Anaerobically digested dairy as a renewable substitution for peat in media for nursery production

Anaerobically digested dairy as a renewable substitution for peat in media for nursery production

Summary

Anaerobically digested dairy fiber (ADDF), a byproduct of methane production from dairy manure, shows promise as a locally sourced alternative to peat for growers in the Northeast.  ADDF has been shown to be a viable alternative to peat in a variety of greenhouse crop trials but there is currently no information published on using ADDF as a component of SPM for nursery crops.  Using ADDF in nursery media has the potential to benefit growers (by providing a locally sourced, inexpensive, sustainable alternative to peat), dairy farmers (by turning ADDF into a valuable horticultural product and providing a lucrative waste management solution) and the environment (by incentivizing dairy farmers to adopt anaerobic digesters and recycling nutrients in manure into horticultural crops)

Long and short term plant growth trials were conducted to evaluate the performance of a variety of woody and herbacious nursery crops in different stages of production grown in ADDF- or peat-containing media. Various parameters, such as plant size, stem caliper and number of shoots were measured regularly to evaluate plant growth over time.  Leaf tissue samples were analyzed for nutrient concentration to estimate nutrient uptake.  Water holding capacity, bulk density and shrinkage were measured throughout trials compare the durability of the mixes over time.  Aqueous extracts were used to regularly to monitor pH, EC and nutrient status of potting media in situ.

Plants grown in the ADDF-containing mixes grew to be of equal or better size and quality to those grown in the peat-containing mixes.  Both mixes had similar changes to physical proporties over time.  Both mixes underwent the same amount of shrinkage during two growing seasons but the ADDF mix experienced more shrinkage under overhead irrigation than the peat mix.  Significant amounts of phosphate and nitrate leach from ADDF mixes.  Nitrate and phosphate from ADDF could contribute to plant nutrition but could also be a an environmental issue.

This project began during the summer of 2014 and concluded in December 2015. This study was conducted in the greenhouses, research fields and laboratories located at the University of Connecticut.

Objectives/Performance Targets

1. To evaluate ADDF as a substitute peat in soilless potting media mixes.
2. To evaluate nutrient availability in ADDF over time.
3. To evaluate physical changes to ADDF over time

Accomplishments/Milestones

Media Analysis
Physical and chemical properties of two nursery mixes were evaluated.  The two mixes were composed of either bark-peat-sand (peat mix) or bark-ADDF-sand (ADDF mix) in a ratio of 4:2:1.  The peat mix was amended with 2.5g L^-1 and the ADDF mix was amended with 4g L^-1 gypsum. The physical properties of This mixes used in this trial were evaluated using the techniques described by Elliott (1992b): Media put in pots with known dimensions (truncated cone with height (H) of 120mm, bottom radius (R_b) of 30mm and top radius (R_t) of 40cm).  Pots were weighed at the start of the trial and were subsequently irrigated, drained and weighed several times a week until the irrigated mass reached equilibrium.  Equilibrated irrigated mass was used to derive effective water holding capacity (EWHC) using the equation (net weight after irrigation – initial dry weight).  Pots were then saturated with subirrigation for 24 hours, then weighed before and after draining.  Saturated and drained masses were used to derive container capacity (CCAP) using the equation (net weight after saturated and drained – initial dry weight).  The volume of media in each pot after saturation was derived by measuring the height of the media in the pot and calculating volume as a function of height using the formula for a truncated cone: V=πH(R_b²+ R_b+ R_t+ R_t²).  Dry bulk densities were obtained by weighing a given volume of each media before and after drying and using the formula (initial dry weight)/(volume). There were no significant differences in physical properties and pH between mixes, however EC was slightly higher for the ADDF mix.

