Developing quality standards for physical properties of mineral wool plugs

The KIWA certification guidelines for mineral wool products contain standards for slabs and blocks. Propagators would like to introduce quality standards for mineral wool plugs as well. Main concerns were effects of too dense plugs on plant growth, and handling problems with too fluffy or broken plugs. Other concerns were too deep or shallow seed holes and handling problems with ill centred seed (holes). A number of test productions of plugs with different densities and seed hole depths were used to design new methods. One part of the test productions was used for growth experiments at two propagators. The other part was used in the laboratory to develop methods to determine material density, shape and volume of the seed hole, pene¬trability and the number of broken plugs. It was found that plant growth decreases with a material density of more than 85 kg•m-3 dry matter and the optimum material density is between 70 and 85 kg•m-3 dry matter. Methods for measuring the material density, the seed hole depth and the centricity of the seed hole were successfully developed and accepted by propagators and producers. Methods for penetrability and a tentative breakage test were not reliable enough yet. Based on the new methods, applicable standards for plug performance were introduced as part of the KIWA certification.