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.
DATE: January 1995
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