Woody Nursery Crops
Liners of button bush (Cephalanthus occidentalis) and silky dogwood (Cornus amomum) were transplanted into #2 pots (7.3L volume) containing either the  4:2:1 bark-ADDF-sand amended with 4g/L gypsum or 4:2:1 bark-peat-sand amended with 2.5g/L dolomitic lime.  Plants were fertilized with a top dressing of Osmocote 18-6-12 (Everris NA, Inc.) at a rate of 30g per pot. Plants were grown outdoors with natural season lighting and irrigated with drip irrigation during the first season’s growth.  Plants were overwintered in an unheated hoop house.  Plants were moved from the hoop house and forced out of dormancy in a double polyethylene film greenhouse with overhead irrigation.  At the end of the first growing season, plant height, width, thickest stem caliper and number of shoots were measured.  After leafing out at the beginning of the second season (approximately 10 weeks after being moved into greenhouse) plant height and shoot fresh weight and dry weight were measured.  Leaf tissue samples were taken for nutrient analysis at the end of the first season before plants began to enter dormancy and at the beginning of the second season after plants had leafed out. SME samples were taken from media before planting to measure pH and electrical conductivity (EC) using Twin pH/conductivity meters (Horiba Corp., Kyoto, Japan).  PourThru samples were taken regularly and analyzed for pH, EC and nutrient concentrations during the first season.  Physical properties and volume shrinkage (Based on the formula of a truncated cone described above) of the mixes in this trial were measured again at the end of the woody shrub growth trial to evaluate long-term use of ADDF for nursery crops.

Measurements taken at the end of the first season of growth after transplanting show no differences in size, stem caliper or number of stems between button bush and silky dogwood grown in the bark-peat-sand mix and bark-ADDF-sand mix. Analysis of PourThru samples show elevated levels of orthophosphate in the bark-ADDF-sand mix for approximately 8 weeks after planting. Upon harvest after leafing out in the second season there were no differences in height, maximum caliper, dry weight, new shoots or visual appearance  between plants grown in the two mixes.

There were no significant differences in initial physical properties or pH (5.53 for bark-peat-sand and 5.66 for bark-ADDF-sand) between the mixes. The bark-ADDF-sand mix had a higher mean EC (1230µS vs. 156µS).  At the beginning of the second growing season the bark-ADDF-sand mix had significantly lower EWHC and CCAP than the bark-peat-sand mix.  Both mixes did, however, have similar Db upon final measure.  The bark-peat-sand mix had significantly more shrinkage in the first season under drip irrigation whereas the bark-ADDF media had significantly more shrinkage upon final measure after approximately 10 weeks of overhead irrigation.  Both mixes had the same total amount of shrinkage.

Woody Nursery Crops from Cuttings
Cuttings from ninebark (Physocarpus opulifolius) and cranberry bush viburnum (Viburnum opulus) were rooted in sand under intermittent mist with bottom heat.  The rooted cuttings were then transplanted into 2.5” establishment pots (approximately 250 ml volume) containing the same mixes as in the previous woody plant trial.  Plants were grown outdoors with overhead irrigation for one growing season.  Plants were overwintered in a cold frame and forced out of dormancy in a greenhouse under overhead irrigation in early spring.  Plants were evaluated after leafing out by measuring dry weight, height, above ground tissue concentrations and with a subjective visual evaluation.

At the end of the first growing season there were no noticeable differences between plants in the two mixes.  At the beginning of the second season both ninebark and viburnum grown in the bark-ADDF-sand mix appeared to break from dormancy more quickly, vigorously and with darker foliage than those grown in the bark-peat-sand mix.  The plants grown in the bark-ADDF-sand mix were also significantly taller than the plants grown in bark-peat-sand mix.  Ninebark grown in the bark-ADDF-mix had a greater dry weight but there was no difference in dry weight between viburnum grown in either mix.

Herbaceous Nursery Crops
A variety of representative herbaceous perennials were used in this trial. Plugs of brunnera (Brunnera macrophylla ‘Jack Frost’), Shasta daisy (Lucanthemum superbum ‘Whoops-a-Daisy’) and rooted cuttings of phlox (Phlox paniculata ‘David’), liatris (Liatris spicata ‘Kobold Original) and coreopsis (Coreopsis verticillata ‘Moonbeam’) were transplanted in pots (approximately 2.8L volume) containing either 4:2:1 bark-ADDF-perlite amended with 4g/L gypsum or 4:2:1 bark-peat-perlite amended with 2.5g/L dolomitic lime.  Plants were fertilized with a top dressing of Osmocote 18-6-12 at a rate of 6g per pot.  Plants were grown in a glass greenhouse.  Plants were overhead irrigated and leachate was collected for nutrient analysis to calculate cumulative quantities of nutrients leached per pot.  Growth and quality of each species was evaluated based on quantitative parameters appropriate its growth habits and a subjective visual evaluation.  For brunnera, the number of flower spikes, maximum flower spike length and canopy volume (as described in previous trials) were measured.  For coreopsis, dry weight was measured.  For Shasta daisy dry weight and number of flowers were measured.  For liatris, dry weight, number of flower stems and maximum height were measured.  For phlox, dry weight, number of stems and maximum height were measured.  Leachate from each irrigation event was collected, measured gravimetrically and analyzed for PO₄-P using colorimetric methods to calculate the cumulative amount of these nutrients leached. 

