Evaluation of potting media for commercial nursery production of container-grown plants

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Abstract
Some physical and chemical characteristics of three New Zealand commercial nursery potting media (peat-sand, peat-sand-sawdust, and peat-sand-soil) and their constituents were determined. Two kinds of peat were used, one weakly decomposed (Mataura) and the other strongly humified (Dipton). Mataura peat had lower bulk density, cation exchange capacity CEC), soluble salts, total carbon (C) and nitrogen (N), and lower carbon/nitrogen (C/N) ratio but higher porosity, air or container capacity, water-holding capacity, and pH than Dipton peat. The media reflected these properties except water-holding capacity: at low moisture suction the Dipton peat media held more water than the Mataura ones. This difference was related to the different physical structures of the peats and the effects of amendments on these structures. Sawdust had a high total pore space, air space, percent pore space, and air capacity. However, the easily available water (EAW) and water buffering capacity (WBC) were low, indicating that sawdust does not retain much water for plant use. Added sand reduced total pore space, air capacity, EAW, and WBC thus reducing aeration and water available for plant use. Soil had low values for air capacity and EAW although its WBC was high because of its micropore structure. However, in peat-sand-soil media soil effects were not expressed, as these media had air and water characteristics similar to those for the peat-sand media. Peat and sawdust had a low pH, sand a high pH, and soil an intermediate pH. The CEC for sand and sawdust was extremely low. As expected, soluble salt levels of all media and their constituents were low and well below critical levels. The C/N ratio of sawdust was extremely high and this was reflected in high C/N values in peat-sand-sawdust media.