Cotton Waste Amended Substrates Impact Azalea and Juniper Growth

Alternative substrates that replace pine bark (PB) completely or partially are needed as PB supplies are running short in some areas of the country and prices are increasing.  Cotton stalks (CS) and cotton gin trash (CGT) are plentiful waste products of the cotton industry and have shown promise as substrate amendments.  Substrates constructed from whole pine trees (PTS) have also shown promise as substrates.  Cotton wastes have not been evaluated when mixed with pine tree based substrates.  Rhododendron obtusum ‘Sunglow’ and Juniperus conferta ‘Blue Pacific’ were potted on May 7^th, 2010 into 2.8 L black plastic containers filled with either PB or PT based substrates that had been amended (v/v) with cotton stalks composted without an N source (CS), cotton stalks composted with an N source (Daddy Pete’s Plant Pleaser, 0.5-0.5-0.5, Stony Point, NC) (CS+N), or cotton gin trash (CGT). The pine tree substrate base was produced from freshly harvested loblolly pine trees (Pinus taeda) that were delimbed, chipped, and ground in a hammer mill through a 5/8” screen.  A factorial treatment arrangement of these substrate bases (PB and PT) and amendments (CS, CS+N, and CGT) resulted in six substrates: 4:1 PB : CS (PBCS), 4:1 PB : CS+N (PBCS+N), 9:1PB : CGT (PBCGT), 1:1 PT : CS (PTCS), 1:1 PT : CS+N (PTCS+N), and 4:1 PT : CGT (PTCGT) arranged in a RCBD.  Additions of CS, CS+N, and CGT were made to PB or PT to achieve similar water holding capacities. An industry control of 100 % PB substrate was included in the experimental design for comparisons.  All substrates were amended with 3.0 lbs/yd³ (1.4 kg^.m³) dolomitic lime at mixing.  On May 17^th, PB-based substrates and the 100% PB control were topdressed with 2.6 g N and PT-based substrates were topdressed with 3.4 g N supplied by a polymer-coated, slow release fertilizer, 17-5-10 (17N-2.2P-0.83K) (Harrell’s, Sylacauga, AL). Shoot and root growth of azalea and juniper were greatest with PB amended with CGT and smallest with PTS amended with CGT.  Substrate bases of PB and PTS blended with composted CS either with or without an additional nitrogen (N) source produced similar shoot and root growth in both azalea and juniper.