DOING YOUR OWN FIELD TRIALS WITH COMPOSTS
Pam Pittaway, PhD
Chrysalis Landscape Consultants www.grubbclc.com.au
31 Douglas McInnes Dve Laidley,
phone – (07) 5465 2017
e-mail, grubbclc@bigpond.com
Can do
Sheet
No. 5
A compost is the end-product of the microbial breakdown of organic matter, in
the presence of ample air and water. Heat is produced, favouring the activity of
heat-tolerant microbes. The microbial diversity in a compost is renowned for
destroying toxic chemicals such as pesticides. Vermicomposts (with worms) are
excluded from this definition as worms cannot tolerate heat, and raw material
requirements for processing are more restricted compared to conventional
composts (see ‘Can Do’ No 7 - ‘What is a quality vermicompost’).
BIOLOGICAL TRANSFORMATIONS IN A MATURE COMPOST
The three stages of composting are:
1) consolidation when raw materials (feedstock) are mixed with water (to field capacity),
formed into a windrow and the bulk density of the windrow is established;
2) active stage when the core temperature is maintained ideally between 50 and 650C; and
3) curing stage when the core temperature drops and nutrients are released in the inorganic
form (figure 1 and refer to ‘Can Do’ sheet no 4‘A Practical Guide to On-Farm Cocomposting’). In a mature compost, microbes have converted the readily available organic
matter into humic acids and microbial cells. In contrast immature composts may contain
partially broken down chemicals that may be toxic, and excessive microbial activity which
may induce nutrient draw-down if the compost is applied to growing plants.
Figure 1: Temperature change over time in a cotton trash windrow, describing the three
stages of composting (Consolidation, Active and Curing). Bars show changes in the total
phosphorus concentration over the three stages, with mineral P (available) in yellow,
and organic P (slow-release) in blue. Windrows were turned only once every 5 weeks.
If the compost is well mixed during the Active stage, then the survival of weed seeds and
plant and animal pathogens will be minimal. Plant residues can be composted in their own
right (figure 1), to provide stabilized organic carbon to improve soil structure and provide an
alternative form of nutrient exchange. Composts based on animal manures typically contain
higher phosphorus (P) and calcium (Ca) concentrations. For example, adding feedlot manure
to cotton trash (1:2 volumetric ratio) increased total P from 3.1 kg/t to 5.0 kg/t.
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CHARACTERISTICS OF A QUALITY COMPOST
A quality compost should have up to 50% of the P in the mineral (plant-available) form
(figure 1). To achieve this the compost must have undergone more than 4 weeks curing,
during which time the water content of the mix will also decrease. At sale the water content
of the product should be about 25-35% by weight, providing enough moisture to minimise
dust during handling. At this stage, microbial activity will have degraded any toxic chemicals
originally present in the raw inputs, and sufficient P and K will be available for plant growth.
However, N will still be predominantly in the organic form (figure 2), which plants cannot
use. Therefore, if used as an alternative to conventional fertilisers, additional mineral N
fertiliser may need to be applied (eg. urea, or certified organic inputs eg blood or fish meal).
The compost should be screened before sale, to remove any stones or other large particles.
Produce and
CO2
Non-fertiliser
(manure, erosion)
Fertiliser
(NPK)
Volatilisation
Shoots, roots,
residues
Inorganic nutrients (plant-available)
Soil organisms
Fixed to clays
Leaching
Humus pool
Figure 2: Cycling of nutrients in the soil. Straight arrows show the pathways of
inorganic (plant-available) nutrients. Curved arrows show organic nutrient pathways.
To become available for plant uptake, organic nutrients must be converted to inorganic
forms by soil organisms (insects and other arthropods, and microbes).
Composts based on raw inputs such as bark or timber, contain a higher proportion of lignin
and other plant phenolics. In the immature state these compounds contribute to plant
toxicity. However, after decomposition, they assist in controlling soilborne diseases. For this
reason, mature timber-based composts are preferred in potting media for plant nurseries. Raw
materials high in lignin also contribute higher concentrations of humus-like (Figure 2),
compounds which improve soil structure, and its capacity to retain nutrients and water.
STANDARDS FOR AUSTRALIAN COMPOSTS
Composting is a robust biological process, based on the microbial recycling of organic
matter. For this reason, many different raw materials ranging from crop residues to dead
animals to industrial sludges may be included. In poorly managed composts, animal and/or
plant pathogens, heavy metals and/or chemical residues may not be adequately broken down,
and excessive microbial activity may starve plants of nutrients such as N and P.
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Controlling the quality of the raw materials, the temperature and turning dynamics of a
compost, will produce a more uniform product. Hence quality assured composts should
specify both the raw material inputs and the process control methods used. Monitoring the
temperature is the most direct way to verify the level of process control achieved.
Standards Australia has developed a standard for composts, soil conditioners and mulches
(Australian Standard 4454 2003). Accepted ranges for pH, heavy metal content, high
temperature treatment and nutrient content are specified. However, most of these ranges are
generic, and do not relate to the limitations and/or requirements of specific soil types and
crops. In contrast, the standard for potting mixes (AS 3743 1996) is more prescriptive,
focussing on the needs of container-grown plants. Standards for fertilisers and soil
conditioners have also been developed by the organics industry. For example Australian
Certified Organic (ACO 2003)) permits the use of compost from organically sourced
materials, but inputs from non-organic sources must be tested to ensure that levels of
pesticide residues, heavy metals or other potential contaminants do not jeopardize the
certification status of organic farmers. However, certification under AS and ACO does not
guarantee that the product will be biologically stable and suitable for growing crops.
