Effect of Substrate Storage Time On Marigold Growth In Pine Tree Substrate

Pine tree substrate (PTS) is a relatively new alternative to pine bark or peat based substrates.  The objective of this research was to determine the effect of PTS storage time with and without lime and peat amendments to PTS on plant growth, topics that have not been extensively researched. PTS was manufactured from freshly harvested loblolly pine trees (Pinus taeda L.), and amended with 25% peat (v/v, PTSP).  A peat/perlite (4:1 v/v, PL) treatment served as a control.  The PTS and PTSP substrates were amended with dolomitic limestone at rates of 0, 1, 2, 4, or 6 kg·m^-3; PL was amended with 6 kg·m^-3.  Substrates were placed in 0.08 m³ plastic bags and stored on shelves in an open shed in Blacksburg, Virginia.  Substrates were subsampled at 1, 42, 84, 168, 270, and 365 days.  At each subsampling, six 1-L containers of each treatment were potted with 2 week-old marigold (Tagetes erecta ‘Inca Gold’) seedlings.  Marigolds were fertilized at each watering with 250 mL of a 300 mg·L^-1 nitrogen 20-10-20 fertilizer solution (Jack’s Professional, Allentown, PA).  After three weeks, substrate solution pH and marigold dry weight (DW) were determined.  Marigold DW was lower in freshly manufactured (day 1) PTS and PTSP for all lime treatments than DW values for PL (all pH values ≥ 5.5).  At subsampling day 42, only the DW of the PTSP no lime treatment (pH 5) was less than PL (all other pH values ≥ 5.8).  Day 84 DW values were higher for PTS with 1, 2, or 4 kg·m^-3 lime treatments (pH values ≥ 6.3) than for PL (pH 5.7).  At day 168 there were no DW differences for any PTS or PTSP treatment compared to PL, however DW was low in all treatments presumably due to a seasonal effect. At day 270 only the marigold DW of unlimed PTS (pH 5.4) and PTSP (pH 4.0) treatments were less than PL.  By day 365 only DW of PTSP no lime (pH 4.3) was lower than PL (pH 5.9).  Although no clear trends were found in marigold DW relative to substrate solution pH, lime amendment was necessary for maximum marigold growth in stored PTS, and especially for PTS amended with peat.  DW values for limed PTS and limed PTSP treatments were only lower than PL treatments at day 1 indicating that by day 42 and thereafter some inhibitory influence of PTS was absent.