Too much manure Many soil manufacturers think that if some is good, more is better – so their soils feature up to 20 per cent poultry manure or cow manure when 5–10 per cent would provide perfectly adequate fertility levels. I had a gardener once who lost everything except tomatoes and capsicums when they planted a vegetable garden in a commercial soil. (Tomatoes and capsicums are very tolerant of salt and ammonia, which is why they survived in a mix that was around 20 per cent manure.) Manufacturers that analyse their products in a laboratory and can provide test certificates, or those that have demonstration plots of their products at the yard, are more trustworthy, but do obtain a statement of how much manure is in the mix if you can. If it’s any more than 10 per cent, then ask if you can have a sample and do a germination test.
Alkaline soil Lime from cement and mortar is everywhere in urban areas, and it is also prevalent in marine deposits that are often mined as sand. Soils from building and construction areas are often screened and sold as commercial soils, and these are always high in lime and gypsum from plasterboard. Unfortunately, there is so much lime (5–10 per cent) in these soils, they cannot be acidified in an economic way. Iron-inefficient (acid-loving) plants cannot be grown in these alkaline soils, so it’s a good idea to always do a pH test on commercial soils (see Testing the pH of Your Soil). You can also do a lime test . Place some of the soil in a diluted acid, such as one part hydrochloric acid (sold as swimming-pool acid) and five parts water. If it fizzes, then the soil is probably more trouble than it’s worth. Note that strong acid can burn the skin and eyes, so always wear rubber gloves and eye protection when diluting acids. Vinegar is a weaker acid that will work in a pinch, but don’t dilute it; look carefully after the soil has been added, because the fizz reaction is not so strong as with mineral acid.
Rooftop gardening demands a premium growing medium, especially given the large investment of time and money that is necessary to create and maintain the garden beds.
DISCOVER NO-DIG GARDENS
No-dig gardens can be set up anywhere you like – just ensure that the plants growing in the bed have access to enough full sunlight.
Where a site has no useful soil, there is an insufficient depth of soil or the plot is on a constructed surface, such as a concrete slab or roof, it will be necessary to construct a soil profile. A very simple and effective constructed soil is a system called the ‘no-dig garden’, developed by Australian gardener Esther Deans in the 1970s.
In a no-dig garden, an edge – which is usually made of timber – is constructed, and then a thick blanket of newspaper is laid down on the existing soil to suppress weeds. The newspaper is covered with alternating layers of manure, organic fertiliser, straw and compost. Little holes are made in the growing-medium ‘sandwich’, into which are placed handfuls of compost followed by the seeds or seedlings. As the name suggests, there is no need to dig the garden bed – effectively, earthworms do the digging for you. Over time, the soil beneath is enriched and the organic matter is incorporated. Straw and compost are added regularly to top up the level as the ‘soil’ breaks down.
No-dig gardens are useful for placing over existing soils that are compacted or otherwise infertile. In addition, they can be used over a constructed surface such as a concrete slab if the garden bed is 300–500 millimetres in depth.
One big advantage of raised beds is that you do not have to bend down as far to cultivate the bed or harvest your crops.
MAKING RAISED BEDS
Creating raised beds is a great way to control the fertility and drainage capacity of your growing medium, and you can make sure that the beds are located in places where they will receive optimum sunlight levels. If you are making raised beds on soil (rather than sowing plants directly into the ground), ideally they will be at least 300 millimetres deep.
When raised beds are placed not on soil but on rooftops, concrete slabs, compacted fill or some other impermeable material, then they need to be at least 500 millimetres deep. If the beds are any deeper than this, then adequate drainage can become a major problem – there is always a ‘wet’ zone in deep beds following rain or irrigation. To solve this issue, place at least 200 millimetres of a low-organic and freely draining sandy medium in the bottom of the bed – a coarse layer will help keep plant roots happy.
Where raised beds are 300 millimetres or shallower – such as container plantings – using soil as your growing medium is not a great idea. There are three reasons soil cannot be utilised in these situations:
Soil becomes waterlogged very easily in shallow containers and pots. These types of raised beds require a growing medium with more porosity than soil and which includes coarse particulate matter such as pine bark, ash and perlite. Look at any modern potting mix, and you will see how open it is compared to soil.
Shallow beds need to retain some – but not all – water. Soil-less growing media such as peat moss are ideal because of the suitable water-holding ability of these natural materials.
Soil can become compacted and heavy in shallow raised beds. However, containers often need to be lightweight, especially when they are located on rooftops or must be moved – so a lighter growing medium is preferable.
For best results, you must utilise a very open and porous medium such as potting mix. One that incorporates ash, sand and compost works well and does not lose its structure. If you use a pure organic potting mix, remember that it will lose volume with time as the coarse particles decay. The bed will need regular topping up with potting mix as this shrinkage occurs.
Raised beds can be made from a variety of materials, such as corrugated iron or stone, but timber creates a solid, cost-effective and aesthetic option.