Composts containing biosolids (treated industrial and domestic waste sludges) are the most
likely to have a high heavy metal content. Slurry from intensively farmed livestock may
have cadmium present, as a contaminant of the superphosphate included in some feed
rations. Metal concentrations will be highest in sludge accumulating in anaerobic ponds. For
one-off applications for land reclamation or for continued use in forestry plantations heavy
metal contamination may not be an issue. However some food crops such as peanuts,
preferentially accumulate heavy metals, and relatively low concentrations of zinc, copper,
nickel and cadmium can reduce populations of beneficial soil bacteria. The level of repeated
applications of a compost containing heavy metals to a soil, is determined by the capacity of
the soil to fix the metal to clay particles (figure 2), and the ease of uptake by plant roots.
Therefore if using a compost based on biosolids, seek advice on the potential of your crop
plants to bioaccumulate heavy metals in your particular soil type.
GETTING THE MOST OUT OF A COMPOST APPLICATION TO SOIL
Quality assured compost costs more, to recover the costs of monitoring the composting
process, and testing for nitrogen draw-down (NDI), concentration of total NPK, available P &
K (Colwell method), organic carbon (wet oxidation method), pH and chloride concentration,
electrical conductivity, cation exchange capacity and exchangeable cations (Rayment and
Higginson Australian Laboratory Handbook of Soil and Water Chemical Methods should be
specified). If buying cheap compost or stockpiled manure, expect to undertake a chemical
analysis of the compost yourself, and have a ‘Plan B’ if your crop suffers from nitrogen
draw-down as a consequence of the biological instability of the product.
Soil testing is recommended to calculate the immediate crop requirements for NP and K
(mineral component), and to adjust for future crop requirements as the organic (slow-release)
component. To avoid excessive nutrient build-up in soil, we recommend that compost should
be used only after several weeks of curing, when up to 50% of the P is available (figure 1).
The upper limit of application should be based on either the mineral (available) P and/or K
requirement for crop growth. Use the nitrate N analysis of the compost to adjust the rate of
application of conventional nitrogenous fertiliser.
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If the compost is mature (biologically stabilized), the ammonia N concentration will be very
low, and the total N concentration (predominantly organic) will be very high relative to the
nitrate (available) concentration. Any nutrients in the organic form (most of the N, 50% of
the P, but only 10-20% of the K) will become mineralised over time, depending on the
temperature, water content and extent of microbial activity in the soil. Conventional soil tests
are the only way to measure the rates of mineralisation. Soil tests for available NPK levels
taken at the start of the next cropping cycle, will enable you to get the most out of the slowrelease component of earlier compost applications. Experience in some soils indicates that
the benefits of a high rate of compost application may be measurable for up to 5 years later.
However, in the years following the initial application, using soil test results and the analysis
of available nutrients in the compost to adjust the fertiliser requirements for the next crop,
will optimise both the environmental and the economic benefits of the compost.
BENCHMARKING THE PERFORMANCE OF A COMPOST
Composts provide a range of soil health benefits in addition to nutrient inputs. To test for
evidence of these, select a paddock on your property where the organic carbon levels have
declined (sometimes associated with increases in root disease severity), or where
recommendations for the application of gypsum have been made. Select the upper limit for
compost application based on either the fertiliser P or the K requirement for the early
growth of the crop.
Crops known to be strongly dependent on mycorrhizae may better respond to compost
application than to inorganic fertilisers, given that half of the P is in the slow-release form.
Mycorrhizae are beneficial root-inhabiting fungi that improve the uptake of nutrients such as
P and zinc, and assist in the control of root diseases. Chick-pea and other legumes, sunflower,
cucumbers, capsicum, onion, sorghum and cotton are known to be dependent on mycorrhizae.
Monitoring the performance of these crops in the short term, comparing responses to compost
versus conventional fertilisers, may provide evidence of the soil health benefits of compost.
Apply biologically stable compost to half of the paddock, topping-up with inorganic fertiliser
N and P to meet crop requirements (application rates of below about 5 t/ha are unlikely to be
detected in soil test results in the short term). Compare the performance of the crop on the
other half of the paddock by applying conventional inorganic NPK fertiliser. All other
management operations must be the same for both halves of the paddock. Compare rates
of emergence and seedling vigour, the incidence of disease and insect attack, and of course,
plant yield at harvest. The ability of a plant to respond successfully to attack by pathogens
and to repair insect damage depends on the availability of circulating carbohydrates. Some
organic growers use a refractometer, to compare the health of crop plants. Refractometers
are used routinely in the horticultural industry, to measure the concentration of sugar in
ripening fruit. To access plant sap cut a leaf off at the petiole, or use a garlic crusher to
extract sap from several young expanded leaves. Place 3 drops of sap onto the refractometer.
Test 10 plants in the compost section, then repeat the testing in the conventionally fertilised
section. Are the compost values higher?
Be prepared to undertake field trials for at least 3 or 4 growing seasons. Selection of a
responsive crop species, and a paddock with a history of soil health problems will
provide the best test conditions to compare the efficacy of compost on your property.
But above all, improve your chances of success by selecting a defined, QA product!
Sheet updated 02/2007
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