The Promise of Pomace (SA Fall, 1992 (v5n1))

Fall, 1992 (v5n1)

The Promise of Pomace
by Chuck Ingels, SAREP

Viticulturists and enologists have long known of the fertilizer value of grape pomace, the seeds and skins left over after grapes have been processed in wineries. In the scanty literature on the subject, the value of pomace has been described primarily in terms of its nitrogen contribution, with less emphasis on other nutrients and characteristics. A benefit of raw grape pomace that has often been overlooked is the effect of the added organic matter on soil structure, water penetration, and enhancement of nutrient availability. Composted grape pomace, however, provides nutrients in a more concentrated and stable form than raw pomace.

Depending on the variety, for every ton of hand-harvested grapes crushed, 20 to 100 pounds of stems and 160 to 240 pounds of pomace are produced. The stems, which can be used in composted pomace, are usually removed before crushing. (Before air quality controls were in place, much of the stem material was dried and burned.)

Pomace is used in many ways. Most of the waste is spread back into vineyards. It is used as livestock feed, and in some areas (primarily the Central Valley), it is burned in cogeneration plants to produce energy. Increasingly, winegrape waste is composted for use in gardens, landscapes, vineyards, and other crops.

Table 1. Typical moisture and nutrient contents of grape wastes.¹

Average Content (% of fresh weight)

Moisture Nitrogen Potassium Phosphorus

Stems 65 0.9 1.2 0.1

Seeds 30 1.0 0.6 0.5

Skins 70 0.3 0.6 0.1

Wet Pomace 50 0.9 1.0 0.25

Unblended

Pomace

Compost 30 1.5 2.0 0.5

1These figures are derived from a very limited number of sources. Nutrient analysis of soil amendments should be determined before purchase.

Large quantities of grape seeds are usually present in both pomace and compost. According to Greg Young, agronomist at Y & B Agricultural Services in Redwood Valley, many growers purchasing compost question whether the material is "finished" (i.e., has undergone the high-temperature thermophilic process). To alleviate grower concerns, and to make a better product, Young blends the raw pomace with organic wastes from mushroom production in a 1:2 ratio before composting. Weed problems could occur in landscaped areas if large numbers of grape seedlings germinate. Young notes that an application of raw pomace could result in several dozen seedlings in a ten square foot area; a similar application of finished compost of unblended pomace may yield only one or two seedlings.

FOR MORE INFORMATION:
Jacob, H.E. 1947. Fertilizing value of pomace. Grape Grower, December, 1947.
Kissler, J.J. 1978. Fertilizer value of grape pomace. Manuscript, UC Cooperative Extension, an Joaquin County.
Logsdon, G. 1992. Pomace is a grape resource. BioCycle 33(2):40-41.

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