Substrate pH Affects Nutrient Availability in Fertilized Douglas Fir Bark Substrates

Abstract

An experiment was conducted to determine how pH and nutrient availability in douglas fir bark (DFB) substrates respond to lime and sulfur (S) rates. The treatment design was a two-by-nine factorial arrangement with two substrate types and nine pH-altering amendments. The two substrates were 100% DFB or 75 DFB:15 sphagnum peatmoss:10 pumice (by volume). Substrate pH-altering amendments included elemental S amended at either 0.6 or 2.4 kg·m−3; calcium carbonate amended at 0.6, 1.5, and 5.9 kg·m−3; calcium hydroxide amended at 4.4, 8.9, or 23.7 kg·m−3; and a nonamended control. All substrates were amended by incorporating 0.9 kg·m−3 Micromax micronutrients before potting and topdressing 8 g/pot of 14N–4.2P–11.6K Osmocote controlled-release fertilizer after potting. A group of controls was also maintained for each substrate that received no fertilizer amendment (no S, lime, Micromax, or Osmocote). Four containers of each treatment were randomly selected and harvested 4 and 8 weeks after potting. Amendment with S decreased pH with increasing rate, whereas both lime types increased pH with increasing rate. The two substrates in general responded similarly to S and lime amendments, although there were some significant effects and interactions caused by substrate type. Ammonium-N and NO3-N both decreased exponentially with increasing substrate pH, whereas water-extractable phosphorus decreased linearly with increasing pH. Water-extractable potassium, calcium, magnesium, and sodium responded quadratically to increasing pH by initially decreasing and then increasing. The micronutrients boron and iron decreased with increasing pH, whereas DTPA extractions of manganese, zinc, and copper initially increased and then decreased over the range of observed pH.