Solid compost is best dug in to the soil to some extent, because that’s where the roots are. However, even if it is applied to the surface, it will be carried down slowly by rainfall and worm activity – so it’s not useless. For poor soils with low levels of organic matter, results will certainly be much faster if the compost is dug in prior to planting.
Compost ‘tea’ for liquid feeding Well-matured compost has a lot of elements in highly soluble form that are easily extracted by steeping the compost (like making ‘tea’) or by leaching them out into an instantly available liquid feed. These liquid feeds can be applied strategically at the times they are most needed, for example to boost the growth of young seedlings, or to obtain a very dramatic and rapid response from plants that are starting to fruit. Ensure that your leachate or ‘tea’ is not so dark that you can’t see through it – as this indicates that it is too strong to put straight onto plants. If the solution is too concentrated, add water until you can see through it – and you can be assured that you will not cause any fertiliser burn on your plants. This is particularly important for soft young seedlings, but it also applies to more mature plants.
An on-ground cold-composting bin allows worms to enter from the surrounding soil. The flap at the bottom gives the urban farmer easy access to the matured compost.
CASE STUDY:
INNER-CITY FARMING
FareShare and 3000acres
FareShare (www.fareshare.net.au) has extended itself far beyond the usual community-garden concept. As well as growing highly productive kitchen gardens on vacant parcels of disused city land, such as areas near train tracks and airports, the charity also sources food that might otherwise be wasted in supermarkets and restaurants to cook and distribute over a million meals a year to those in need around Melbourne.
3000acres (www.3000acres.org) is a not-for-profit organisation that seeks to influence ‘the cultural and regulatory environment to make it easier to grow food’. Their goal is to link landholders with those who want to become urban farmers, and also to team up these people with those who can provide other necessary resources.
One of the most impressive urban-farm projects in Melbourne is one that resulted from a collaboration between FareShare and 3000acres. FareShare wanted to increase its access to fresh produce for its meal service, so they enlisted the expertise of 3000acres to create an urban farm near its kitchens in inner-city Abbotsford. Sponsored by the Royal Automobile Club of Victoria and Gandel Philanthropy, volunteers created 70 garden beds on land owned by VicTrack (the organisation that manages land for Victoria’s public transport network), adjacent to Collingwood Football Club’s famous Victoria Park. Where it was not possible to sow directly into the soil, raised beds were built to grow a range of crops including zucchini, turnips, carrots, silverbeets, capsicums, sweet potatoes, eggplants, beans and herbs.
Large compost bins were manufactured from recycled timber to provide fertiliser and soil-conditioning humus, while a couple of greenhouses were constructed for propagation needs. The freshly harvested produce goes directly to FareShare’s nearby kitchens, with minimal delay between reaping and use as ingredients in the meals that FareShare makes and hands out on a daily basis.
This is a perfect example of how innovative organisations can coordinate and assist various stakeholders so they can take advantage of vacant community land in Australian cities. Urban farmers need resources other than land as well, and there is an increasing awareness in modern society of the opportunities that various corporate and philanthropic bodies can provide to the willing urban-farming workforce in our cities. The desire of budding urban farmers to grow more and more food plants is a powerful driver of these important initiatives.
Top to Bottom: Capsicums are protected by a thick layer of straw mulch; an eggplant crop grows well in raised beds.
Raised beds that have been built from recycled timber look rustic and appealing.
Sunflowers provide both visual attraction and grain.
Large composting bays are made from leftover wood.
MULCHING
Mulch is a protective blanket of material that sits on top of the soil or growing medium. It acts as an insulating layer – keeping the soil cooler in summer and conserving the summer heat into winter – and, most importantly, it reduces evaporation and keeps the soil moist, which conserves precious irrigation water. By stopping temperature levels around the roots from reaching extremes of hot or cold, mulch promotes root growth and creates an environment where worms can thrive. Mulch also prevents or at least slows the growth of weeds, as it smothers their seeds and stops most (if not all) of them from germinating.
Plants always grow better with mulch than without it. We generally use mulch more for its physical effects than for its impact on chemical fertility; however, moist soil means better nutrient uptake, so mulch does have an indirect influence on a soil’s chemical fertility. Some mulches can actually improve fertility by conditioning and/or adding nutrients to the soil. For example, lucerne hay is excellent mulch that stimulates worms and releases nutrients into the soil.
Usually, mulch should be coarse rather than fine, so that it rapidly admits water and does not soak it up. We also don’t want rainfall or irrigation to sit on top of the mulch, where it will just evaporate – we need the water to run through the mulch easily and enter the soil below. The ‘blanket’ layer of air trapped between the mulch particles will then stop the water from evaporating. If you want to conserve moisture, it is not a good idea to use fine compost as mulch; however, if you’ve got plenty of water in the soil and/or a humid climate, this becomes less important – you can use fine mulch such as compost to insulate the soil and add nutrients.
Sugar cane can be purchased quite cheaply, and it makes for an excellent temporary mulch for quick-turnover crops.
