From the Archives
of the Rare Fruit Council of Australia, inc.
by Phil Thomas
Mulching Sub-Tropical Fruit Trees
Having just read two references to mulching fruit trees in the July
newsletter, I thought I would pass on some information I have gained
over the years on the role of mulch from both personal experience and
from various readings.
Firstly in response to Mr. Whitman's
comments about mulch spreading virus and fungus diseases, I have been
mulching trees now for over 12 years and have never had any of the
problems he cites. I now have over 1000 fruit trees mulched including
citrus trees. Rots can develop in tree trunks because: (a) fresh
organic material gives off heat which can burn the trunk and provide an
entry point for fungus and (b) organisms which naturally occur in mulch
and soil, especially in wet conditions can promote fungal or bacterial
rots. These problems can be avoided by keeping all mulching material at
least 10 cm from the tree to prevent the trunk from staying moist for
too long.
Organic mulching is the covering of soil surface with
crop residues, legume or grass hay, there are also of course such
non-organic mulches as plastic. Mulching is a method of improving the
physical condition and fertility of the soil and changing its
microclimate. Subtropical fruit trees have evolved in a microclimate
very different to that of the conventional orchard. Using mulches while
the canopy cover is being established can buffer the soil against
nutritional, biological and ecological disorders or imbalances which
are detrimental to tree health.
Mulching results in optimum
temperature and moisture conditions for root growth, improved soil
structure and physical penetration of plant roots, increased
infiltration capacity and erosion control, higher organic matter
content and improved weed control. Organic matter can also act as a
buffer against the effects of excessive fertilizer applications. In
addition to improving the soil structure, readily decomposable organic
mulches release nutrients to plants and increase the biological
activity of the soil. Grass or trash mulches have been shown to
increase available moisture - soil moisture levels and infiltration
rates were shown to increase from 30% on bare ground to 90% on mulched
soil. Unwatered straw mulches have been shown to have increased soil
moisture in the top 15 cm of soil compared with regularly watered bare
soil.
Research in South Africa with avocado, macadamia and
citrus trees have shown that mulch treatments promoted development of
vigorous roots on the surface, soil moisture was much increased and
daily soil temperature varied much less under mulches. Young macadamia
trees cropped better with mulching. In Russia annual mulching of tea
increased the yield by 76% and also reduced weed growth. Dried grass
mulch 8-l0 cm thick increased coffee yields by 173-233% in Africa.
Yield increases of 146% have been obtained using a straw mulch on peach
trees. In another test a layer of straw 4 cm thick reduced evaporation
by 73%. A layer 16 cm thick reduced evaporation by only a further 7%.
This confirms that more is not always better, in fact thick mulches are
undesirable as they hold a lot of water, so reducing the proportion of
water reaching roots.Organic mulches reduce the soil temperature which
means that in the summer plant roots can grow right up into the moist
cool surface layer of soil. In cooler areas mulch has a major
disadvantage - frost damage may be increased. The answer here is to
remove your mulch during frost-prone periods.
All of my trees
are mulched and although they don't get the care and attention they
deserve due to other commitments (family, work, house building, etc.) I
have a survival rate that I am very pleased with and I attribute this
to mulching. Last year prior to a particularly dry spell I planted 64
persimmon trees. I watered them, mulched them and left them to it. On
checking them several months later, the only tree that had died was one
that I had missed with the mulch.
Given the fact that organic
mulches consist of bulky animal or vegetable matter, readers could be
excused for being confused by Sandra Clayton's conclusion in the
paragraph on mulching fruit trees the statement that lime, urine and
wood ashes are also good. They may be good, but they are NOT mulches,
The best use for urine and wood ashes are in the compost heap.
Urine
is considered by some to be the very best compost activator. It is
definitely an under-utilized resource. It is sterile (W.W.1 soldiers
are said to have bathed their wounds with it), it contains
approximately 94% of the total NPK excreted by - humans (500 litres
contains 5.6 kg N, 0.4 kg P, 7.0 kg K) and is free from heavy metals
such as mercury, lead and cadmium (these are fixed in the liver and
kidney).
Wood ashes have an average composition of 1.5% P, 7.O%
K, their potassium content very useful in the compost heap of the
organic farmer, as most animal manures are rather low in this element.
My ceramics friends who use wood ash glazes tell me that wood ashes are
very alkaline and caustic and that gloves and a dust mask are essential
when handling this material. Perhaps a safer source of K for the
compost heap is molasses (approx 5%K) which has the added advantage,
like urine, of being a great activator, but then compost-making is
another story altogether!
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