From the Archives
of the Rare Fruit Council of Australia, inc.
by Dave Romney
Mulch - Is it as
Beneficial as We Think?
(Some thought-provoking observations)
Many of us assume that mulch must be beneficial. We believe it to
be one of the essentials of good farming, like selected plant
varieties, fertilizers, weeding, irrigation, pruning and insect and
pest control. But mulch is a heavy, bulky material; even if it is
beneficial, it seems that we should at least evaluate all the 'pros'
and 'cons' before carrying out such an onerous and probably expensive
operation.
For myself, I think of mulch primarily for its
ability to reduce evaporation from the soil, this by keeping the sun
and drying air away from the soil surface. To achieve this, one could
use such materials as cut weeds, coconut husk, sawdust or polythene;
each of these have numerous examples of success. In east Africa, there
are trials in progress to test a layer of gravel as a mulch. Many
growers consider that tilling the surface soil to a depth of about
one-half-inch produces a natural soil mulch; this has also been
reported as successful.
Having worked with coconut palms for
many years, I look to them for examples of response to mulch. Young
coconut palms in Jamaica showed significant growth response when
mulched with husk or coir dust, although black polythene laid over the
same area around the palm produced the same benefit much more easily:
in fact, bare soil as a result of using herbicide also gave the same
growth improvement as mulch. Evidently the upper half-inch of soil was
not being used by the roots.
On the coral sands of the islands
of Belize, coconut palms improve their appearance markedly when mulched
with sea-weed, but improved equally when bare-weeded. Coconut palms
circle-weeded by shallow hoeing in dry areas of Tanzania grew and
yielded significantly in weeded circles of radius 12 feet compared to
radius 6 feet. These are all examples of mulch acting as a preventative
of evaporation.
A striking experiment on the effect of mulch on
coconut palms was conducted by the author, also in a very dry part of
Tanzania. First cross MAWA hybrids between Malayan Dwarf and West
African Tall were planted in 1981. In January, 1983, cut grass and
weeds were applied as a mulch to 30 groups of three palms each, over a
circle of radius 12 feet, to a depth of 1 inch, around each palm. (This
was the amount of material available from between the circles.) Thirty
other groups of similar palms were left un-mulched but with weed free
circles. All palms were fertilized every six months, so that any
nutrients supplied by the rotting mulch would not be a factor. (Foliar
analysis showed that nutrient levels were good).
By July 1985,
there was no difference in onset of flowering or frond production as a
result of the mulch. Hence the depth of mulch was increased to about
six inches, and mulch reapplied every three months because termites
soon ate it. The mulch was temporarily taken up from a few palms, after
it had dried for a week, and found to weigh over 100 pounds per palm.
Such heavy mulch application involved carrying most of it in from
outside the field (the 100 pounds of dry mulch weighed about 400 pounds
when it was cut and carried in). Four years later (see Table) the
mulched palms tended to yield more nuts, but the difference was not
statistically significant compared with the natural differences in
yield between the palms.
MEAN HARVEST PER PALM, JULY 1988 to JUNE 1989
Number of nuts Weight per nut (gm)
Mulched palms 38* 596*
Unmulched palms 31 526
*These differences were not significant at P=0.05
Those
readers conversant with statistical analysis will know that such
differences, though carefully measured, are not meaningful if
mathematical analysis shows that they are not statistically
significant. The P=0.05 used in the analysis infers that similar
differences in yield would occur with these palms 19 times out of 20
whether mulched or not. Even if the palms had been less variable and
the differences significant, the extra seven nuts per palm per year
would not have compensated the farmer for all the effort of mulching
(assuming that he could find enough mulch and had the time to move it).
Of course, the unmulched palms were kept weed-free - a much easier task
than mulching in most soils.
By now, readers are probably making such remarks as:
"You
haven't mentioned that mulch protects the soil from beating rain and
helps to maintain soil structure and reduce erosion when the soil is
bare."
"What about the improvement of soil organic matter by the decomposition
of mulch?"
"Remember that mulch will break down to provide plants with valuable
nutrients, especially trace elements."
"You've forgotten that mulch will help control weeds by smothering
them."
"You
have used the coconut palm as an example but there are some species of
plant that benefit especially from added organic matter."
Now I
don't want to be completely negative towards mulching, but I'd like to
present the other side of the subject. The above remarks are valid in
certain instances, but let's not get carried away.
First there
are some soils that benefit from protection from the type of heavy rain
that we experience in South Florida. Data presented recently by Bob
Rosenstein in "Coccoloba" (Vol. 7 No. 11, 1991) illustrate how heavy
mulching increased five-fold the rate of rain percolation into the soil
compared with bare soil, lowered the temperature of the surface soil
and decreased evaporation. However, I suggest that the stony clay-loams
of south Dade and the deep sands of Broward are flat and so
free-draining that soil structure and reduction of erosion are not
important factors. Of course, maybe you are brave enough to try to grow
a crop in the Florida dry season using mulch without irrigation.
However, there are occasions when a light rain, needed though it may
be, is sopped up by the mulch and never drains down to the plant roots.
Secondly,
organic matter added to the soil in the tropics breaks down very fast;
if it does not, it is probably too woody to be of much benefit to the
clay-humus complex with its important nutrient-holding capacity. Most
mulch materials will certainly yield some plant nutrients as they break
down, but rotting is usually slow and the quantity of nutrients small
compared with the needs of plants; especially crops where large
quantities are carried off the field at harvest. These days we have a
multitude of sources of plant food more concentrated than most mulch
materials and of known nutrient content: some, such as urea, are indeed
manufactured, but the CO(NH2)2 so made is exactly the same as the
CO(NH2)2 occurring in the nitrogen cycle in nature; many other
fertilizers are natural materials dug out of the ground, e.g. potassium
chloride and sulfate, rock phosphate, magnesium carbonate and others.
Of course, one may use too much of one of these relatively concentrated
sources of plant food and damage the plants, but this is where the
skill and good sense of the grower is called upon.
Mulch does
indeed suppress weed growth, but I hope that my readers never have to
deal with prostrate weeds growing through the mulch! Neither would I
wish upon them a mulch of cut grass which has sprouted out and taken
root at the nodes! How many have tried to locate and check dripline
emitters under mulch? Sure some plants that originate in the forest
floor will enjoy organic mulch on the soil surface, but, after they
have happily put their roots up into this artificial forest floor, the
grower had better continue to add good quality mulch regularly to
prevent the roots being left high and dry when the mulch has rotted
away.
And you all know that a high-carbon mulch (which most
usually are) may take nitrogen out of the soil and away from the plants
during the process of decomposition.
When I make observations
such as the above, I like to look for a moral in the story. The moral
here is to consider the disadvantages of mulch as well as the
advantages. When planning to use mulch, let's analyse our reasons for
using it. When we use it (and I trust that you do at least utilise your
kitchen and garden refuse rather than consigning it to a landfill)
let's try plants and compare this with the benefit. With crop plants,
mulches alone can hardly replace all the plant food carried off the
field at harvest, so we need to use fertilizer judiciously to obtain
good yields. If all we want from mulch is moisture retention, we can
use polythene, cardboard, gravel, or even old carpeting. (I know one
grower who does so!)
Meanwhile, please excuse me - I have to go out and spread some mulch
.
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