Permanent vs seasonal
It’s useful to divide mulches into two different types: permanent and seasonal (or one-crop). Permanent mulch is used for weed control and moisture retention around long-term perennial crops, such as fruit trees, vines, biennials and herbaceous perennials. In this case, you want mulch that will last as long as possible and is usually topped up only once each year. These mulches should be woody (such as coarse pine bark or woodchips), or, for large-scale enterprises, they can be synthetic (such as weed mat or organic ‘rolls’ made from coconut coir, jute and paper/cardboard fibre).
Permanent mulches should be kept around 50–75 millimetres deep by adding new mulch each year as the bottom layer decays, but very woody and bark mulches will often last two years or more before they need to be topped up. Note that woody mulch often causes nitrogen drawdown of the Fertilisers chapter). This can be overcome by applying nitrogen in solution over the mulch, or urea at 20 grams per square metre watered in lightly. The commercially composted woody mulches that come from the large-scale composting of garden green waste don’t produce the same significant nitrogen drawdown as raw-wood mulches, and they are much more environmentally friendly because they comprise recycled materials. They are also cost effective.
Seasonal or one-crop mulches, as the name implies, are used on annual crops – such as most vegetables – where the growing cycle is usually 6–12 weeks. When we turn over the crops in a working vegetable-growing area, we often cultivate the soil as well. You don’t want long-lasting woody mulches here, as they just get dug in and incorporated into the soil every time you replant. It’s preferable to have readily decomposable mulches such as hay, straw, sugar cane and, where water is not a problem, fine compost.
It’s not always necessary to mulch vegetable gardens. It can be very difficult, costly and time consuming to mulch very fast turnover crops such as coriander and lettuce, and downright fiddly with fine plants such as chives and spring onions. With the close spacing used between vegetable crops, and the shade cast by taller crops such as tomato and eggplant, the benefits of mulching are marginal if the soil is composted regularly and fed properly. It is important to water appropriately i
n such situations, so the surface soil does not stay permanently wet.
Types of mulch
There are many different materials that can be successfully used as mulch. Organically derived options include leaves, straw, hay, chipped bark, woodchips, newspaper and cardboard. Inorganic materials, such as gravel or synthetic weed mats, can also be useful for certain applications. Small, decorative stones are utilised mainly for moisture retention rather than thermal insulation (although the white ones do reflect heat) – but they are only suitable for use as permanent mulches. They are most effective in formal gardens, such as parterres and Italian- or Greek-themed gardens; keep in mind that formal gardens can be both beautiful and very productive. Here is a list of commonly available materials for use as mulches.
Lawn clippings Many gardeners have access to a regular supply of freshly cut grass, either from their own lawn or from their neighbours. Green clippings have a fairly high nutrient content, but they are rather fine so it is a good idea to mix in some dead leaves or other coarse material, such as chopped straw (run over the straw with your lawn mower to shred it). This will help to open out the mulch and stop the lawn clippings from matting down and becoming impervious to water. There are many advantages to using lawn clippings: they are free, they do not need to be transported, and they will help add nutrients to your growing beds. Ensure that there are no weeds going to seed in the lawn being mowed. If you do have a lot of weeds in the lawn, the clippings are best composted.
Lawn clippings can be used as mulch if they are mixed with coarse leaves and twigs. If they contain weeds, the clippings should be composted.
Pine bark is a popular mulch choice, but it needs an application of nitrogen to prevent drawdown.
Homemade compost Making your own compost from various organic materials is a fantastic way to generate mulch that has the potential to both feed and insulate your soil (see the Compostable Materials table). It’s important to note that when using homemade compost as mulch, the larger the particle size, the better. We recommend sieving your compost into two particle sizes: coarse (preferably several millimetres in diameter) and fine (all the tiny particles). The coarse-grade compost can be used as mulch, while the finer particles can be dug in as fertiliser or soil conditioner. Homemade compost mulches are free to make, and you can be sure that they are ‘organic’ because you know that they do not have any artificial chemicals. They also add nutrients to the soil if you have properly composted them, and thus your garden beds will not usually require the application of additional fertilisers.
Stones come in various colours, so they are often used on beds as a decorative permanent mulch.
Raw woodchips Mixed tree loppings can often be obtained cheaply (or for free) from tree-lopping crews working in your local area. They must be composted first to get rid of weeds and to allow any natural toxins, such as tannins, to break down and dissipate. There is usually enough leaf material to promote composting, otherwise add some green matter to help the composting process. Wet the material up in a heap, and allow it to break down for a month or so before use. Woodchips are also available commercially as forestry residues, both as pine and hardwood chips and flakes, and these do not have weed or disease problems. Woodchips are good as long-term mulches for permanent plantings, such as fruit trees. All uncomposted woody mulches will cause nitrogen drawdown, which is overcome by applying some nitrogen (commonly urea). Blood and bone and other organic sources of nitrogen can also be used. Alternatively, you can compost them before use by adding urea at 100 grams per 100 litres, wetting them up and letting them break down for a month.
Containers should be mulched to prevent excessive temperatures in the growing medium, and to conserve water.
