Over 2016 and 2017, the project team planted three trial crops at the Greater Sydney LLS Demonstration Farm in Hobartville, near Richmond, north-west of Sydney:
- A late-summer capsicum trial, with seedlings planted in beds treated with:
- recycled organic (RO) compost at two different rates of application;
- NextGen compost (RO compost augmented with UF38 and nutrients), at two rates of application;
- chicken manure, applied at the typical grower rate; and
- inorganic fertiliser.
- A spring planting of capsicums eight months later, using the same beds as those used for the first demonstration trials, to evaluate the longer-term benefits of using RO compost (such as release of nitrogen into the soil); and
- A corn crop grown in seven beds, replicated in two areas – one that had been treated with compost in previous years; the other that had never been treated with compost. In both plots, each bed received a different application:
- recycled organic compost at different rates of application;
- NextGen compost at different rates of application;
- a blend of RO compost and chicken manure; or
- inorganic fertiliser.
Data collection, measurement and analysis
All three demonstration trial crops were monitored extensively, with plant and soil analyses conducted. Monitoring included measurement of soil solutes, soil water, soil biological activity, fruit numbers, fruit weight and total yield, as well as chemical assays of the resulting produce to compare fruit grown using RO compost blends with those grown using inorganic fertiliser or poultry manure as is typical farmer practice.
Soil solution was collected at regular intervals and tested for pH, EC, nitrate, potassium and calcium. Greater Sydney Local Land Services staff monitored all three compost trials with a variety of soil moisture monitoring methods. Rainfall and irrigation times were also recorded.
Representative samples of the most recent fully expanded leaves from 10 capsicum plants were collected from each of the six treatments 86 days after planting. These samples were analysed for the full range of major and minor plant nutrients. Published standards for capsicum were used to interpret the results. Similar sampling was done with the trial corn crop.
Plant growth and productivity was assessed at the second capsicum trial crop’s final harvest on 25 January 2017, 127 days post-planting, with five representative plants from each treatment plot cut at ground level and weighed. All large fruit (>80mm in length) was then stripped from the plants, counted and weighed; and the number of small fruit each had produced was recorded.
First trial capsicum crop: Initial planting post-compost needs extra N
The first trial identified some significant strengths and weaknesses of using compost in commercial vegetable production:
- Incorporating recycled organic (green waste) compost into the root zone soil can inhibit growth and yield of the first crop, at best providing no benefit over current grower practice – generally attributed to the low N content of RO compost.
- In this trial, growth inhibition was caused primarily by N drawdown – that is, microorganisms that decompose organic matter, reducing plant-available nitrogen in the soil. As N drawdown decreases with compost maturity, this finding highlights the importance of vegetable growers using high-grade compost compliant with the Australian Standard (AS 445-2003).
- Adding slow-release N to recycled organic compost prior to spreading it can prevent drawdown-induced inhibition in plant growth. The first capsicum trial showed plant growth and yields can be maintained even in the initial crop post-application by amending RO compost with urea formaldyhyde (UF38), a synthetic slow-release nitrogen fertiliser.
- Adding compost to soil can improve its biological health. This trial showed that adding RO compost to the soil increased the numbers and activity of microorganisms responsible for nutrient solubilisation, disease resistance, nutrient accessibility, nutrient cycling, drought resistance and residue breakdown, with the benefit increasing with higher rates of compost application. Poultry manure also improved soil biological health but to a lesser extent.
- Poultry manure is a wasteful and potentially damaging nutrient source used in vegetable production. The nutrients it supplies are readily available to plants and highly mobile in light textured soils, with around 60 percent of its nitrogen released in the first season. This means nutrients in poultry manure are readily leached from the root zone and can contaminate groundwater, with additional losses in runoff and into the air. The nutrients in poultry manure can damage seedlings by increasing soil salinity and by contributing toxic ions.
- Adding UF38 to compost may slightly suppress the development of beneficial soil biology – perhaps because UF38 is transformed into ammonium, which is toxic at high rates; or because added nitrogen stimulates growth in other organisms that competitively inhibit beneficial ones.
