Anaerobically Digested Dairy Fiber: A Renewable Substitute for Peat By John R. Lamont and George C. Elliott

Four plant trials were conducted at the University of Connecticut to evaluate this peat alternative. Here are the results.

Peat has played an important role in horticulture for decades, but concerns over rising costs and environmental effects caused by its harvest have spurred on efforts to find alternative media components. Similarly, environmental issues related to dairy manure management have led to initiatives to find better ways to handle dairy waste. Fortunately, both of these problems may be alleviated with one solution: the use of anaerobically digested dairy fiber (ADDF) as a replacement for peat in potting mixes.

Anaerobic digestion is a more environmentally friendly way to manage dairy manure in which dairy manure is “composted” under anaerobic (without oxygen) conditions to capture methane for use as a biofuel rather than let it escape into the atmosphere as a greenhouse gas. The solid and liquid fractions remaining after digestion are sepa- rated and the solid fraction is ADDF. The ability to market ADDF as a horticultural product could incentivize the adoption of anaerobic digesters on dairy farms, solve some problems associated with dairy manure management and generate additional revenue for dairy farmers.

Peat is also formed under natural anaerobic conditions in peat bogs. The similari- ties in water holding capacity, porosity and bulk density between peat and ADDF suggest that ADDF could be a suitable replacement for peat in potting mixes. Previous trials using ADDF as a peat replacement in media for floriculture and greenhouse crops have had promising results but issues with stunted root systems likely related to calcium deficiencies still
needed to be overcome.

Greenhouse growers could also benefit from using ADDF in potting media. Plants grown in ADDF could be branded as more “sustainable” or “local” than plants grown in peat from Canada. Currently, ADDF is a waste liability for dairy farmers so it would likely be less costly than peat, which is harvested specifically as a horticultural product.

Four plant growth trials were conducted at the University of Connecticut to evaluate ADDF as a potential replacement for peat in potting media. Garden chrysanthemum, cyclamen, poinsettia and a variety of bedding plants were tested in several ADDF-containing mixes.

Media Formulation and Analysis

Five soilless potting mixes (SPM) were formulated containing peat-ADDF-perlite, peat-ADDF- parboiled rice hulls (PBRH), coir-ADDF-perlite and coir-ADDF-PBRH each in a 2:2:1 ratio amended with 4g/L gypsum. The ADDF was obtained from Freund’s Farm in East Canaan, Connecticut. The control mix was composed of peat and perlite in a 4:1 ratio amended with 2.5 g/L dolomitic lime. All mixes had similar porosity and water holding capacity however, mixes con- taining PBRH had slightly greater porosity than other mixes. ADDF is slightly alkaline but when it is mixed with acidic peat, the two neutralize either other to arrive at a pH suitable for plant growth. Gypsum was used to supply calcium in ADDF mixes without raising the pH. Coir pH is close to neutral so both coir-ADDF mixes had pHs slightly above the ideal range for plant growth.

Bedding Plant Trials

Seedlings of pansy (Viola x wittrockiana ‘Karma White’), viola (Viola cornuta ‘Penny-Jump-Up’), petunia (Petunia x hybrida ‘Fuseable Vogue’) and rooted cuttings of geranium (Pelargonium x hortorum ‘Patriot Red’) were planted in pots containing the peat-ADDF-perlite, peat-ADDF-PBRH and control mixes described previously. Pansies and petunias were planted in Nu-Pots 4, geraniums were planted in Nu-Pots 3 and violas were planted in #3 CowPots.

Plants were overhead irrigated without fertilizer for 12 days and then sub irrigated with constant liquid feed and 100-ppm nitrogen delivered from PlantexTM 19-2-19 for the remainder of the experiment. Trials were conducted in a computer-controlled greenhouse covered with corrugated polycarbonate. Plants were harvested approximately eight weeks after planting and fresh weight, dry weight and tissue nutrient concentrations were measured.

