Initial chemical and physical properties of potting mixes amended with anaerobically digested cattle biosolids

The objective of this study was to determine the effects of anaerobically digested cattle biosolids on initial physical and chemical characteristics of potting mixes used for container production of landscape plants. Five potting mixes containing 0, 15, 30, 45 or 60% composted biosolids (by volume), 10% sand, and 90, 75, 60, 45 and 30% aged pine bark, respectively, were used in this study. Initial air filled porosity, container capacity, total porosity, and bulk density of the media were analyzed. Initial chemical properties, including pH, electrical conductivity (EC), cation exchange capacity, C:N ratio, available K, P, and B, nitrate-N, ammonium-N, and sulfate, as well as extractable Ca, Mg, K, Na, and Cl-, were determined for the mixes. All mixes containing biosolids had good air filled capacity and container capacity (means of 30 and 49%, respectively) and low bulk density (<0.35 g cm-3). Anaerobically digested biosolids raised the pH of the bark-sand (control) mix from 4.05 to 7.2 for the 60% medium. The C:N ratio decreased with increasing biosolids concentration from 103 to 22. Electrical conductivity was high (>or=5.0 dS m-1) in media containing >or=30% composted biosolids. Mineral concentrations in the mixes increased with increasing percentages of biosolids. Concentrations of some minerals (e.g., ammonium-N) in the 60% biosolids mix were double the concentrations of those in the control mix, whereas available K and P concentrations in the 60% mix increased at least 15- and 17-fold, respectively. Extractable Na and Cl- in the 60% biosolids mix were at least 16- and 117-fold higher, respectively, than those levels in the control mix. Use of anaerobically digested cattle biosolids in potting mixes resulted in media with suitable physical properties, but initial mineral levels in mixes that contained 30% or more biosolids could cause problems for plants due to the high concentrations of K, P, Na and Cl-.