Chemical composition of composted grape marc.

Composted grape marc, produced at the vineyard using different procedures, is generally returned into the vineyard. Information on the chemical composition of these grape marc composts is lacking, particularly the variability that may result from different composting practices. In this study, grape marc composts, varying in age from 3 months to 3 years, were collected from four different vineyards and subjected to detailed chemical analysis. The chemical analysis revealed that all the grape marc composts contained levels of free potassium, in the range of 2-3% w/w. Plant macronutrients such as Ca, S, Mg were present at low levels (<1% w/w), while phosphorus (0.1-0.3% w/w) and nitrogen (1-2% w/w) levels were not very high and mainly present in "plant unavailable" form. The levels of soluble salts, though not excessive, were elevated in all the composts. These salt levels could cause a problem in some soils. Heavy metals were not present in any significant levels. All the grape marc composts analysed provided some benefit in returning nutrients into the vineyard and all were significant potassium sources.