Grow Your Own
Page 19
The most important decision is to select a system that suits the size of your urban farm, the amount and types of organic matter that are available to you, and the amount of time you can put into composting. For example, don’t buy a 100-litre compost bin if you have a small balcony, and don’t use ‘cold’ composting methods if you have a lot of weeds, dog manure or meat scraps to add to the pile.
A compost heap goes through various stages of decomposition, and it is critical to understand when the compost is fit for your specific purposes. Composts can be used as soil conditioners, fertilisers or mulches (or combinations thereof). Those that are to be used as soil conditioners and/or fertilisers for seeds and seedlings need to be fully ‘mature’ (well rotted), while mulches that will, by definition, just sit on the soil surface and act as an insulating blanket do not need to be fully broken down; in fact, mulches should contain a lot of coarse fibrous matter, not fine and earthy humus that will germinate any weed seeds that may blow in.
‘Mature’ vs ‘immature’
The major difference between a ‘mature’ compost and an ‘immature’ compost is that a ‘mature’ one will be suitable for very sensitive seedlings and seed germination, whereas an ‘immature’ compost will not. If an ‘immature’ compost is dug into a soil, it is likely to contain ammonium nitrogen that is potentially toxic to sensitive seedlings. If it is low in nitrogen, various composting microbes will decompose it further; in doing so they will be competing with plant roots for the available nutrients and oxygen in the soil. By the time composts are ‘mature’, ammonium nitrogen is no longer present and the potential for nitrogen drawdown is reduced. ‘Immature’ compost can be used, but only on established robust crops and as a pre-plant fertiliser at least two weeks before seeding or transplanting.
‘Mature’ and ‘immature’ composts both contribute plenty of humus to the soil, but ‘immature’ compost takes longer. Humus is a kind of biochemical end point in the breakdown of organic matter. It is a very complex substance that is made up of numerous individual organic substances and is therefore very difficult to quantify. Each batch of humus isolated from compost will likely be slightly different in chemical composition and nutrient content, depending on what went into the compost in the first place. However, the overall properties of humus are what make it an essential substance for urban farmers.
Humus creates excellent soil structure by holding soil particles together to form aggregates, or peds. Particles of clay, silt and sand all benefit from this capacity. If your soil has lots of crumb-sized peds, then water can drain around the peds – this leads to good soil aeration. The peds themselves absorb and store some water, which is used for plant growth; thus humus improves a soil’s water- and nutrient-holding ability.
A dark colour and a somewhat spongy texture are evidence that a soil has a good level of organic matter.
Compost bins and worm farms come in a variety of shapes and sizes – choose the system that is most appropriate for your urban farm.
A large system with bays for holding the various ingredients for compost as well as the actual compost heap is ideal for big farming plots.
IGNORE EXTRAVAGANT MARKETING CLAIMS
Despite what you may hear or read to the contrary, there is no proof whatsoever that it is beneficial to add any particular strain of microbes to your compost. The various types of organisms that are responsible for composting are a normal part of the environment, and they will colonise your compost in a variety and form determined by what you put into the compost. So please don’t waste any of your money on so-called compost ‘activators’, because you actually get them for free with everything you put into your heap.
If you are genuinely worried about the number of organisms living in your compost, place a handful of a previous compost batch, some worm castings (if you have a worm farm) or even soil into your new mix, and your compost is sure to get all the beneficial microbes it could ever possibly need.
MAKING A BALANCED COMPOST
Like us, composting microbes and worms need a balanced diet to operate at peak efficiency. They also require the right mix of water and air. If compost gets too wet, for example, the composting process shuts down and the worms all try to escape from the heap because there isn’t enough air to breathe.
The nutrient value of your finished compost will vary depending on inputs. As with plant growth, the first and most important element is always nitrogen. It stands to reason that if we use organic matter that is high in nitrogen, then the compost will also be high in nitrogen. If we use woody (high-carbon) waste that has a poor nitrogen level, then the compost will be low in nitrogen and could, in fact, consume nitrogen when applied to soil (a process known as nitrogen drawdown). The same principle applies to the other essential nutrients for plant growth: the nutrient value that goes into the compost will reflect what comes out in the ‘mature’ compost.
In practice, even if we get the mix wrong, nature will always guide the compost towards a natural balance of nutrients. This is easy to demonstrate. If we put too much nutrient-rich manure in compost, it will stink of ammonia. This is the excess nitrogen that the microbes can’t use being driven off as gas, or ‘volatilised’.
On the other hand, if we put high-carbon compost on the garden, the decomposing microbes will rob the soil of nitrogen so they can continue to break down the woody material in the compost, eventually causing a nitrogen deficiency in our plants. This is nature readjusting the nutrient balance. It is also why it is impossible to make ‘mature’ and stable compost that has any more than 1.5–2 per cent nitrogen in it. If you compost pure manure, which has 3–3.5 per cent nitrogen, it will actually decline in fertiliser value as ammonium is removed as a gas.
Getting the mix right
Loss of ammonium nitrogen is a waste of precious fertiliser nitrogen, while hungry carbon-rich compost will actually rob your plants of nitrogen. To avoid these two major problems, we have to be careful when we choose the raw ingredients that we include in the compost.