- Compost is a costly source of nitrogen compared with common chemical fertilisers and poultry manure. It’s more than four times the price of urea and six times that of poultry manure; and the cost of applying 10t/ha of compost to a paddock is more than double that of poultry manure. Further, no more than 10 percent of the N in a typical application is available to crops in the first year. More research is required on the mineralisation rate of N in compost, especially for repeat applications at low concentrations.
- High concentrations of compost are required to make measurable changes in soil physical properties, making current costs prohibitive for vegetable growers.
A key takeaway from the first demonstration trial was that recycled organic compost needs to be augmented with a nitrogen-based fertiliser if vegetable growers are to maintain yields, hence profitability, in the first crop planted post-application.
Second trial capsicum crop: Evidence of compost's benefits
The second demonstration capsicum trial provided strong evidence that the benefits from using compost in vegetable production accrue over time, becoming more apparent in second and subsequent crops; and that if you compensate adequately for initial N drawdown, there’s no ‘lag time’ in plant health or yields.
- Plants in all compost-treated ground were larger and yielded more fruit than those produced on ‘control’ plots treated only with inorganic fertiliser. The second capsicum crop in RO compost-treated ground was not inhibited significantly by N drawdown, possibly because plant performance was influenced by treatment effects on soil biology.
- More N appears to have been available to crops in the compost-treated ground. This is suggested by the leaf N results and by soil-solution nitrate concentrations, which were relatively high in the two compost treatments. The additional nitrogen could have come from the soil organic pool that was built up by the compost and added to by fertiliser used to compensate for N drawdown. If so, the initial N drawdown can be considered a mechanism that helps to capture nitrogen that might otherwise be lost through denitrification or leaching.
- Leaching losses of nitrate nitrogen were lower in soil that had received NextGen compost treatments. This could be because the slow-release N added to NextGen compost had inhibited bacteria responsible for mineralising and recycling N in the soil. Further research is needed to ascertain whether this inhibition is avoidable if a different type of synthetic fertiliser is mixed through the compost.
- The apparent increase in capsicum yields in the second trial crop is one of several possible benefits of compost use that help justify its cost. Though the yield increases could not be confirmed statistically, the trend was strong, and was consistent with the experience of growers who’ve used compost over a number of years. Observations suggested that treating soil with recycled organic compost has additional benefits, including improving its biological health, influencing root development and reducing nitrate leaching. Moreover, the abundant concentrations of nitrogen found in the leaves sampled from the second capsicum crop indicate that it may be possible to lower the level of synthetic nitrogen fertiliser augmentation, reducing costs and chemical inputs.
- Much of the soluble nutrient contributed to the soil by poultry manure was gone by the time the second crop was planted. Seedlings established quickly with no apparent inhibition in growth; and leaf manganese levels were no longer toxic, as they had been in the first capsicum crop. These findings support the practice of mixing poultry manure in thoroughly before any crop is grown, and of irrigating generously. Poultry manure is a fast-releasing source of nutrients and should be used cautiously.
Trial corn crop: Residual benefits from prior compost application
This study was intended as a farm demonstration trial; as such it was not replicated adequately to make reliable statistical analysis feasible; hence discussion is limited to observed trends.
Key observational findings included that:
- Augmenting compost with particular nitrogen sources led to greater changes in soil chemical properties. Poultry manure had the biggest impact, markedly raising soil pH, EC, and concentrations of ammonium and phosphorus; generally, the extent of change correlated positively with the rate of addition. Urea and slow-release UF38 had much smaller effects.
- N-augmented compost produced healthier plant growth. Plants grown on soil that had received either NextGen compost with UF 38; the 50/50 mix of compost and poultry manure; compost side-dressed with urea; or NextGen compost with urea were the most vigorous.
- Some significant residual benefit from compost application in previous years was evident in the corn trials. Two plots were used to replicate the treatments: one plot had never received compost; the other had been used by Yin Chan for compost trials nine years previously and had also received compost mulch three years before on a pumpkin crop. Though there was no marked change in the soil’s appearance or water-holding capacity, the best growth rates were achieved in the block where compost had previously been applied.
