Rockwool

The OMRI Generic Materials List© says that rockwool is synthetic and thus prohibited for use in organic agriculture. After doing some research, it looks to me like it is made from a natural basaltic rock. Why does OMRI consider it synthetic?

By Lindsay Fernandez-Salvador

Rockwool, also called mineral, rock, or slag wool, is one of the most popular growing mediums in hydroponics operations due to its versatility and water retention characteristics. It is manufactured from melting basaltic rock and spinning the molten rock into fibers. After spinning, a binder is added to the fibers and they are compressed and cured into large slabs, which can be later cut up into convenient sizes and shapes. This process changes the functional properties of the basaltic rock to make the final substance an exceptional substrate for water and air exchange. This process is also the main reason that rockwool is considered synthetic.

NOP Rule §205.105(a) prohibits the use of “synthetic substances and ingredients, except as provided in NOP rule §205.601 or §205.603.” OMRI uses the definition of “synthetic” as it appears in §205.2, as well as in the March 2006, NOP-proposed decision tree to determine if a given substance is synthetic or nonsynthetic. When using the decision tree to determine if the manufacturing process renders rockwool synthetic, three questions apply:

1. Does the substance contain any synthetic substances not on the National List? Maybe; various binders are used in the manufacturing of rockwool, which could be either synthetic or nonsynthetic. The binder would need to be disclosed to know for sure.
2. Has the substance been transformed into a different substance via a chemical change? No; basaltic rock undergoes a physical change from solid rock to fibrous strands through heat and mechanical processes. 
3. Are any important functional properties of the substance altered by the manufacturing process? Yes; basaltic rock has little water and air retention capacity, while rockwool has exceptional water and air retention capacity.

Answering “Yes” to any of these three questions means the substance is synthetic. Since the answers to questions 1 and 3 are “Maybe” and “Yes,” rockwool is classified as synthetic and thus prohibited for use in organic agriculture.    

Revised and updated in September 2016 by OMRI Technical Director Johanna Mirenda. This article was originally published in the Winter 2010 edition of the OMRI Materials Review newsletter.