We also want to prevent bad smells from permeating urban environments, so it is very important to avoid adding any material with excessive nitrogen to your compost. Too much water or too much wet vegetable matter leads to poorly aerated (anaerobic) compost, which also produces foul odours – and you risk becoming alienated from your neighbours!
The best, fastest and easiest compost to create and manage – without odour and offence – has the right balance of carbon, nitrogen and other nutrients necessary for your particular urban farm. The Compostable Materials table shows you what can be added to compost, and the nutrient value of each material.
Your job in composting is to find the right blend of the ingredients listed in the Compostable Materials table to optimise the moisture content and nutrient levels (especially nitrogen) for the composting organisms or worms. Any material that has once been living is capable of being composted, but some materials are much easier than others.
The mix should look damp but not shiny with wetness, and it should only just moisten the hands when you touch it. For composting, if it glistens in the sun and you can squeeze any water out of it, the compost is definitely too wet. For worm farms, you should be able to squeeze just a few drops of water out of it to indicate the moisture content is about right. If the compost looks wet and smells rotten, then it is anaerobic. If it smells of ammonia, then you have too much nitrogen and not enough carbon in the mix.
Compost should be open and porous so that it admits air, and it should look moist but not wet.
CARBON VS NITROGEN
Effective composting is largely a balancing act. We need to consider the carbon to nitrogen ratio of the materials to be composted if we are to make compost that supplies sufficient nitrogen for plant growth. The woody parts of plants are rich in cellulose and lignin, which makes them very high in carbon but low in nitrogen. Wood has a carbon to nitrogen ratio between 100:1 and 500:1; dead leaves are between 40:1 and 80:1. At the other end of the scale, vegetable
and fruit scraps have a carbon to nitrogen ratio between 15:1 and 20:1, coffee grounds are 20:1 and manure is between 5:1 and 25:1. Ideally, when a compost pile is mixed together, it will average out to have a carbon to nitrogen ratio between 20:1 and 30:1.
COMPOSTABLE MATERIALS
ORGANIC WASTE MOISTURE CONTENT NITROGEN CONTENT NUTRIENT CONTENT
Paper and cardboard Very low Very low Very low; high carbon to nitrogen ratio
Sawdust, woodchips, tree bark, straw Low Very low Very low; high carbon to nitrogen ratio; indigestible
Dead leaves (including sclero-
phyllous leaves, such as those from eucalypts), sticks, stems Low Low Very low to low in all nutrients; digestible carbon
Mixed chipped garden waste Low to moderate Moderate Moderate and well-balanced in all major nutrients; reasonably digestible, except the woody matter
Fresh green-leaf matter (for example, lawn clippings or leafy prunings) Moderate About right, moderate Moderate and well-balanced in all major nutrients; highly digestible
Fleshy vegetable-type leaf matter High High High in all major and minor plant nutrients
Fruit and tuber wastes (for example, tomato, potato and carrot) Very high High Tubers very high in digestible carbon; fruits very high in nitrogen and potassium; highly digestible carbon
Herbivorous manure (for example, from horses, cows, rabbits or sheep) Low to high High Concentrated nitrogen and other major and minor nutrients; feedlot manures can be poorly balanced and have excessive levels of trace elements; high fertiliser value
Carnivorous manure (for example, from poultry, pigs, dogs or cats) Often high Very high Very high in nitrogen, phosphorus and calcium; high fertiliser value
Meat and bone scraps High Highest of any organic fertiliser Very high in nitrogen, phosphorus and calcium; very high fertiliser value
Left to Right: Alternating layers of rich food scraps and straw or shredded paper is ideal for creating a worm-farm compost. Add a little water if the mix is too dry.
Useful compost blends
There are an infinite number of combinations of organic wastes. However, the following ‘recipes’ usually work well (all proportions by volume).
A typical urban-farm compost
4 parts dry leaves, stems and/or twigs
2 parts grass clippings and fresh weeds
1 part vegetable and fruit scraps
1 part poultry manure
This mix has the right moisture content, is open and porous enough to let in air, and has enough nitrogen and nutrient-rich component. It is also likely to be digestible enough for the composting organisms that are found naturally in the heap to heat it up if conditions are right.
A typical worm-farm compost
4 parts vegetable and fruit scraps
2 parts shredded paper and straw
This is ideal for ‘worm bin’ systems, as it is a little richer and wetter than the typical urban-farm compost. If you can get stable manure, a suitable brew for vermicomposting might be:
4 parts stable horse manure
2 parts rich vegetable and fruit scraps
1 part grass clippings
A typical hot-composting brew
4 parts shredded garden waste
1 part grass clippings and fresh weeds
1 part vegetable and fruit scraps
½ part animal manure
Based on shredded plant material and prunings, this recipe is likely to be a hot (thermal) compost and will sterilise the compost quite well.
BUYING COMPOST?
Commercially available compost usually comes in two types:
Those made from garden waste, which is shredded, composted thermally and then screened to produce compost fines.
Those with food wastes, manures and human biosolids added.
