Various barks, aged and composted to different degrees, are used in potting mixes. These differences have different effects on plant growth. It was observed that electrical conductivities (ECs) of the bark mixes that reduced plant growth were lower when compared to the ECs of the mixes that did not reduce growth, despite the same fertilization. This difference in EC diminished over time, differently for different barks. The decrease in EC was mainly due to a decrease in N. Apparently, nutrients were adsorbed or immobilized, which decreased their availability to the plants. This observation may be used to assess the suitability of a bark. The relative decrease in EC or N of similarly fertilized bark mix vs. no bark, peat mix (that does not reduce EC) may indicate the relative unsuitability of the bark, as related to nutrition. The amount of decrease in EC may also indicate the amount of additional fertilization to be provided to the bark mix during its use. The same method may also be applicable to other wood wastes, such as kenaf, sawdust, etc.