Composting – The Microbial Way

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Soil. The unsung hero of every farm.

Although it is the foundation for all land based living organisms, including ourselves, soil seems to get overlooked when it comes to the role it plays for farmers to generate an income from their product and the quality of the nutrients in the end-product we as consumers eat.

Just as vegetables provide us with our required nutrients to survive and thrive, organisms within our soil require high-quality, nutrient-dense compost to achieve their own nutritional requirements and make their enriching contribution to the soil food web.  

How do we maximise our soil quality through composting?

When it comes to composting for a commercial output, it’s important that we maximise our efforts – making high quality compost, from existing farm inputs, in a short period of time.

Using good compost provides the following benefits:

  • Nutrient provision
  • Greater water retention 
  • Improved soil structure
  • Disease suppression
  • Increased pest tolerance
  • Elevated yields

 Enter…the Controlled Microbial Composting (CMC) method….
This method concentrates on maintaining aerobic conditions during composting.

How to create top quality on-farm compost

Site Selection

Before choosing the materials to compost in your windrow (row of compost) the first thing to consider is the composting site. If using a paddock site to set up windrows the land should be well drained to avoid waterlogged compost.  Ideally, the site should be a slightly sloping (~ 4 degrees) concrete base close to the main growing operation. This will allow ease of movement of machinery without damaging the soil. It will also prevent excessive moisture accumulation and ensuring that monitoring is carried out regularly whilst reducing the man hours spent travelling to and from the windrow for turning. 

Windrow design

When deciding where to situate the windrows on the composting site, thought should be given as to the most efficient way of moving the machinery.  Windrows are usually 50 metres long and should not exceed 2.5 metres in height and 1.2 metres in width. Research has shown these dimensions to be the most effective for making good CMC, helping to develop a homogenous, well aerated and odour free heap. A BobCat or Front-end Loader is the most efficient way of building windrows.  The turning of them should be performed by a specialist piece of windrow turning equipment. These tend to be self-propelled or tractor mounted implements the latter of which requires a creep gear box. The initial layering of the windrow will vary depending on the material used. The following can be used as a guide:

Once the windrow is built it should be covered with a Gortex type breathable cover to prevent rainwater entering but allow oxygen to enter.  Thus preventing the leaching of nutrients in rainy weather and allows carbon dioxide to disperse outwards from the windrow.

Choosing your composting materials

The quality of your compost will depend on the quality of material that is used in the process.  Farmers should always choose organic residues from farm sources, such as manure and feedlot waste, packhouse waste and old tomato plants are all suitable ingredients, as is green waste from parks and woodlands.

If bringing non-farm waste onto the farm for composting it is important that all the associated regulatory and organic standard requirements are met. 

The Carbon (C) to Nitrogen (N) ratio and the moisture content of the starting material will have a bearing on the success of the composting process.  The ideal C:N ratio is between 20:1 – 40:1. The moisture content of the selected materials should ideally be 50 – 65%. You can calculate your C:N ratio input using the figures below.

C:N ratio and moisture contents of compost ingredients

MaterialC:N RatioMoisture content (%)
Vegetable matter11:1 to 13:175
Cattle manure11:1 to 30:167 to 87
Horse manure22:1 to 50:159 to79
Laying hens3:1 to 10:162 to 75
Grass clippings17:182
Hay15:1 to 32:18 to 10
Straw48:1 to 150:14 to 27
Paper pulp90:182
Sawdust200:1 to 750:119 to 65

Source: On farm composting handbook. Values are representative only.

Good compost should contain the highest possible diversity of food resources to stimulate the highest possible diversity of beneficial organisms. It is crucial to add some clay, as no clay humus crumb can be built without it. Application of up to 10% ready-made compost and some soil will also help to facilitate the process ensuring that the desired organisms are present.  A microbial innoculant often referred to as a starter can be applied within two days of composting. This is a mixture of microbes that will help speed up the composting process, there is some debate as to whether or not a starter should be used with some reports that good quality compost can be made without using a starter. 

The breakdown process

After the windrows have been set up temperatures rise, and within 5 days can climb to 60°C or higher.  If care is not taken the windrow can ignite itself. It is important that the windrow is monitored frequently with a temperature probe.  The temperature should not exceed 65°C as valuable carbon sources can be burnt up and beneficial organisms are killed off. If the windrow is getting too hot the pile should be turned to cool it down again.  During the first 7 days of the process temperatures should be maintained between 55°C and 65°C to ensure that all weed seeds and plant pathogens are killed. This is known as the thermophilic stage of the composting process.  During the first few weeks of composting daily turning may be required. After about 14 days the temperature of the heap will start to drop as all organic matter in the heap is broken down.  

The build up process

In the second phase known as the mesophilic stage, new groups of organisms colonise the heap, feeding on the heat-loving organisms. It is during this phase that long chain humic acid substances develop, binding to clay particles in the heap to form the clay-humus crumb. The key measure in assessing that the process is finished is temperature. After the initial rise, temperatures will fall before stabilising at an ambient temperature. With the right combination of ingredients, and regular turning, the active composting phase can be finished in 6 to 8 weeks.

Compost Development

3 Management indicators

As a guideline turning may be required on a daily basis at the beginning of the process reducing to once weekly as the process nears completion.  Turning moments depend on results of measuring temperature, CO2 and moisture.  It is therefore essential that both temperature and CO2 probes are used.  CO2 is released in both the thermophilic and mesophilic stage of the process. If allowed to build up it will sink to the bottom of the pile and intoxicate desired aerobic organisms.  It is therefore important that CO2 levels do not reach more than 20%. Controlling and maintaining the right moisture levels is a third management indicator. Water is required by the organisms in the heap to metabolise organic matter. Biological metabolic decomposition results in high heat, which in turn changes the water in the piles to steam. Protected coverings will help to retain moisture but if levels fall below 60% in the first 3 weeks then moisture needs to be added by spraying the pile during the turning process.

The final product

After the composting process is completed the compost should be:

  • Humified, adding to the soils ‘bank of fertility’
  • Resemble and smell like a forest soil
  • Biologically very active, to suppress plant diseases
  • Free of weed seeds
  • Free of human pathogens, such as E. coli and Salmonella spp
  • Free of plant pests, such as plant parasitic nematodes
  • Free of plant pathogens, such as rhizoctonia, sclerotinia and fusarium.

It will also contain:

  • Macro and micro nutrients in addition to organic Nitrogen, Phosphorus and Potassium (NPK). Compost is an especially good supplier of trace minerals such as boron, cobalt, copper, iodine, manganese, molybdenum, and zinc. The more varied the materials used to make the compost, the greater the variety of nutrients the compost will provide.

Conclusion

While compost production is not a cheap operation it can be justified for high value crops, such as vegetables.  Producing a consistent, high quality compost requires investment in suitable machinery and commitment of management time.  Compost turning equipment ranges from about $11,000 to $300,000. Costs of breathable covers, monitoring equipment, man-hours and the creation of a concrete base (if required) also need to be considered. However, the benefits are more than worth the investment, extending beyond mere NPK input. Improved soil structure, water holding capacity and disease suppression are a few factors that alone justify and provide return on investment. There may be potential for relationships with local councils who want to get rid of green waste.  Composting could also provide a new and lucrative enterprise on the farm if sold to other farmers.

A windrow being turned Well made compost
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