Market Potential and Grape Pomace Composting

As previously noted, the market potential determinations for both red grape skins and grape seed oil were based on 100% utilization. These market potentials decrease when a large portion of the produced grape pomace is used to create compost which is then recycled back into the vineyard. Composting is based on aerobic microbial decomposition. Organic compounds are broken down (decomposed) into natural elements such as carbon and nitrogen. This is a very simplified version of the composting process as a multitude of factors are involved in composting biology and chemistry. For optimal composting, the material being composted must have a high moisture content and contain a sufficient carbon-to-nitrogen (C:N) ratio. The high C:N provides nutrients for the microbes to survive and continue degradation. A study in 2001 provided the chemical characterization of fresh grape pomace, which is displayed in Table 15. The chemical composition after composting is also displayed in Table 15, providing a before and after comparison of grape pomace composting.

The optimal initial C:N of a compost is between 25:1 and 35:1. As shown in Table 16, the C:N of grape pomace is approximately 27:1, therefore it qualifies as an optimal substrate for composting. After composting, the nitrogen content increased to 2.35 (wt%). This is beneficial as nitrogen content in soil is influential in  determining vineyard growth. The theoretical average amount of nitrogen needed for a vineyard is 55 kg N/ha. This is equivalent to 22.267 kg N/acre (1 ha = 2.47 acres). As stated, the market potential of grape pomace as a suitable supplement decreases as increased pomace is utilized as compost. Although composting grape pomace increases the nitrogen content, using composted grape pomace alone to supply vineyards with nitrogen is neither feasible nor economical. To validate 1) due to the significant amount of composted grape pomace needed to supply 22.267 kg N/acre and 2) the other nutrients needed for vineyard growth, i.e., phosphorus, potassium and magnesium, are not adequately supplied by grape pomace compost.