The pH, electrical conductivity, and primary macronutrient concentration of diary biofiber-containing root substrates over time in a greenhouse environment

Root substrates were formulated to contain 0, 30, 40, 50, 60, 70, or 80% dairy biofiber (DBF) and 20% perlite, bark or parboiled rice hulls (PBH) with the remainder being sphagnum peat (peat). The pH, EC and primary macroelement (N, P, K) concentrations were determined for the different substrates at 0, 4 and 8 weeks in a greenhouse environment. For all substrates, the pH and EC increased as the amount of DBF increased. The maximum pH and EC levels were 7.9 and 1.8 dS��m-1, respectively. The initial ammonium (NH 4 +) concentrations for the substrates amended with perlite, bark and PBH were 10.0, 10.2 and 6.0 mg.L-1, respectively. Over time, the NH4 + concentrations decreased in all substrates. For perlite and bark-containing substrates, nitrate (NO 3 -) concentrations increased as the concentration of DBF increased to 40% after which the NO3 - concentrations decreased. Over time, NO3 - concentrations increased for all DBF-containing substrates. The maximum NO3 - concentrations were 38, 35 and 24 mg.L-1 for perlite, bark and PBH-containing substrates, respectively. The phosphorus (P) and potassium (K) concentrations for the DBF-containing root substrates increased as the concentration of DBF increased. The maximum P concentrations occurred for the 80% DBF substrates and were 10, 14 and 9 mg.L-1 for perlite, bark and PBH-containing substrates, respectively. The maximum K concentrations occurred for the 80% DBF substrates and were 225, 200 and 190 mg.L-1 for perlite, bark and PBH-containing substrates, respectively. The EC and primary macronutrient concentrations were within commonly recommended ranges for most greenhouse crops. Because of the high pH of the DBF, the DBF would need to be mixed with unamended sphagnum peat at not more than 30% of the total substrate volume or amended with a compound such as iron sulfate or elemental sulfur to produce a substrate with an acceptable pH.