Of all plant growth parameters measured, the only significant differences between plants grown in bark-peat-perlite and bark-ADDF-perlite were greater fresh and dry weights in Shasta daisy grown in the bark-ADDf-perlite mix.  While no differences were found in measured parameters there were some visible differences in plant growth and development between the two treatments in some species.  Coreopsis grown in the bark-ADDF-perlite mix was slightly chlorotic and less dense. Brunnera grown in the bark-ADDF-perlite mix had slightly chlorotic leaf margins and a deeper blue flower color than those grown in the bark-peat-perlite mix.  Overall, plants growth in the bark-ADDF-perlite mix were more variable than in the control mix with many plants in the bark-ADDF-perlite mix being of higher quality and larger size than those grown in the bark-peat-perlite but others being severely stunted. 

A much greater amount of phosphate was leached from pots containing the bark-ADDF-perlite mix and continued to be released through the growing season of all crops tested.  In the unplanted pot leaching, phosphate was continually released from the raw ADDF and the bark-ADDF-perlite mix throughout the trial and only began to plateau at the end.  Peat and the bark-peat-perlite mix released negligible amounts of phosphate.  Virtually all nitrate was released after the fourth leaching event.  The raw ADDF released far more nitrate than the bark-ADDF-perlite.  The bark-ADDF-perlite mix is slightly less than 30% ADDF but only releases approximately 17% of the nitrate released from the raw ADDF.  This suggests that mixing ADDF with bark immobilizes nitrogen and reduces nitrate leaching. Peat and the bark-peat-perlite mix released negligible amounts of nitrate.  Ammonium was slowly released by all mixes with the ADDF mixes releasing the greatest quantities.  There was an initial dip in pH followed by a rise and a leveling off.  Most soluble salts were leached in the first few leaching events.  The rapid release of nitrate and soluble salts in ADDF paired with the steady release of phosphate suggest ADDF has a large store of adsorbed labile phosphate.

Impacts and Contributions/Outcomes

The results of this project show that ADDF can be used as a 1:1 replacement for peat in nursery mixes.  ADDF can be used as a renewable, locally sourced media component for nursery growers of the Northeast.  Adoption of ADDF in nursery mixes in the Northeast would decrease the demand for peat and reduce the environmental impact of peat harvest and transport.  ADDF will likely be less expensive than peat because it is currently a waste management liability for dairy farmers.  Additional nutrients provided from ADDF could also decrease fertilizer requirement for crops grown in ADDF mixes. 

ADDF can also be generate additional income for dairy farmers with anaerobic digesters as a high quality horticultural product.  Adding value to ADDF will further incentivize more dairy farmers to adopt anaerobic digestion systems for waste management. 

Manuscripts containing the results of this research have been prepared for submission to a scientific peer reviewed journal and are in the process of being submitted.  Several articles based on the results of this project were published in trade journals aimed at a wide range of audiences.  “Anaerobically digested dairy fiber: A Renewable Substitute for Peat” appeared in the September 2015 issue of Greenhouse Product News and will reach members of the greenhouse and floriculture industry.  “Alternatives to Peat” was published in the October 2015 edition of Permaculture whose audience is composed of small scale growers and home gardeners interested in sustainable agriculture.  The potential of ADDF as a horticultural product was communicated to the dairy industry via “Anaerobically Digested Dairy Fiber: A Valuable Product” in the November 2015 edition of Progressive Dairyman.  “Unconventional Components” appeared in the December 2015 issue of Nursery Management and communicated the results of this project with the nursery industry.  Another article is scheduled to appear in Acres USA, a sustainable agriculture magazine, in early 2016.

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