Creating raised beds is a great way to control the fertility and drainage capacity of your growing medium.
Soil-less growing media
When it comes to soil-less growing media, there are many different choices. The Soil-less Media Options lists the properties of some of the most common materials used to make growing media that are highly porous yet hold water well.
Clever concoctions
For a lightweight growing medium that has a longevity of up to five years – making it useful for containers located on balconies and rooftops – a good mix of components would be:
a premium-grade commercial potting mix (20 per cent by volume)
vermicompost or garden compost (20 per cent by volume)
sand (20 per cent by volume)
perlite (coarse grade) or horticultural ash (40 per cent by volume).
Plants will thrive in a raised bed if the growing medium is well drained. If the bed is on soil, it is also a good idea to ensure that it is no shallower than 300 millimetres.
SOIL-LESS MEDIA OPTIONS
MATERIAL DESCRIPTION BENEFITS AND ISSUES
Composted pine-bark fines Usually the less than 12-mm fraction of composted plantation pine Lightweight, high water-holding capacity, medium cost
Composted coarse pine bark Usually the 12–20-mm fraction of composted plantation pine Lightweight, very high porosity, poor water-holding capacity, medium cost
Composted sawdust Composted sawdust from timber sawmills Lightweight, high water-holding capacity, high nitrogen drawdown, medium cost
Coarse sand A sand predominantly in the 0.5–2-mm size range Heavyweight, poor water-holding capacity, good aeration and root anchorage, high longevity (years), coarse and abrasive, low cost
Boiler ash, washed and screened, or power station bottom ash Coarse fused mineral material from high-temperature combustion of coal Lightweight, reasonable water-holding capacity, high longevity (years), coarse and abrasive, low cost
Perlite Fused aerated silica balls Very lightweight, very high water-holding capacity, high longevity (several years), high cost
Pumice and scoria Aerated volcanic extrusion Variously lightweight, high water-holding capacity, high longevity (many years), not readily available, high cost
Diatomaceous earth Fossilised siliceous skele
tons of diatoms laid down in almost pure beds Very lightweight, excellent water-holding capacity and longevity, high cost
Composted garden-waste fines Council collection yard or garden waste that is chipped, composted and screened into compost fines and mulch fractions Good nutrient content, high water-holding capacity, readily available, poor aeration, low cost
Coconut coir The pith surrounding the shell of the coconut, milled into various-sized fractions (usually around 2 mm) Very lightweight, excellent water-holding capacity, reasonable longevity (two years), high cost
Peat moss Preserved remains of plants growing in saturated environments Very low density, high water-holding capacity, high cost
Vermiculite Expanded mica clay High cation and water-holding capacity, high cost
Plastic foams Expanded balls of either polystyrene (Styrofoam) or phenol formaldehydes Very lightweight, Styrofoam is inert but formaldehyde foams hold water, low cost if recycled
Zeolites Hard aluminosilicate clays Good cation and water-holding capacity, high cost
‘Soil’* Usually comprises mined alluvium from river terraces Provides mass and water-holding capacity, poor aeration, low cost
*Note: The ‘soil’ we obtain commercially is quarried soil-like material and not actually real topsoil, so it is still technically a soil-less medium.
Perlite is a soil-less media option that offers a lightweight way of improving drainage in growing media as an alternative to coarse sand.
Such a mix would usually not need any lime or other conditioning agent, as the compost provides a balance of all the required nutrients. However, because they just don’t have the acid-buffering capacity of natural soils, artificial mixes such as the one detailed here can quickly change their pH level. A pH check should be performed at least once a year, and a lime/dolomite mix applied if the pH measures 5.5 or less. Otherwise, there is no reason why artificial mixes should not last many years and maintain good porosity.
There is an almost infinite array of potting mixes that you can create as soil substitutes. One that is widely used in the United States is equal parts peat moss, good-quality compost and vermiculite. This is a relatively expensive option, but it will certainly give excellent results. By studying the Soil-less Media Options table on the opposite page, we can work out which materials will perform the specific functions we require of growing media, such as water holding, nutrient storage, aeration and being lightweight. If we have a ready supply of ingredients (preferably free ones), such as compost, we can then experiment and create our own ‘home brews’. This not only saves us money, but also allows for recycling of household organic materials. When designing artificial soils for constructed gardens, remember the golden rule: the shallower the bed, the more open and porous the mix must be.
LOOKING AT THE FUTURE OF SMALL-SPACE GROWING
As a species, humans have always been incredibly inventive and agile when our survival depends on it. Hence, we predict the emergence of all sorts of flexible and adaptable technologies for growing and fertilising plants in smaller and smaller spaces. A recent innovation that is available for purchase is an Australian-made growing system called Composta, which enables us to utilise any extremely small space (such as on rooftops or balconies) for growing food in the urban farm.
