Native Plant Production in Potting Media Amended with Anaerobic Digested Cattle Biosolids

Anaerobically digested cattle biosolids are a by-product of cattle waste processing for pollution control and biogas production, yet this material has potential as an economical component of potting mixes used for sustainable nursery container production. As an animal waste, this material could have valuable fertilizer properties that could reduce the amount of supplemental fertilizer applied to nursery crops. In this study, anaerobically digested cattle biosolids were added to a bark and sand mixture in 0, 15, 30, 45, or 60% concentrations (by volume) to make five potting mixes. Four species of native perennials—Mimulus guttatus (golden/seep monkey flower), Holodiscus discolor (oceanspray), Philadelphus lewisii (syringa/mockorange), and Penstemon strictus (Rocky Mt. penstemon)—were planted into the various mixes and grown for four months in 1-gallon containers. Initial physical and chemical properties of the media were analyzed including aeration, water-holding capacity, total porosity, bulk density, C:N ratio, electrical conductivity (EC), pH, cation exchange capacity (CEC), and mineral concentrations including nitrate-N, ammonium-N, available P, K, B, S, and extractable K, Ca, Mg, Na, and Cl. Media pH and EC were analyzed monthly by the pour-through extraction method, and all plant heights and penstemon widths were measured every three weeks. At the end of the experiment, plant leaves from 60 plants per species were collected for foliar mineral analysis, and the plant shoots were harvested for biomass to compare growth among the mixes. All mixes containing biosolids had good air and water-holding capacity (30% and 50%, respectively) and low bulk density (< 0.4 g cm). Anaerobically digested biosolids neutralized the otherwise acid pH of the bark sand mixture from 4.8 to 5.9 for the 15% biosolids medium and 7.2 for the 60% medium. The C:N ratio decreased with increasing biosolids concentration from 103 to 22. Electrical conductivity was high (5.0 30% biosolids and some oceanspray plant mortality in the 60% biosolids mix. Some of the salt and chloride problem, however, was reduced by leaching the media, and eventually EC was reduced to Native Plants Production in Potting Media Amended with Anaerobic Digested Cattle Biosolids 9 December 22, 2008 below desired levels. An overall increase in pH overtime, probably the result of breakdown of the organic matter from the biosolids, could potentially have limited nutrient availability over time, reducing the effectiveness of the biosolids as a fertilizer source, but the presence of controlled-release fertilizer somewhat controlled rising pH by acidification (by release of ammonium or urea). Supplemental fertilization most likely offset the any early nutrient imbalances from high salt levels and late season nutrient deficiencies, for the most part, since foliar mineral analysis failed to reveal any serious deficiencies. Further research is needed to determine the best overall percentage of biosolids required for optimum plant growth for domestic species and larger trees and shrubs, particularly salt sensitive species. Further study is also recommended to determine the best methods of mitigating excessive sodium and chloride levels in order to maximize the potential of anaerobically digested cattle biosolids as a container media amendment. Overall, this study demonstrated that anaerobically digested cattle biosolids could be substituted for at least 30% potting mix volume for producing selected species of native plants. Native Plants Production in Potting Media Amended with Anaerobic Digested Cattle Biosolids 10 December 22, 2008 Table 1. Initial physical properties of media amended with various percentages (by volume) of anaerobically digested cattle biosolids. Biosolids Air Capacity Water-holding Capacity Total Porosity Bulk Density (%) (%) (g·cm ̄3) 0 30.9 ab* 45.7 d 76.6 b 0.356 a 15 32.5 a 46.6 cd 79.1 ab 0.335 bc 30 29.5 b 48.2 bc 77.8 ab 0.344 b 45 30.3 ab 49.8 ab 79.2 ab 0.329 c 60 30.8 ab 50.7 a 81.5 a 0.330 c *Means followed by different letters within a column are significantly different at P<0.05 protected Fisher's LSD test (n = 4). Table 2. Initial chemical properties of potting mixes amended with various percentages (by volume) of anaerobically digested cattle biosolids. Biosolids pH EC CEC C:N (%) (dS·m ̄1) (cmol(+) kg ̄1) Ratio 0 4.8 e* 1.6 e 23 103 a 15 5.9 d 3.6 d 26 47 b 30 6.4 c 5.0 c 26 37 c 45 6.9 b 7.0 b 23 28 d 60 7.2 a 7.9 a 25 22 d *Means followed by different letters within a column are significantly different at P<0.05 protected Fisher's LSD test (n=4). Table 3. Initial mineral concentrations in potting mixes amended with various percentages (by volume) of anaerobically digested cattle biosolids. Available Available Available Nitrate Ammonium Sulfate Biosolids K P B N N S (%) (ppm) 0 268 e* 50 e 3.7 d 1.4 e 11.0 c 315 d 15 1175 d 245 d 5.7 c 107.5 d 22.3 b 380 c 3