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

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

† 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.