Comparison of different bioassays for testing the phytotoxicity of growing media

Two international interlaboratory trials for the analysis of growing media and soil improvers were performed. Samples of white sphagnum peat, coir dust, composted green material, uncomposted bark, peat mixture and a peat and clay mixture were tested. Of the materials the pH, electrical conductivity, organic matter content and ash, dry matter content, moisture content, laboratory compacted bulk density, dry bulk density, air volume, water volume, shrinkage value and total pore space were determined using European Standards. The results give an idea about the excitability and reliability of the methods. Results were variable. As a reference a test performed for European standardization was used. Results less good as obtained in the former trial were found for the pH of bark. This appeared to be a difficult material for the pH determination. Also the repeatability and reproducibility results of the organic matter determination of the peat and clay mixture were bad. This can either be caused by the fact that laboratories do not exactly follow the procedure, or the method is more reliable for high organic matter contents and should perhaps be adapted for materials with a low organic matter content. The laboratory compacted bulk density results of the performed test are in most cases satisfactory. The determination of physical parameters were not always good. The repeatability and reproducibility of dry bulk density of composted green material were extremely high. Probably due to the constitution of this compost, dry bulk density is difficult to determine. The method is not suitable for this type of material. Results for bark and peat and clay mixture are also rather poor. For particle density it appeared that some laboratories used another formula as prescribed. Next trial and an amendment will have to be made about how total pore space has to be calculated from particle density. For water and air volume the repeatability and reproducibility results of one of the interlaboratory trials were not good when compared to the former trial. There is not a clear reason but perhaps laboratories need to gain more experience for these determinations. The results show that it is essential to have interlaboratory trials for the physical and chemical characterization of growing media and soil improvers. The procedures still need further improvement. The only way to find out for a laboratory if it is producing the right data, is to participate in these trials. If the standards are to be submitted for national or international regulations this is even more important.