Spent litter and composted farm by-products can also contain other contaminants such as metals (copper, zinc, cadmium etc), which may limit the application rate. The contaminant levels in spent litter are generally well below the maximum acceptable limits set by various state guidelines (see Table 6), except for copper and zinc. But because many Australian agricultural soils are deficient in copper and zinc, this is unlikely to be a concern. Therefore, heavy metals will exist in the soil at safe levels at application rates that meet plant phosphorus requirements (Wiedemann et al., 2015). Soil testing is recommended if regular applications are made to the same paddocks to ensure there is no build-up of heavy metals.
The levels of heavy metals in spent litter are well below the maximum acceptable limits set by various state guidelines, except for copper and zinc. As many agricultural soils are deficient in copper and zinc, this is unlikely to be a concern. Therefore, heavy metals will exist in the soil at safe levels at application rates that meet plant phosphorus requirements.
The Fresh Produce Safety Centre (2019) highlighted cadmium as the heavy metal of most concern to fresh horticultural produce, particularly when used on root and tuber vegetables (e.g. carrots, beetroot, some potato varieties), as well as leafy vegetables and fresh herbs (e.g. spinach, silverbeet). Its uptake can also be increased in soils that are very sandy, saline or irrigated with salty water, acidic, low in zinc, or lacking organic matter. However, Craddock & Hollitt (2010) found levels of cadmium in a variety of spent litter samples well below the Class 1 (unrestricted use) category documented in various guidelines, so this is unlikely to pose a risk when utilising spent litter.
Cadmium levels are the most concern to fresh horticultural produce; however, the levels found in spent litter are well below the Class 1 (unrestricted use) category documented in state guidelines.
Table 6. National and state limits for heavy metals in compost and sewerage biosolids
| Concentration (mg/kg db) | NRMMC (2004)1 | AS 4454 (2012)2 | NSW EPA (1997)3 | EPA VIC (2004)4 | EPA SA (2017)5 | DPIWE (1999)6 | DEC WA (2012)7 | QLD (2019)8 | Craddock & Hollitt (2010)9 |
|---|---|---|---|---|---|---|---|---|---|
| Arsenic (As) | 20 (60) | 20 | 20 | 20 (60) | – | 20 (20) | – | 20 | 1.8–5.4 |
| Boron (B) | – | 100 | – | – | – | – | – | – | – |
| Cadmium (Cd) | 1 (20) | 1 | 3 (20) | 1 (10) | 1 (11) | 3 (20) | 1 (20) | 3 (20) | 0.02–0.06 |
| Chromium Total (Cr) | 100–400 (500–3000) | 100 | 100 (500) | 400 (3000) | – | 100 (500) | – | 100 (500) | 1.7–3.5 |
| Chromium VI (Cr VI) | – | – | – | – | 1 (1) | – | 1 (1) | – | – |
| Copper (Cu) | 100–200 (2500) | 150 | 100 (2000) | 100 (2000) | 100 (750) | 100 (1000) | 100 (2500) | 150 (2000) | 124–147 |
| Lead (Pb) | 150–300 (420) | 150 | 150 (420) | 300 (500) | – | 150 (420) | – | 150 (420) | 0.06–1.4 |
| Mercury (Hg) | 1 (15) | 1 | 1 (15) | 1 (5) | – | 1 (15) | – | 1 (15) | – |
| Nickel (Ni) | 60 (270) | 60 | 60 (270) | 60 (270) | – | 60 (270) | – | 60 (270) | 2.1–7.7 |
| Selenium (Se) | 3 (50) | 5 | 5 (50) | 3 (50) | – | – | – | 5 (50) | – |
| Zinc (Zn) | 200–250 (2500) | 300 | 200 (2500) | 200 (2500) | 200 (1400) | – | 200 (2500) | 300 (2500) | 360–538 |
Note: Limits presented are for Class 1 grade (unrestricted use) and, in brackets, Class 2 grade (agricultural use, forestry, land reuse). Check each state’s guidelines for definitions.
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