feeding them to the tree. Pigeon pea is coppiced
regularly and when it dies in about 2 to 3 years, is cut off at the
base for the roots to decay in situ. Pea and vetch fix nitrogen and
also gradually build up the soil for when the fruit tree grows bigger.
Wattle
(Acacia melonoxylon) are also grown between trees as soil improvers,
and coppiced for mulch until the fruit tree needs the soil. Bananas are
especially valuable if soil conditions permit their cultivation. They
provide a microclimate, develop lots of organic matter, produce a crop
and the residue leaves and stalks are very rich in potash (40-50% dry
matter) which is an element not easily added to the soil except in
soluble chemical form which gives the soil life a hard time. Potassium
deficiency is common also because it is so readily leached. Again, the
roots are left to decay to feed the worms. At Fonterra, we also harvest
the self-sown wattle for mulch, sometimes using a Rover shredder to
make it more manageable for the smaller trees, but in the tropics,
breakdown of whole branches appears to be quick enough. Orchard mowing
is mainly slashing, sometimes picked up with the mulcher, but the
detail around the trees is done with a side-throw ride-on to throw some
mulch onto the trees and to avoid driving the tractor too close to the
tree roots.
Materials with a high carbon content such as bark or
sawdust usually acidify the soil and should be mixed with chook manure
or at least partially composted. Some consideration of the
nitrogen/carbon balance in the mulch allows one to produce a more
nutritious diet for the soil life, much like balancing meals for
humans. This is because the breakdown of the mulch is similar to that
in compost, except that the layer is thin enough not to heat up and
damage the roots.
Some mulches tend to mat and become
impermeable and may need to be mixed with coarser mulches or prunings,
to provide a more open texture and habitat for the decay-promoting
organisms. One grower I spoke to uses lawn clippings from a commercial
lawn care service and spreads it out thinly to avoid this problem.
Bagasse is also susceptible to matting. Whether it be bales of mulch
hay in the shed to a jerry-rigged slasher, each grower needs to look at
his own circumstances, preferences and skills, and hopefully with the
benefit of others experiences, develop his own system of mulching. To
further this process, I would make a plea to all growers to let me know
of any unique or unusual practices so that this can be made available
to all growers. Perhaps we can have a field day in more than one sense!
Of
course, you can always forget to collect the mulch and let the
flowering weeds and aromatic herbs grow wild to encourage predators,
but this is another story for another article.
Back to
Mulch page
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