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Natural does not mean clean: what can ride in on organic inputs

by Seacliff Organics in Information

No. "Natural" and "organic" describe where an input came from, not what is in it. Biosolids, poultry litter, green waste compost, phosphate rock, seaweed, fish and shellfish have all been shown to carry contaminants, and that list includes inputs Seacliff sells. The only thing that settles it is a lab sheet for the batch in the bag.

What the law asks of a seller

Fertilisers need no registration in New Zealand. Since 7 March 2025, though, anyone selling a fertiliser made from organic matter must identify and manage its microbial and chemical contaminants, including heavy metals, pharmaceutical residues and plastics (MPI 2024). The notice sets no heavy metal limits and does not require results to be published. A draft 2023 review for the Ministry for the Environment found that no New Zealand compost or organic waste standard covers PFAS or microplastics (Wilson and Lewis 2023).

Biosolids and biosolid composts

Biosolids are treated sewage sludge. Under New Zealand's guidelines the top "Aa" grade may be sold at retail like any soil conditioner. Its limits include 1 mg/kg cadmium, 20 mg/kg arsenic and 300 mg/kg lead. It has none for PFAS (NZWWA 2003, Table 4.2).

Archived United States biosolids averaged 403 ng/g of PFOS, which is 403 µg/kg (Venkatesan and Halden 2013). Bagged biosolid garden products carried 9.0 to 199 µg/kg of perfluoroalkyl acids, against 0.1 to 1.1 µg/kg for other organic composts (Kim Lazcano et al. 2020). At two New Zealand treatment plants PFAS turned up in every wastewater sample (Lenka et al. 2022). BioGro prohibits biosolids outright, including in compost (BioGro 2026a).

Ask: is any of this sewage derived, and is there a PFAS result?

Poultry litter and manures

Forty poultry litter samples from the south-eastern United States averaged 16 mg/kg arsenic, 479 mg/kg copper and 373 mg/kg zinc, from trace elements added to feed (Jackson et al. 2003). Arsenic based feed drugs kept United States approvals until 2014, and the last was due to go by the end of 2015 (FDA 2022). We could not confirm whether they were ever used in New Zealand.

A review reported E. coli counts above land application limits, antibiotic residues and resistance genes, and no standards for most of these in litter (Kyakuwaire et al. 2019). BioGro requires raw manure to be hot composted and bars factory farmed manure from certified compost (BioGro 2026a; BioGro 2023).

Ask: what were the birds fed and dosed with, was it hot composted, and is there a metals and pathogen test?

Feather meal

All 12 feather meal samples in one study contained antimicrobial drugs, two to ten each (Love et al. 2012). All 12 in a companion study contained arsenic, 44 to 4,100 µg/kg (Nachman et al. 2012). Seacliff does not sell feather meal. The detail is in What is in feather meal?

Blood and bone and rendered by-products

One American living soil retailer warns that blood meal may carry drugs and hormones and bone meal BSE, or mad cow disease (Silva 2013). That is a sales claim. The residue evidence we could load is for feather meal, not blood or bone. New Zealand has never had a case of BSE, has been rated negligible risk since 2007 and bans feeding ruminant protein to ruminants (MPI 2026). Fertiliser that may contain ruminant protein must be labelled as not for feeding to ruminants (MPI no date a).

That settles BSE here, not pathogens, which the 2025 notice puts on the seller (MPI 2024). Seacliff does not sell blood meal.

Ask: is it New Zealand sourced and rendered, and is there a pathogen test?

Municipal green waste compost

Clopyralid, a broadleaf herbicide used on lawns, turf and farms, persists in sprayed clippings and in the manure of animals that ate them. Compost made from either can stunt or kill peas, beans, tomatoes, potatoes and lettuce, so the rules restrict clopyralid to workplaces and bar treated plants from compost and green waste (EPA no date). WasteMINZ puts the damaging level at 1 to 3 parts per billion and says testing is not fail safe at those levels (Stanway 2024). Aminopyralid and picloram also pass through livestock into manure and stay active after composting (Davis et al. 2020).

German biowaste composts and digestates also held 14 to 895 plastic particles larger than 1 mm per kilogram of dry weight (Weithmann et al. 2018).

The compost standard, NZS 4454:2005, is sold, not published, so we could not load it. The 2023 review lists its cadmium limit as 3 mg/kg (Wilson and Lewis 2023). BioGro allows compost 1 mg/kg cadmium, 20 mg/kg arsenic and 250 mg/kg lead, with tighter cadmium and lead limits if household waste is an ingredient (BioGro 2026b).

Ask: what are the feedstocks, and is it tested for clopyralid? Then sow peas or beans in a 1:1 mix of compost and potting mix and watch for two to three weeks (Davis et al. 2020).

Rock phosphate and phosphate fertilisers

Cadmium is a natural impurity in phosphate rock and cannot be removed economically (Rys 2020). The industry's voluntary limit has been 280 mg of cadmium per kg of phosphorus since 1997, and the average is now about 180 (Rys 2020). The voluntary Tiered Fertiliser Management System starts restricting phosphate at 0.6 mg of cadmium per kg of soil and allows no further build-up at 1.8 mg/kg without a site investigation (Sneath 2020). Wheat and spinach have occasionally exceeded food standards on soils under 0.5 mg/kg (Rys 2020).

