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Compost Standards

What standards must compost products meet to be used in organic agriculture?

By Taryn Kennedy

The USDA National Organic Program (NOP) regulations cover on-farm practices such as cultivation, crop rotation, and fertility management. These standards specify what substances can be applied to crops and soil. The NOP publishes guidances that further clarify the standards in relation to a specific substance or practice. NOP Guidance 5021 covers the production and use of compost and vermicompost. To summarize the standards, organic matter must be applied in a manner that prevents contamination of crops, soil and water. Organic matter can be added as uncomposted plant materials and animal manure, or composted plant and animal materials; each substance carries a restricted or unrestricted status depending on its nature and how it was produced. 

Compost for organic farms must be made from allowed feedstocks. These include nonsynthetic substances not prohibited at §205.602, including crop residues and other plant material, and/or synthetic materials approved for use as plant or soil amendments, including synthetic nutrients (Table 1). Agricultural feedstocks do not have to be certified organic. Compost must be managed to achieve temperatures of 131–170°F for a minimum of three days. Materials can be managed in windrows, static aerated piles or any other management system that achieves the minimum time and temperature requirements. Compost made from allowed feedstocks is unrestricted if it meets these standards, meaning there is no pre-harvest interval (i.e., time between application and crop harvest). 

To prevent pathogen contamination of crops, uncomposted animal manure carries a “days to harvest” application restriction. Products that contain this material must be applied before harvest within a certain time period: >90 days for crops that do not come into contact the soil surface or soil particles (tree fruit) or >120 days for crops that come into contact with the soil surface or soil particles (beets, lettuce or carrots). 

OMRI classifies compost products according to the categories and requirements outlined in the OMRI Generic Materials List® and summarized in Table 2. In OMRI’s review, suppliers must: 1) declare the composting method and the type and source of feedstocks; 2) submit compost logs that document daily temperature readings and frequency of turnings; 3) provide lab analyses that report certain heavy metal content (As, Cd and Pb) and pathogen levels (fecal coliform and salmonella); and 4) describe how foreign contaminants are removed. An OMRI Listed compost product is either: "Allowed" without any use restriction; “Allowed with Restrictions” requiring that the user follow the “days to harvest” application guidelines; or “Prohibited” due to the presence of prohibited substances (Table 2). OMRI Listed compost may also carry a caution based on heavy metal and pathogen test results. 

Table 1: Allowed and prohibited materials for use as Compost Feedstocks

InputTypeFeedstock ExamplesNOP Compliance
Food Waste and Green Waste GreenwasteFood residues, paper, food soiled paper (coffee filters, paper cups, plates), raw wood (sawdust, chips), stumps, prunings, bark, grass clippingsNO:
  • plastic utensils
  • bioplastics
  • Styrofoam
  • plastic mulches
  • materials recovered from mixed municipal solid waste

Food Processing Waste / AG ByproductsTomato puree, grape pomace, rice hulls, mushroom growing media; cannery wastes & cannery waste waterNO additives added after material becomes waste stream

Aquatic plantsSeaweed, kelp; aquatic plantsNO synthetics unless allowed

Processed WoodConstruction woodNO:
  • painted, stained or treated wood
  • plywood
  • pressboard

Unprocessed Slaughter By-productsSkulls, bones, blood, flesh, urine, eggshells, whole eggs, feathers, hair, sea creaturesNO additives added after material becomes waste stream
ManureAnimal ManuresAnimal manure with bedding, manure tea, urine NO:
  • biosolids/sewage sludge
  • manure ash
OTHERMineralNonsynthetic limestoneNO synthetic calcium carbonate (beet lime, PCC), quicklime, sheetrock/drywall

Allowed syntheticsNewspaper, Paper with some recycled content, unwaxed cardboard

Aquatic plant extracts, Elemental sulfur, Humic acids, Lignin sulfonate, MgSO4, Micronutrients, Liquid fish products, Vitamins
NO glossy, colored or waxed paper products

Additional Information:
  • Extracted with potassium or sodium hydroxide only
  • Lignin sulfonate allowed only as a chelating agent or dust suppressant
  • Vitamins B1, C and E

Formulated/Processed Feedstocks (non-wastestream)Fertilizers (bloodmeal, alfalfa meal), plant extracts, inoculants, odor controls (lime, ash)NO: 
  • kiln dust
  • lye
  • synthetic preservatives
  • prohibited processing aids

AshPlant/animal ash, biocharNO
  • ash produced from treated or painted wood
  • manure ash

Commercial/IndustrialAgricultural wastewater products, grease trap oilsNO nonagricultural industrial byproducts are prohibited unless explicitly allowed


Table 2:
Product requirements and usage allowance of Compost Categories in the OMRI Generic Materials Lis

CategoryRequirementsAllowance†
Compost – in-vessel or static aerated pile (plant & animal materials) 
  • C:N ratio‡ 
  • 131-170°F for 3 consecutive days
A
Compost – other (plant & animal materials)
  • 131-170°F for 3 consecutive days
A
Compost – plant materials
  • 131-170°F for 3 consecutive days
A
Compost – windrow (plant & animal materials)
  • C:N ratio‡
  • 131-170°F for 15 consecutive days
  • 5 turnings during that 15 day period
A
Compost-mushroom media waste 
  • Mushroom media waste feedstock must be derived from allowed materials
  • 131-170°F for 3 consecutive days
A
Worm Castings 
  • Allowed feedstocks
  • Aerobic conditions
  • 70-90% moisture level
A
Manure – processed
  • Minimum 150°F for one hour or 165°F
  • Maximum 12% Moisture
A
Anaerobic Digestate – plant materials 
  • Allowed feedstocks of non-animal manure origin 
A
Compost Tea 
  • Allowed feedstocks
R
Manure Tea 
  • Statement of additives required 
R
Manure – Raw, Uncomposted 
  • Statement of additives required
R
Anaerobic Digestate – from manure feedstock
  • Allowed feedstocks of animal manure origin 
R
Manure Ash
  • Burning of manure is prohibited
P

† A = Allowed; R = Allowed with Restrictions; P = Prohibited
‡ C:N ratio of feedstocks prior to composting (25:1 – 40:1)

Revised and updated in May 2019 by OMRI Technical Director Doug Currier. This article was originally published in the Winter 2014 edition of the OMRI Materials Review newsletter.