Effects of hardwood sawdust in potting media containing biosolids compost on plant growth, fertilizer needs, and nitrogen leaching

Biosolids compost is used in media to grow potted plants. Nitrogen (N) in media leachate may contribute to nitrate (NO3‐N) contamination of surface or ground water. Addition of sawdust to potting media containing biosolids compost will increase the carbon (C) to nitrogen ratio and could prevent N leaching without adversely affecting plant growth. A control medium containing 0% sawdust (v/v), 30% perlite, 50% municipal biosolids compost, and 20% sand was modified to contain either 10, 20, or 30% (v/v) fresh hardwood sawdust. The sawdust replaced either 1/3, 2/3, or all of the perlite in the control medium. Slow release fertilizer, slow plus quick release fertilizer, or no fertilizer was added to each of the four media to determine how the sawdust affected fertilizer needs. Coreopsis (Coreopsis grandiflora L.) and Rudbeckia (Rudbeckia hirta L. ’Goldstrum') were grown in pots for five months. Leachate was tested for NO3‐N and ammonium N (NH4‐N). Increasing amounts of sawdust produced no differences in growth of Coreopsis and few differences in the growth of Rudbeckia. The addition of slow or slow plus quick release fertilizer had little effect on the growth of Coreopsis and a greater effect on the growth of Rudbeckia. Sawdust and fertilizer had no effect on the leaching of N. Nitrogen leached primarily as NH4‐N during the first four weeks of the experiment.