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Title: Substrate pH Affects Nutrient Availability in Fertilized Douglas Fir Bark Substrates

item Altland, James

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/8/2008
Publication Date: 12/1/2008
Citation: Altland, J.E., Buamscha, G., Horneck, D. 2008. Substrate pH Affects Nutrient Availability in Fertilized Douglas Fir Bark Substrates. HortScience. 43:2171-2178.

Interpretive Summary: Little is known about the chemistry of Douglas fir bark used in nursery production. These experiments were conducted to determine which nutrients are available from Douglas fir bark, and in what quantity, over a wide range of substrate pH. The information presented would be valuable for extension agents, fertilizer consultants, and knowledgeable nursery growers as they formulate fertilizer packages and make lime/fertilizer recommendations for container nursery crops. Like many farmers, nursery growers routinely adjust pH to levels between 6 and 7. Our research shows that pH between 4 and 5 would provide a greater amount of nutrients for crop growth. Most importantly, it shows greater amounts of nitrogen and phosphorus are available at low substrate pH. Lower pH should not only improve crop growth with reduced fertilizer inputs, but also reduce the amount of nitrogen and phosphorus leaching from containers and potentially contaminating surface and ground waters. Nitrogen and phosphorus are often identified as the most serious pollutants of surface and ground waters.

Technical Abstract: An experiment was conducted to determine how pH and nutrient availability in Douglas fir bark 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 peat moss: 10 pumice (by volume) (hereafter referred to as BPP). 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 non-amended control. All substrates were amended by incorporating 0.9 kg.m-3 Micromax micronutrients prior to potting, and topdressing 8 g.pot-1 of 14N-4.2P-11.6K Osmocote controlled release fertilzer after potting. A group of controls were 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 (WAP). Amendment with S decreased pH with increasing rate, while 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, while water-extractable P decreased linearly with increasing pH. Water-extractable K, Ca, Mg, and Na responded quadratically to increasing pH by initially decreasing and then increasing. The micronutrients B and Fe decreased with increasing pH, while, DTPA extractions of Mn, Zn, and Cu initially increased and then decreased over the range of observed pH.