Soft rock phosphate is still phosphate rock, and "natural" does not exempt it. Seacliff sells BioPhos and two soft rock phosphates. [LAB RESULT NEEDED: BioPhos, Colloidal Soft Rock Phosphate, MSR: phosphorus %, cadmium (mg/kg and per kg phosphorus), arsenic, lead, mercury]

Ask: how many mg of cadmium per kg of phosphorus?

Seaweed and kelp

Seaweed can hold up to 100 mg/kg of arsenic, mostly as arsenosugars, which are relatively non-toxic to people but break down in soil to inorganic arsenic. Commercial seaweed fertilisers in the same study held 10 to 50 mg/kg (Castlehouse et al. 2003). The top of that range is above BioGro's 20 mg/kg arsenic limit for compost (BioGro 2026b). Seaweed is also rich in iodine, 16 to 8,165 µg/g across 12 edible species (Teas et al. 2004), and it grows in salt water.

Seacliff sells Acadian Kelp Meal and Tri-Kelp. [LAB RESULT NEEDED: Acadian Kelp Meal, Tri-Kelp: total and inorganic arsenic, cadmium, lead, mercury, iodine, sodium]

Ask: for those figures on your batch.

Fish and shellfish meals

MPI notes naturally high cadmium in some shellfish and crustaceans (MPI no date b). One Bluff oyster can more than double an adult's normal daily cadmium intake (McKenzie-Parnell et al. 1988). That finding is for the flesh, not the shell, so crushed oyster shell needs a test, not an assumption. European feed law caps fish derived feed materials at 25 mg/kg total arsenic, 0.5 mg/kg mercury and 2 mg/kg cadmium (EU 2002).

Seacliff sells four inputs in this category. [LAB RESULT NEEDED: Mussel Meal, Fish Meal, Fish Bone Meal, Crushed Bluff Oyster Shells: cadmium, arsenic, lead, mercury, sodium, pathogen screen]

Ask: for those figures on your batch.

Wood ash and treated timber

A Christchurch compost study measured about 133,000 to 179,000 mg/kg of arsenic in ash from H3 to H5 treated timber, and about 96 mg/kg even in ash from untreated timber. Compost arsenic rose in the cold months, when log burner ash was going into green bins (Safa et al. 2020).

Ask yourself: what exactly was burned? If you cannot say, keep the ash out of the garden.

Bagged blends with undeclared ingredients

The biosolid products in the PFAS study were ordinary commercial bags (Kim Lazcano et al. 2020), and New Zealand's guidelines record biosolids sold through garden centres (NZWWA 2003). The label must state nutrient content and the form each nutrient is in (MPI 2024); we found nothing in the notice requiring every raw material to be listed.

Ask: every raw material by name and origin. If the seller will not say, do not buy.

Worked example: concentration is not dose

Cadmium from phosphate. Take a phosphate on the 280 mg/kg P limit at 45 kg of phosphorus per hectare a year, the top superphosphate rate the tiered system allows (Sneath 2020).

  • 45,000 g P ÷ 10,000 m² = 4.5 g P per square metre.
  • Cadmium applied: 0.0045 kg P × 280 mg/kg = 1.26 mg per square metre a year.
  • Soil in the top 150 mm: 1 m² × 0.15 m = 0.15 m³, or 150 kg at an assumed bulk density of 1.0 t/m³.
  • Rise in soil cadmium: 1.26 mg ÷ 150 kg = 0.0084 mg/kg a year.
  • Years from 0.6 to 1.8 mg/kg with no losses: 1.2 ÷ 0.0084 = 143.

Arsenic from kelp. Take a kelp meal at 50 mg/kg, the top of the published range, at 50 g per square metre, the heaviest kelp rate in our fertiliser calculator.

  • Arsenic applied: 0.05 kg × 50 mg/kg = 2.5 mg per square metre.
  • Rise in the same 150 kg of soil: 2.5 mg ÷ 150 kg = 0.017 mg/kg per application, against BioGro's soil limit of 20 mg/kg (BioGro 2026b).

Both are small per application and both add up. Use your own lab figure and rate.

If you grow medical cannabis under licence

New Zealand's minimum quality standard sets limits of 3.0 ppm arsenic, 0.5 ppm cadmium, 5.0 ppm lead and 0.5 ppm mercury, and requires tests for microbial contamination, pesticides, aflatoxins and ochratoxin A (New Zealand Government 2019, regulation 7). Clopyralid is not among the 16 pesticides listed for New Zealand grown product, and there is no PFAS test. A pass on the product does not prove the inputs were clean. Test inputs and soil first.

What to do

We are publishing the lab reports we hold on the Lab tests page. Until a result for a product is there, treat our kelp, fish, shellfish and phosphate inputs like anyone else's.

  1. Get the full list of raw materials and where each came from.
  2. Get a batch lab sheet: arsenic, cadmium, lead and mercury as a minimum, plus the extras named above.
  3. Work out the dose at your rate, not just the concentration.
  4. Run a pea or bean test on any compost or manure.
  5. Keep ash from unknown or treated wood out of the garden.
  6. Test your soil first. If it already has enough phosphorus, the cleanest phosphate is the one you do not apply. See where to test your soil.

References

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