Mulch

Mulching simply means putting a covering on the surface of the soil. The word is thought to have derived from a German word molsch, meaning 'soft' or 'beginning of decay'.

There are three main reasons why we mulch, retention of moisture, suppresion of weeds and to assist in keeping the soil temperature at an acceptable level.

We use a variety of items for mulch. Newspapers, cardboard, stable manure and a variety of straws including pea, wheat, oat and barley. Newspapers and cardboard we get for the asking at the local newsagents.

The benefits of mulching

Retaining moisture
In the reasonably harsh Australian climate, a layer of mulch at least 5-7cm but preferably 15cm. thick helps to retain moisture in the soil by preventing evaporation through heat and drying winds. Studies have shown that mulching to a depth of 30mm may reduce the amount of water evaporation from the surface by up to 50%. Deeper mulches are still more effective. In the long term, the increased biological activity under a mulch leads to an increase in the water retention rate, thereby reducing surface runoff and soil erosion. In areas of high evaporation a mulch will indirectly reduce the soluble-salt level that results from deposits of salts in the upper soil layer.

Preventing a crust on the soil's surface
The crust caused by evaporation of moisture and the drying effect of winds can stop water entering the soil. This crust also restricts natural air flow to the plants. This aeration of the surface soil and the enhancement of ammonification is equally important, and crops growing during spring and summer in properly mulched land are much less dependent of nitrogenous fertilisers.

Nutrients
A layer of mulch also prevents nutrients from washing away and eroding the soil's fertility during heavy rains.

Insulation
The value of mulch as an insulator has been recognised for a number of years. Experiments on maize crops in during the daily maximum average summer temperature the top 2.5cm of soil was 31.2°C if the surface was bare, whereas it was only 23.6°C under a mulch of 20 t/ha of straw.

Whether you live in a hot or cold climate matters little as mulching assists in both.

In the winter a layer of mulch insulates and keep the ground warm, thereby lessening or preventing frost damage. In hot climates, the same insulating layer of mulch protects shallow rooted plants and trees from the burning rays of the sun. Bare, dry soil under bright sunshine can reach temperatures in excess of 45°C, which is too high for the normal functioning of plant roots. The cooling effect of mulch is therefore very valuable.

Mulches applied to the surface not only affect the amount of heat, but also how it is dissipated. Mulches of dead vegetation do immobilise to some extent the air within the mulch, and because still air has a very low thermal conductivity, heat is only slowly transmitted from the surface of the mulch down to the soil surface. Hence, soils under mulches remain cold in spring in regions which have a cold winter, and germination of seed sown under such mulches may be delayed. However, you can overcome this by raising the seeds elsewhere and transplanting them into mulched areas when the ground begins to warm up.

Macroscopic fauna
Surface mulching also has a significant effect of macroscopic fauna. Mulching improves their environment during winter in a temperate climate, enabling them to remain active in the topsoil. The increased temperature caused by the mulch reduces the risk of their death which may occur in low temperatures. The overall effect of increased earthworm activity is more stable in the top 200mm of soil.

WHAT TO USE?

There are two types of mulching materials: biodegradable and non-biodegradable.

Biodegradable mulches
Within the biodegradable category, mulch may be coarse, dense or living.

Coarse mulch
This includes stalks, leaves, straw, seaweed, hay, grass clippings, bark, twigs and leaves.

Dense mulch
Dense mulch includes sawdust, bagasse (the residual product of sugar cane), dense lawn clippings and almost any other organic material which forms a more compact cover.

Living mulch
Organic matter such as grass and legumes growing under the trees or in the spaces between rows has proved to be beneficial. This is being used increasingly in organic orchards where it has been found that old methods of using herbicides and the continual movement of heavy machinery between tree rows has caused depletion of soil organic matter and the eventual loss of soil structure and fertility.

Non-biodegradable mulches
These include stones, gravel, black polythene, metal foil and even bitumen emulsion which is sprayed over rows after seeding. Theses have specific uses but do not break down or add nutrients to the soil. Stones and gravel insulate and protect under certain conditions and black polythene, if laid over bare soil, excludes all light and is effective in weed control. Beware of polythenes which claim to be biodegradable as they tend to deteriorate into strips rather than disappear altogether, becoming a visual and physical nuisance. Planting through holes in black or other polythene may cause roots of plants (such as strawberries) underneath it to burn, and rain cannot penetrate except around the holes allowed for the plants. A soak-it hose may need to be laid beneath the polythene to allow the plants adequate water. It also prevents air reaching the soil and may also harbour pests.

Producing and obtaining mulching materials
The best materials to use are aged manure and compost. These not only assist drainage and water-holding capacity but, if used in sufficient quantities, will improve and maintain the fertility of the soil.