Composted, screened, recycled woodchips This is the coarse fraction (usually > 20 millimetres) of composted green waste from kerbside collection and drop-off at waste recycling stations. It is usually well composted in large heaps, so it has been through the heating process and is thus free of weeds and pathogens. A low-cost and excellent long-term mulch, it can be a bit dull coloured for decorative use – but that’s not our primary interest on urban farms. Since it’s composted, this mulch does not generally cause nitrogen drawdown.
Newspaper and cardboard This is waste paper and cardboard that is either shredded or unshredded. It can pack down and exclude air and water, so the shredded form is best for thermal insulation. Flat sheets of cardboard are excellent for suppressing weeds around tree crops (such as citrus), provided that there is adequate air and water movement through the holes in the cardboard created for your plants. In the past, this mulch was often obtained from office waste, but these days most of that goes to the paper recyclers – so you might have to make your own. Shredded paper and cardboard can blow around and look trashy; cover the mulch with another decorative and heavier material (such as woodchips) to solve this problem.
Pine bark This is the most common decorative mulch available in the landscaping industry, and it comes in various sizes (just don’t use the fines for mulch). A recycled forestry residue, it is both sustainable and renewable. It makes for excellent long-term mulch (especially around fruit trees) as it does not have excessive nitrogen drawdown, but it is more expensive than woodchips.
Sugar cane The remains of sugar cane after the juice is squeezed out, it is present in abundant quantities in sugar-growing regions and is exported in compressed bales as mulch. It’s a very good thermal insulator and does not create excessive nitrogen drawdown, so it makes for a good short-term mulch around vegetable crops.
SOIL AS MULCH
A layer of dry soil on the surface acts as mulch, especially if it is light and airy. Farmers will often cultivate the top 50 millimetres of the soil just to fluff it up and let it dry. This is also why watering deeply and less often is better for the garden. It allows the surface soil to dry out and conserve moisture at depth. A constantly wet surface acts like a wick, conducting water from below.
Blanching is a form of mulching where soil is used around plants to stop sunlight from turning stems green. Commercial asparagus growers use a ‘hilling’ plough to build up the soil around the crown of the plants. Pickers then harvest the white asparagus spears by utilising a chisel-pointed knife to cut the stems off below the ground.
Gravel A decorative option, gravel is available in a wide range of colours and rock sizes. The best gravel for mulching purposes is 10–20 millimetres in size, and it should be applied to a depth of 50–75 millimetres. Only use it in permanently mulched areas, such as orchards, where the appearance of mulch is as important as its practical considerations.
Crop residues These include straw as well as maize and sorghum stover, and are usually high in carbon. They are ideal for annual crops such as vegetables, as they decay quickly and add valuable organic matter to soils. However, they often cause nitrogen drawdown; this can be overcome by applying nitrogenous fertiliser, such as urea or chicken manure, to the soil surface before putting the mulch down, and then watering thoroughly.
Lucerne hay This is the premium ‘mulch and feed’ material, and it stimulates both root growth and worm activity. This is not surprising really, as this stock feed is high in the nutrients and protein that plants and worms love. Unlike other carbon-based mulches, which can cause nitrogen drawdown, lucerne hay is quite high in nitrogen and actually adds this nutrient to the soil. It is especially useful for really fast-growing annual crops, such as tomatoes and root vegetables.
Landscape fabrics Plastic weed mats and cellulose (coconut coir) mats are excellent for long-term weed control, and they provide some moisture conservation but not much thermal insulation. This is not a problem if you want to warm soil (they are often black, so they absorb heat), but it can be counterproductive if you want to keep the soil cool. They are also very cheap, unlike woven and matted fibre products (such as jute matting), which provide excellent weed, water and heat management, but are costly. Historically, these latter products were developed for erosion control.
Selecting mulch
The choice of mulch material will be influenced by a number of factors, such as the kinds of plants you are growing. Other factors include:
The availability of materials – you might have your own source, such as lawn clippings mixed with dead leaves, or there may be a ready source of recycled or free material available locally. For example, some councils give away chipped green-waste mulch. However, always compost ‘giveaway’ chipped green waste for at least six weeks, as it will contain weed seeds and plant pathogens that can only be destroyed during a period of hot composting. Screen out the coarse fractions for mulch and use the fines as compost, or just put the whole lot on the garden as a ‘mulch and feed’.
The need for weed control – for the most effective weed control, it is best to create a continuous barrier (perhaps using cardboard sheets from recycled boxes). These mulches are very effective for new ground that is full of weed seeds, but they do exclude water so you will have to adapt your watering to suit (for example, don’t use a sprinkler – place drippers next to the plant instead). Weed-free composted woodchips and bark are also very effective weed-control mulches.
The wish to control water and temperature – for water efficiency and the moderation of temperature extremes, a thick layer of coarse, chunky organic materials will give the best results. Straw is very effective here, as its low density and coarse nature traps air for maximum insulation.
The desire to feed plants – if you wish to fertilise your plants at the same time as gaining the other benefits of mulching (which is known as ‘mulch and feed’), use a material that has a relatively high nutrient level, such as lucerne hay, pea straw or a mix of woody and nutrient-rich compost.
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