An excellent solution for inner-city urban farmers, Composta comprises a large bowl of potting mix with a cylindrical worm farm embedded in the centre. The cylinder has holes at its base that remain covered with potting mix at all times – this means that the worms can move easily between the worm farm and the potting mix, distributing nutrients and beneficial microbes (which they produce whenever they feed) as they go. The bowl has a single large hole that not only drains the unit, but also allows you to collect the liquid that leaves the system. This drainage liquid is still rich in nutrients and can be used to fertilise other plants on your urban farm.
A clever idea for an apartment balcony, Composta is a worm farm that transforms kitchen scraps into useful fertiliser for plants growing in it.
Containers of any size can be used on the urban farm, as long as you’re mindful of the drainage capacity and fertility of the growing medium you place in them.
It’s handy to grow herbs in pots or a raised bed close to the kitchen, so you can access them quickly and easily when cooking.
CASE STUDY:
RAISED BEDS
Helene’s Garden at Berowra
Helene moved from inner-city Sydney because the gardens there were not big enough to cater for her aspiration of growing all her own vegetables and fruits, and owning chickens that would supply her with eggs. Her father was an agronomist from Russia, and this initially sparked her interest in food-plant growing when she was young.
Because the soil is so thin and poor on the sandstone ridges around Sydney, she decided to go with raised beds, including water tank-type planters, timber-edged beds, old wine barrels cut in half and borders against the house. The ‘soil’ in her planters originally comprised purchased potting mix, but it is now entirely made up of her own compost. She has three large compost heaps that were constructed from landscape sleepers by her WWOOFer (Willing Workers on Organic Farms) friends. She still hosts WWOOFers from Europe, Japan and Australia to this day.
Helene is now pretty much self-sufficient in vegetables; the only thing she buys is lettuce. She has stone fruit, mango and persimmon trees and passionfruit vines for fruit production, but is thinking of pulling out the stone fruit trees due to a persistent fruit fly problem as well as the lack of ‘chill factor’ in the suburbs these days, which results in poor fruiting. To deal with pests, Helene uses the herb tansy to repel flies and leaf-eating insects, traps to catch fruit flies, and occasionally a little pyrethrum spray. She physically nets the mangoes to prevent damage from fruit flies and possums, believing that such large fruits are worth the effort.
After noticing pale leaves on her plants, we tested her soil and leaf tissue and worked out that she had an inadequate level of manganese in her garden. Sandstone soils are notoriously low in manganese, so we recommended she use some manganese sulphate to correct the deficiency.
The raised beds help prevent damage from the chooks that roam at will during the day. She has four Buff Orpingtons, one Black Orpington and one ISA Brown that provide all the eggs she and her WWOOFer visitors need. She often has extra eggs and vegetables that she gives away to friends or takes to the local market.
Top to Bottom: Corrugated iron beds are a cost-effective alternative to timber; fruit trees are grown in sunny corners of the garden.
Clockwise Left to Right: half wine barrels and railway sleepers are more traditional ways of creating raised beds; this ISA Brown hen helps keeps the weeds down between Helene’s beds; sweet potato is a great crop to grow in raised beds, if it is given room to move.
Virtually every space in the garden is devoted to food production.
Helene learned farming skills from her father.
GROWING IN BAGS OR BOXES
A system being adopted more and more in the commercial production of vegetables and cut flowers is where plants are grown in a ‘semi-hydroponic’ way; this is particularly suited to trellis crops such as cucumber. Seedlings are planted in 25-litre bags of potting mix that have the top cut open and holes pierced in the bottom. Some people use Styrofoam boxes filled with potting mix. Seedlings grow and develop on trellises, and are nourished by liquid feeds that are virtually identical to hydroponic feeds. Controlled-release fertilisers can also be used as a base in the growing medium.
The nutrient solution does not need to be recycled continuously, and it can be alternated with irrigation water. The good water-holding capacity of the potting mix means that plants don’t die if the pumps fail, unlike with hydroponic crops. Bags can be re-used for up to five crop cycles before the structure of the potting mix collapses; hygiene and weed problems are much easier to control than when planting in natural soil.
This ‘grow bag’ utilises perlite as the growing m
edium to create a lightweight system ideal for rooftops and courtyards.
Hydroponics uses a recirculating … solution designed to provide all the nourishment that the plants need.
The dark hue of the mustard greens plants sits in striking contrast to the white PVC pipes used in hydroponic production.
HYDROPONICS
Essentially the science of growing plants without soil, hydroponics uses a recirculating nutrient solution designed to provide all the nourishment that the plants need. Conventionally it utilises refined soluble mineral fertilisers, as their composition is known and the balance of essential nutrients can be precisely calculated to suit plant needs. It is possible to make organic nutrient solutions, but several problems must be overcome.
Organic sources of nutrients, such as manure, blood and bone, fishmeal, and hoof and horn, are not soluble, so they have to be ‘steeped’ in water for some time, usually a week or so. Essentially, this means they decompose anaerobically – which can lead to odour issues.
Blockages can occur if any undissolved organic matter is not filtered out.
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