Urea Hydrolysis In Pine Tree Substrate

Pine tree substrate (PTS) is a relatively new container substrate that is an alternative to peat (P) and pine bark (PB).  The unique physical and chemical characteristics of PTS compared to P and PB warrant research to understand how to manage the fertilization of PTS- grown crops.  The relatively high carbon to nitrogen ratio (C:N) ratio of PTS requires the application of more N fertilizer than used with P and PB since a significant portion of applied N is lost to immobilization.  If PTS were pre-charged with urea, the least expensive fertilizer N source, then growers could inexpensively fertilize to compensate for immobilization.  The rate of urea hydrolysis (conversion to ammonium) is unknown in PTS.  Thus, the objective of this experiment was to determine the urea hydrolysis rate in PTS over time.  PTS was manufactured from freshly harvested loblolly pine trees (Pinus taeda L.), chipped and hammer-milled to a 15.8 mm particle size.   PTS was tested for urea hydrolysis at 0, 4, 24, and 48 h.  Urea (46% N; 1mg N/g dry weight) was added in solution (1000 mg N∙L^-1) to 1 g dry weight   substrate.  At each sampling time, substrate was saturated with 30 mL 2M KCl-phenylmercuric acetate solution which inhibited urease activity.  Substrate samples were shaken for 1 h, and solutions filtered; an aliquot of each sample (n=4) was set aside for urea analysis.  Urea analysis was conducted using the colorimetric Quantichrom® Urea Assay Kit and a UV-vis spectrophotometer.  At 0, 4, 24, and 48 h, 0, 7, 48, and 72% of the initial amount of urea was hydrolyzed, respectively.  The remaining amount of urea over time was described by the linear relationship y = -1.5329x + 97.528; R² = 0.9157.  Using this regression equation, all the urea is predicted to be hydrolyzed in 64 h. Hence, urea appears to be a suitable and economical way to pre-charge PTS to satisfy immobilization so that growers can fertilize PTS as other conventional substrates.