Violas and petunias grown in both ADDF mixes were significantly larger and more mature at harvest than those grown in the control mix. In contrast, pansies grown in the control mix were larger and had deeper green foliage than those in the ADDF mixes. Fresh weights of geraniums were similar in all mixes, however, plants grown in the control mix were darker green in appearance than those grown in ADDF mixes. Plants grown in mixes with ADDF had higher tissue phosphorus content at harvest for all species (Figure 1).

Garden Chrysanthemums

Rooted cuttings of chrysanthemum (Chrysanthemum morifolium ‘Hankie Yellow’) were transplanted into 8-inch pans containing the peat-ADDF- perlite, peat-ADDF-PBRH, coir-ADDF-perlite, coir-ADDF-PBRH or the control mix described previously. One cutting was planted in each pan and grown, unpinched, outdoors with natural season photoperiod. Plants were overhead irrigated for one week and then drip irrigated with a constant liquid feed at the rate of 100-ppm nitrogen with Plant- exTM 19-2-19. The shoot fresh weight and volume were measured 12 weeks post-transplant.

At harvest, size and maturity were greatest for plants in the control mix (Figure 2). Plants in peat:ADDF:perlite or coir:ADDF:perlite were similar to controls, but plants in PBRH mixes were noticeably smaller and less developed.

Cyclamen (Cyclamen persicum) ‘Silver Heart White’ and ‘Winfall White’ seedlings were transplanted into 4-inch pots containing the same media described for the garden mum trial. Pots were placed in flood and drain trays and irrigated with 100-ppm nitrogen from PlantexTM 19-2-19. At the end of the trial, approximately nine weeks post-transplant, plants were evaluated qualitatively based on appearance and canopy volume. Peat-ADDF-perlite and peat-ADDF-PBRH produced plants equal in size to the controls while the coir containing mixes produced smaller plants (Figure 2).


Rooted cuttings of poinsettia (Euphorbia pulcherrima ‘Classic Red’) were transplanted into 6-inch jumbo azalea pots containing the peat- ADDF-perlite and control mixes previously described. Pots were placed in flood and drain trays and irrigated with 100-ppm nitrogen from PlantexTM 19-2-19. At commercial maturity 15 weeks post-transplant, plant growth was evaluated qualitatively based on appearance. Plants grown in the ADDF mix were significantly larger and denser than the controls (Figure 3).


ADDF can be used as a partial replacement for peat in SPM for a variety of floriculture crops, however, pH, fertilizer and irrigation manage- ment must be considered carefully when using an ADDF mix. When other alternative media com- ponents, like coir and PBRH were used in concert with ADDF, results were less favorable. Generally, the more a mix deviated from a standard peat- based mix, the more likely it was to have unfavor- able results. For example, the coir-ADDF-PBRH mix that was used in the chrysanthemum and cyclamen trials contained none of the same media components as the control, and did not yield any favorable results. The unfavorable results from the coir and PBRH mixes were likely due to the physical properties of these materials. PBRH had high porosity, which might reduce hydraulic con-ductivity, making the drip irrigation used in the chrysanthemum trial ineffective. Coir-containing mixes retained a high proportion of water, which may have led to reduced aeration.

In these trials, management decisions were based on the control mix. In many cases, irrigation management that was optimal for the control mix was less than optimal for other treatments. Better results may be produced with individualized management decisions based on differences in mixes. Additionally, ADDF is a significant source of phosphate and fertilization regimes should be adjusted accordingly.

As with other alternative media components more research is needed to establish the best ways to manage ADDF in media and to process ADDF into a consistent horticultural material. Apart from establishing best practices for ADDF, it is an acceptable replacement for peat media for a variety of floriculture crops.

John R. Lamont and George C. Elliott

John R. Lamont is a graduate student and George C. Elliott is associate professor in the department of plant science and landscape architecture at University of Connecticut. Lamont can be reached at