The Next Gen Compost project, delivered by Greater Sydney Local Land Services (LLS), NSW Farmers and UTS’s Institute for Sustainable Futures under the NSW Environment Protection Authority’s ‘Waste Less, Recycle More’ initiative, explored demand for recycled organics (RO) in commercial vegetable production. A major part of the project was providing documented evidence of the productivity benefits of applying RO compost to soil used to grow vegetable crops.
Over 2016 and 2017, two capsicum crops and a corn crop were grown in trial plots at the Greater Sydney LLS Demonstration Farm Various RO compost blends, with and without augmentation, were applied to the trial beds prior to planting at differing rates of application; ‘control’ beds received conventional fertiliser or poultry manure.
All trial crops were monitored extensively, with soil solutes, soil water, soil biological activity, fruit numbers, fruit weight and total yield measured. Laboratory analyses of soil and plant matter, including chemical assays, were conducted to compare soil, plant matter and vegetables under the various treatments.
First trial capsicum crop: Initial planting post-compost needs extra N
A key finding from the first capsicum trials 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.”