Urease activity and pH changes in a peat moss-based potting medium as influenced by lime source and lime rate following the addition of urea

Reagent grade CaCO3 or Ca(OH)2 was mixed at various rates into a medium consisting of 50% Canadian Sphagnum peat moss, 35% coarse perlite, and 15% sterile sand by volume. After 2 weeks incubation at 20C, medium pH levels were 4.32, 5.00, 6.55, and 7.22 for CaCO3‐amended media, 3.98, 5.13, 6.30 and 6.75 for Ca(OH)2‐amended media, and 3.42 for an unlimed control. Thirty ml of a fertilizer solution (P, K, and trace elements) with or without 500 μg/ml urea‐N was applied to 125cc samples from each lime treatment, then incubated at 20C/95% RH for 20 days. Medium pH was monitored on days 0, 1, 3, 5, 10, and 20. Medium pH increased for all treatments and remained at an elevated level for the entire 20‐day period except for the highest rate of CaCO3. Medium pH increases were dependent upon lime rate. Greater pH increases were found for media amended with Ca(OH)2. Urease activity was assayed on day 0. Urease activity was affected by lime rate with the greatest activity found at a medium pH of 6.30 to 6.55.