Purchased compost that has been enriched with food wastes and manures is likely to have a high nutrient content, especially nitrogen. Garden-waste compost is well balanced and has a particularly good potassium content, but is not so high in nitrogen and usually needs supplementing. Always ask what is in the compost being sold; if food and other types of rich wastes have been added, expect to reduce the need for other fertilisers.
This compost bay has no floor, so it has been invaded by living plant roots. This has caused the compost to lose nutrients and water.
TYPES OF COMPOSTS
There are two main types of composts:
Hot (thermal) composts.
Cold (non-thermophilic) composts (worm composts, or vermicomposts, also fall into this category).
Each has its pros and cons, but they are both designed to achieve the same aim – to increase the fertility of the soil. The most important thing is to select one that suits the scale of your garden or farming enterprise and the types of organic products available to you.
If, for example, you have a balcony set-up and only have access to your own kitchen scraps and maybe some lawn clippings, it will be difficult to get sufficient mass to ensure hot composting. Make do with cold composting, or use a small worm farm. Since you won’t have much of a weed problem, hot composting – which kills weeds – is not necessary anyway.
Hot (thermal) composts
Most of us have experienced the heat build-up in a pile of grass clippings from a recently mown lawn. Within hours, that heap of grass clippings can turn into a hot, steaming pile. What we are seeing in this situation is the energy generated by the explosive growth of groups of microorganisms called mesophiles (those that grow at moderately high temperatures) and thermophiles (those that grow at high to very high temperatures).
Hot composting is a method where we construct a heap that favours the development of both mesophilic and thermophilic microbes. It has the advantage that the high temperatures generated within the heap act to pasteurise the finished compost, and this process kills the vast majority of weeds (both vegetative structures, such as runners and bulbs, as well as seeds), pests and plant pathogens. However, it should be noted that hard-coated seeds (such as those from many legumes) often survive the pasteurisation process.
Sturdy and easy to use, a proprietary compost ‘screw’ is ideal for turning over small amounts of compost.
Left to Right: Looking like something out of Star Wars, this ball-shaped composting bin makes it easy to turn and aerate small volumes of compost, and it keeps vermin out.
A successful hot-compost heap goes through several phases. The first one is monopolised by thermophilic microbes, which bring the temperature up to levels where pasteurisation occurs (50–70°C); this eliminates most of the harmful organisms that may be present. The second phase involves the dominance of mesophilic microbes, which complete the breakdown of organic materials, and proceeds at temperatures of up to 35°C. The third and final phase involves the maturation of the pile, and it begins as the nutrient supply starts to dwindle. Non-thermal organisms start to take over, as the temperature drops to more moderate levels. At this stage, many different microbes – such as actinomycetes (a group of fungus-like microbes) – start to appear that are associated with disease suppressiveness. These types of organisms are thought to be the best ones for stopping plant pathogens, such as root-rotting fungi.
In order to achieve a temperature that is high enough to kill weed seeds (50–60ºC) with small volumes of compost, it is necessary to contain the compost in some kind of vessel, such as proprietary plastic bins or homemade structures (for example, bays made of bricks). In a commercial composting facility, a large yet simple windrow is quite sufficient to maintain heat; in fact, these compost rows often have the problem of excessive heat. In cold climates or where you have a real weed problem and need reliable heat, an insulated vessel can be used.
A means of occasionally turning the compost pile is also important. Either utilise a compost tumbler, which you can rotate with ease, or make a bay with three sides and a concrete or brick floor, so you can access the compost and turn it with a shovel or pitchfork.
Cold (non-thermophilic) compost
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People often become concerned when their compost does not heat up. It does not need to be hot to produce good compost. Natural decay processes will eliminate most plant diseases (compost is not the natural home of diseases that directly parasitise plants), and if you don’t put a lot of weeds in the compost, then this should not be a real issue, either.
Cold-compost bins are available in all sorts of shapes and sizes, with most looking something like a Dalek from Doctor Who. They are a great option for many households, especially if they produce a moderate amount of organic matter from the kitchen and yard. If you take a little care as to what material goes in, they will produce a balanced compost that doubles as both fertiliser and soil conditioner.
Trench composting is one of the simplest and easiest methods of cold composting, as it involves burying organic materials in soil that you want to enrich. Earthworms and various other macroorganisms, such as soldier fly larvae, will often breed in large numbers while this food is available. And, of course, microbes such as bacteria and fungi will arrive en masse to complete the composting task. It is not hot composting, however; it is best considered a type of vermicomposting. If the organic materials are buried deep enough, this will prevent the germination of any weed seeds.
In most natural soil profiles, there is an organic (‘O’) soil horizon above the mineral soil. The ‘O’ horizon is composed of leaf litter, bark, animal manure and so on, and it acts as a mini compost layer that is constantly adding small amounts of humus to the mineral soil in the ‘A’ horizon below. Trench composting is, in some respects, like creating a very thick ‘O’ horizon in your soil. Provided you are using a good balance of nutrient-rich materials with carbon-rich ingredients, your soil will obtain the maximum benefit – as any nutrients that are leached out of the compost will be deposited in the soil below for the subsequent crop.