Mulching materials are everywhere, often free for the taking or the asking, or inexpensive to buy. They all require a little energy to cart and/or convert, but the increased productivity of the garden, allied with the decreasing need to water and weed, makes this all worth the effort. Here are some mulching materials to choose from:

Grass clippings/cuttings
These can be obtained straight from your own lawn or, if you know someone with a lawn mowing service, they would probably welcome someone saving them a trip to the tip. If you do obtain clippings from elsewhere, it might be prudent to find out if pesticides and herbicides were used on the lawns in question before you risk them in your organic environment.

If lawn clippings are applied thickly enough they are very effective for weed control. However, if they are applied so thickly that no air can reach the bottom, they may become slimy and smelly and at that stage useless as a soil conditioner. If lawn clippings are spread thinly around trees or shrubs, or mixed with other mulches before placing them in the vegetable garden, they can be very beneficial. However, you may find that your own lawn clippings are better left on the lawn where they fall, thus reducing the need to water the lawn.

Leaves
Where you have deciduous trees you have heaps of leaves, which many gardeners have hitherto raked up and burnt! Leaves are ideal mulching material, or they can be placed in the compost heap if they are likely to be blown around. A few strategically placed branches, tomato stakes or old hessian will help to anchor them down. Leaf mulch is especially useful around soft fruiting bushes such as raspberries as they protect the soil against frost and help retain warmth which encourages worms. Leaves from the bush (eucalypts etc.) are ideal for placing around other native plants as these should be given a mulch as close to their native habitat as possible.

Straw and hay
Masanobu Fukuoka, whose revolutionary philosophy about natural farming gave impetus to the organic movement, maintains that scattering straw maintains soil structure and enriches the earth so that prepared fertiliser becomes unnecessary. His fields in Japan have not been ploughed for over 30 years, and he tells us that his yields are high and that the quality of soil improves with every season.

When it is plentiful, straw is quite cheap, so that is the time to buy. As with grass clippings, it might be prudent to enquire as to whether or not the straw has come from a field which has been sprayed with herbicide before accepting it. Sometimes straw and hay contains seeds, so it is wise to spread it out and fork it through so that the seeds fall to the ground before using it. If you have chickens, let them work it over for you. They will eat the seeds and manure the mulching material at the same time. Place the straw or hay underneath the hen roosts, or on the floor of the duck house before it goes onto the garden.

Sawdust
Most sawmill operators are happy to have people take away trailer loads of sawdust at no or minimal cost. Old, weathered sawdust is best, but new sawdust can be useful if mixed with other things such as chicken manure, or it can be used for pathways. If the sawdust is from treated timber do not use it .

Sawdust mulch is also an excellent snail and slug deterrent as these creatures do not like moving over its surface. Put sawdust mulch around cabbages, cauliflowers, broccoli or anything else you wish to protect from these pests.

Bark and wood chips
Shredded or chipped bark mulches are usually regarded as ornamental mulches and used mostly in bush gardens, but these can also be used elsewhere to control weeds.

Bark mulches can be expensive to buy already prepared, but it is inexpensive to make your own. An electric or petrol-driven shredder or mulching machine will pay for itself in supplying all kinds of mulching material, including bark and wood chips. Wood chips are an excellent mulch for roses and flower gardens, although they are not recommended for the vegetable garden. They do take a long time to break down naturally.

Large woody prunings, including those of trees and shrubs, lantana and corn stalks, can be put through a shredder, and either used as is for mulch or allowed to break down into a humus using an activator (such as Eokomit) before being applied to the vegetable garden.

Seaweed
Seaweed is very rich in minerals and nutrients, and if taken from the high-water mark will not need much washing. Otherwise it can be hosed down or left for the rain to wash out the salt in the next downpour before you put it on the garden. It can be put around salt-tolerant crops such as asparagus as it is. Because of its salt content, it is a very effective weed control agent.

Bagasse
This is a residual product of sugar cane and although it was once cheap, it is now becoming less available and more expensive as mills use it for fuel. It is, however, an ideal mulch around trees.

Pine needles
Wherever there is a pine forest there are heaps of pine needles for the, taking. They are acidic so use them with caution. They are an ideal mulch for acid-loving plants such as strawberries, azaleas, camellias and rhododendrons. You could also put them around the perimeter of the garden to assist the control of weeds and invasive grasses.

Bracken fern
This seems to grow everywhere and is generally regarded as a pest. However, because it is rich in potassium it is excellent for mulching, especially potatoes.

Newspapers
Bundles of old newspapers seem to accumulate everywhere and are usually free for the asking. They can be used in flat sheets for weed control, but water will not penetrate easily and it will be a long time before they break down. It is probably better if they can be shredded or screwed up and used with other mulching material.

Mushroom compost
Mushroom growers have recently become aware of just how valuable their spent compost is to gardeners, and have begun selling mushroom compost in bags or as loads. It is well worth the money to buy some of this and spread it on the garden. Recently, home mushroom kits have appeared on the market and have become very popular. Spent compost from these kits finds a ready place as mulch in vegetable gardens. There is, however, some hesitancy on the part of organic growers to use mushroom compost as there is a strong belief that it may contain chemical residues.

