Why Look at Grapevine Prunings As a Compost Feedstock?
The objectives of this research and Extension project were:
- To determine the impact of mixing grape vine prunings with dairy manure in a compost mix on the composting process and final product.
- In particular, we were interested in determining if nitrogen gets fixated into the compost mix with increased carbon content.
- To evaluate if composting is a workable alternative to annual grape vine prunings burning. Stopping this annual burning will reduce vineyards environmental footprint.
- To demonstrate three different on-farm composting techniques. Mechanically turned (MT), passive aerated (PA), and forced aerated composting (FA).
What did we do?
We teamed up with a grape and a dairy producer and we built a series of windrows to showcase the three different composting techniques and to research the effects of mixing both waste streams. Grape vine prunings were grounded and mixed with open lot dairy manure. Carbon content of the mix was adjusted to meet organic production standards since the vineyard hosting the project was certified organic. Since the carbon to nitrogen ratio (C:N) of the grounded grape vine prunings was on the low side (80:1), horse stable sawdust and straw from the local county fairgrounds were also used to help increase the C:N. Three replications of each system (MT, PA, and FA) were built with the enhanced carbon mix. A third set of three replications with dairy manure as received (some straw but no added carbon) were built using the mechanically turned system (MTMA) to serve as a control and comparison for that system. In addition to collecting data to evaluate th e effects of the added carbon, the project included two field days where all the systems, how to construct them, and their advantages and challenges were showcased.
What have we learned?
The initial feedstock mix C:N was significantly higher in the carbon enhanced windrows as expected, but the final C:N ratio of the compost was not significantly different among most systems and between the enhanced mix and the just manure mix (Figure 1). The C:N reduction between the initial mix and the final compost was significant in all systems of the carbon enhanced windrows, but not significant in the just manure mix (MTMA).
As expected, the initial mix total nitrogen (TN) was significantly lower in the carbon (C) enhanced windrows compared to the just manure windrows (Figure 2). TN in the finished compost had no significant difference among all the systems. The difference between the initial mix and final compost TN wasn’t significant among C enhanced windrows, but highly significant in net values (10.08 Lb/T of N on dry weight basis; p<0.0001) on the just manure windrows. This difference in TN, coupled with the no significant difference in C:N, suggests the loss of nitrogen as ammonia during the composting process in the windrows made of just manure. Net nitrogen loss was significantly lower in the C enhanced windrows (1.45 Lb/Ton).
Salts concentrations (mmhos) difference between initial mixes and final compost was significant in all windrows, with higher values in the final compost as expected due to the concentration effect that composting volume reduction has (Figure 3). Salt concentrations in the just manure windrows were significantly higher compared to the carbon enhanced mix. There is a dilution effect when carbon is added in the initial mix (lower manure mass per initial mix unit). Similar dilution trends were observed for phosphorous (P), potassium (K), and micronutrients. Carrying this dilution effect in the final compost can be beneficial when land applying compost since application rates can be increased, increasing the nitrogen and carbon content of the application (desirable conditions) by the time the limiting components in our soils (usually P, K, or salts) are reached.
Screening of the carbon enhanced windrows generated a refuse (bigger size particles) containing pieces of grape prunings that can be used as mulch to control weeds in the vineyard or other production units. When PFRP is achieved, plant pathogens in the mulch can be considered absent or inhibited, and the mulch will be usable on the same or similar plant species.
The PA and MT windrows with enhanced carbon mix reached USEPA-PFRP. FA system didn’t reach PFRP and had an incomplete composting process because of the lack of moisture in the initial mix due to problems with water supply during their construction. Other studies conducted by the authors using FA with similar feedstock had reached PFRP. MTMA windrows didn’t reach PFRP, a common event in the region due to the low carbon content of dairy manure.
This project demonstrated that composting of dairy and potentially other livestock manures mixed with woody wastes from the grape industry or similar agricultural products is not only feasible but beneficial for both industries. Further research is necessary to determine how different carbon and animal manures sources, especially harder woods, will affect the composting process and the final product.
Mario E. de Haro-Martí. Extension Educator. University of Idaho. firstname.lastname@example.org
Mireille Chahine, Extension Dairy Specialist
Tony McCammon, Extension Educator
Ariel Agenbroad, Extension Educator. University of Idaho
Unpublished data. Please contact the author, Mario E. de Haro-Martí at email@example.com or 208-934-4417.
The authors want to thank the participating grape and dairy producers for their collaboration. This project was funded by an Idaho USDA-NRCS Conservation Innovation Grant (CIG).
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