(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.

 Number of nutsWeight per nut (gm)
Mulched palms38*596*
Unmulched palms31526
*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 .

Dave Romney
Tropical Fruit News Vol.26 No.3

DATE: May 1992

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