- NextGen compost-treated crops were healthier, with fatter cobs. Treatment differences were readily apparent when the crop was viewed from the air and were most noticeable in the plants’ early stages of senescence. At that stage, plants in beds that had been treated with recycled organic, nitrogen-boosted compost were more vigorous and had greener foliage than those in the other treatment beds. Delayed senescence extends the time for kernel filling and protects against lodging.
The test results on the trial corn crop provided “strong circumstantial evidence that high rates of compost may have long-term beneficial effects on soil health and crop performance”, states the final report. “This is the type of information needed to justify to growers the cost of using compost in vegetable production.”
Getting optimal results from recycled organic compost
The sweet corn trial provided insights into how growers can get maximum benefit from using recycled organic compost, even in the first crop post-application.
- Nitrogen drawdown is a significant issue for the first crop following a compost application. The nitrogen supplied in a typical compost application pre-planting may not be adequate to prevent subsequent reductions in plant growth and yield. N drawdown can be managed, however, by mixing a suitable source of nitrogen through the compost prior to application, or by side-dressing the crop as required.
- Phosphorus is too high for optimal crop growth in poultry manure; too low in compost. A 50:50 mixture of poultry manure and compost will supply enough nitrogen to satisfy drawdown while improving the supply of phosphorus to the soil.
- Urea is not a suitable fertiliser to mix with recycled organic compost to compensate for N immobilisation, as it leads to higher losses through leaching and denitrification. Urea can be used successfully to side-dress a crop to minimise inhibition in plant growth following compost use.
- Urea formaldehyde releases gradually over a period of up to three months, with the rate of release determined largely by biological processes. Mixed through recycled organic compost, it can effectively compensate for N immobilisation.
- Compost amended with nitrogen can delay senescence in plants, which can be of benefit in crops such as sweet corn.
- Compost use may have long-term benefits to soil biological health, which leads to increases in plant growth rates and yields
The project’s demonstration component succeeded in showing that productivity can be maintained or improved in comparison with current farmer practices of using inorganic fertiliser or chicken manure if nitrogen drawdown is addressed. This was achieved using compost blended with slow-release nitrogen fertiliser.
This means, however, that building a market for vegetables grown on compost made from recycled organics is more challenging than making the case for other applications, such as landscaping, which do not have the same crop production demands.
Producing recycled organic compost for use in commercial horticulture, particularly if it is to be tailored to specific crop and soil needs, is also more demanding and costly for manufacturers, which results in a higher price point for the product. The challenge is to motivate manufacturers to produce a product that vegetable growers need in the absence of clarity around demand and willingness to pay. A further challenge is developing a persuasive business case for growers considering adopting Next Gen composts in the absence of certainty around price and access to supply volumes.
Collaboration between compost manufacturers and large growers, such as project participant Jeff McSpedden, could help in developing and testing commercial solutions. Such solutions would be underpinned by the science, communication and extension framework developed under the current project, and would entail:
- trialling different nutrients, including chicken manure, as the source of fertiliser for recycled organic compost augmentation;
- systematically evaluating input costs and margins throughout the supply chain to inform the development of robust business cases for manufacturers and growers;
- developing industry standards and guidelines, supplementary to the Australian Standard, around Next Gen products for different horticultural crops, along with indicative price points; and
- testing markets for the resulting products.
AORA could be expected to collaborate in these activities.
Compost blended with chicken manure may be an incentive for some growers to transition from one practice to another. However, a large-scale reduction in the use of chicken manure is unlikely while it is available at low cost, particularly as many growers are resistant to change.
The Next Gen Compost field trials generated some useful data and built a robust evidence base to strengthen extension efforts. They also generated avenues for additional research.In particular, it would be useful to conduct longer-term trials to see how soil organisms respond as the pool of nitrogen in the recycled organic compost is released over time. Another promising line of inquiry identified was compost’s role in soil disease suppression.