The Groundwater Protection Rule restricts application of nitrogen fertilizer in the fall and on frozen soils in vulnerable groundwater areas. The following information provides clarification on situations where compost is considered a nitrogen fertilizer.

Definition of compost

Compost and composting are defined in Minnesota Statute 18C.005. "Compost" is a biologically stable material derived from the composting process. "Composting" is the biological decomposition of organic matter. It is accomplished by mixing and piling in such a way as to promote aerobic or anaerobic decay or both. The process inhibits pathogens, viable weed seeds, and odors.

Definition of nitrogen fertilizer

Nitrogen fertilizer is defined in Minnesota Rule 1573. "Nitrogen fertilizer" means a substance containing nitrogen that is used for its plant nutrient content, is designed for use or claimed to have value in promoting plant growth, and requires a guaranteed analysis under Minnesota Statutes, section 18C.215. Nitrogen fertilizer does not include animal and vegetable manures that are not manipulated, or marl, lime, limestone, biosolids, industrial by-product, industrial wastewater, irrigation water, or other products exempted by the commissioner. Chemicals or substances added to manure during storage to reduce odor or gas emissions or to prevent foaming, or added to manure to extend the time the nitrogen component of manure remains in the soil, are not considered a manipulation of manure.

Registration required

Compost sold in Minnesota must be registered or licensed through the MDA. If compost is given away for free, it is exempt from the licensing or registration requirement. The registration or licensing can be either as a fertilizer, specialty fertilizer or a soil amendment.

If the labeling (including but not limited to product label, promotional pamphlets and brochures, and product websites) primarily emphasizes the fertilizer value of the compost, the MDA considers it a fertilizer or specialty fertilizer. If the labeling primarily emphasizes the soil amending properties of the compost, the MDA considers it a soil/plant amendment.

If claims of the compost’s nitrogen fertilizer value appear on the labeling, the nitrogen content must be guaranteed in the guaranteed analysis statement.

When is compost considered a nitrogen fertilizer?

For the purpose of the Groundwater Protection Rule, the determination of whether compost is a nitrogen fertilizer is as follows:

  1. If the compost is registered as a soil/plant amendment; it is not a nitrogen fertilizer.
  2. If the compost is licensed as a fertilizer or registered as a specialty fertilizer, but has no guarantee for nitrogen on the label; it is not a nitrogen fertilizer.
  3. If the compost is licensed as a fertilizer or registered as a specialty fertilizer, and has a guarantee for nitrogen on the label; it is a nitrogen fertilizer.

If the compost is not considered a nitrogen fertilizer (bullet 1 or 2 above), it is not subject to fall application restrictions.

If the compost is considered a nitrogen fertilizer (bullet 3 above), it is subject to fall application restrictions. If one or more micronutrients are guaranteed, it is subject to the exception in MN Rule 1573.0030 Subp. 3C which states that a responsible party may make a fall application in a vulnerable groundwater area in the following situation (excerpt):

(1) when applying ammoniated phosphate or micronutrient formulations containing nitrogen, so long as the applied nitrogen rate does not exceed an average of 40 pounds per acre in a field. Fields that have had a soil analysis completed by a certified lab and determined to have low to very low phosphorus levels according to the Fertilizer Guidelines for Agronomic Crops in Minnesota are not subject to the 40 pounds per acre total nitrogen rate.