Hair and feathers
If you live near a town, hairdressers and barbers may think you a little strange if you ask for their floor sweepings, but remember that hair is a rich source of nitrogen (3kg of hair contains 500g of nitrogen), which is invaluable to any garden. Mix the hair with some other medium such as sawdust to ensure that it doesn't blow away. Feathers too are rich in nitrogen, so if you collect chicken manure from your own chickens or from a poultry farm, the more feathers it contains the better.

Husks
These include nut husks such as those from macadamias, pecans, walnuts, peanuts etc. Some, such as macadamias, are slow to break down, but they do provide excellent mulching material. However, it is best not to use them around cropping trees of their own variety as it makes the next season's fallen nuts harder to find. Cocobean husks are also excellent mulching material: they are quite alkaline, so use them to advantage on add soil.

Spent hops
If you live near a brewery you may be able to obtain a supply of these, either by getting them yourself (as most breweries are now in cities) or by paying someone to carry a truckload for you, although you will probably have to load the truck yourself. Hops may need to be allowed to remain heaped up and exposed to the elements for a month of two before using them as mulch.

Comfrey This plant is so easy to grow that is can become as invasive as a weed in no time. It will grow from almost any piece of root or shoot. It is a rather controversial plant, so if the medical arguments have deterred you from eating it (because it is said to contain carcinogenic properties) you can use comfrey leaves as a mulch. It too is rich in potassium and potash.

Once established, comfrey plants can be harvested every six weeks until their dormant season. Spread the leaves under fruiting bushed, underneath potatoes when you plant them, on the onion bed, or place about 14kg of comfrey leaves in a 200 litre drum of water, cover for 4 weeks, and use the comfrey brew either straight or diluted as a fertiliser (remember, you cannot burn plants with a comfrey brew).

Weeds
Do not overlook how useful even common weeds can be as mulch (uproot and use them before they set seed). Many of these are deep rooted and bring up minerals that other plants cannot reach. As these weeds decompose, the minerals are returned to the surface where they can be taken up by your crops/ flowers.

Living mulches
Masanobu Fukuoka not only advocates using uncut (unshredded) straw mulch, but he also grows leguminous ground cover crops such as white clover to act as a living mulch for vegetables and fruit trees. The clover enriches the soil and keeps the ground moist and well aerated. He also plants alfalfa and daikon (a Japanese radish) as a living mulch to enrich the soil. With daikon there is the added advantage that its roots deeply penetrate the soil, opening channels for air and water circulation.

Living mulches are inexpensive to grow and nitrogen-fixing pasture grass's such as lucerne (which also encourages earthworms) and clover are probably best. These under-tree grasses not only provide living mulch but also fodder for small animals and geese which can be grazed when the trees have grown sufficiently so as to avoid being damaged by them. Chicory, mustard and broad beans can also be usefully grown and slashed for mulch.

For tree crops which fall, such as nuts, these living mulch crops can be mown and left on the ground around the trees prior to harvest. In other orchards grasses are left to grow quite long to cushion any falling fruit from bruising, then mown and left to lie on the ground.

The simple task of planting leafy crops such as zucchinis close together or planting all under storey crop such as pumpkins beneath corn allows these plants to act as a living mulch by shading the soil and keeping it cool as well as inhibiting weed growth. When the vines dies they can be left on the ground.

Preparing a new garden
If you want to use mulch when preparing a new garden or improving a weed infested one, it's better if you can make the growing area as clean as possible right from the start, before covering the ground with mulch. Dig it over if you call, removing all weeds and paying particular attention to the edges where many grasses and weeds begin their invasion. An excellent mulch for new ground is to first spread fresh chicken manure over the area, as the heat in this will burn off any weeds, spread sawdust over the top and leave it for a couple of months.

Covering the new area with a layer of mulch at least 5-7cm thick will prevent the weeds' access to sunshine and many will die before they reach daylight. If persistent weeds do grow up through the layer, it is a simple matter to pull these out before they become reestablished. The mulch will have weakened their grip on life.

Mulch tillage
Mulch tillage is the use of mulch in larger scale farming. In this system, crop residues are left on the surface, and subsurface tillage leaves them relatively undisturbed. In dryland areas, a maximum amount of mulch is left on the surface whereas in more humid regions, some of the mulch is buried. Planting is done with disk openers that go through several centimetres of mulch.

In areas receiving high rainfall, intercropping extends the protection provided by mulches against erosion. Intercrops used are small grain or sod crops such as lucerne or clover which grow between the rows of the field crops and reach maturity shortly after the field crop has been established. This method enables mulch cover over a prolonged period.

CONCLUSION
Mulch for all seasons and reasons. Apply a mulch to your soil as you would a sun screen oil to your skin under our harsh summer sun; or as you would don a thick coat in winter. As organic gardners, most of the plants we grow are not natives and we need therefore, to control their environment in as natural a way as possible for them to flourish. Whether you apply mulch to establish a garden, to prevent weeds, to improve the soil's fertility, or to control moisture and temperature, the benefits to your soil and growing environment will soon be obvious.