Pomace, which is the skins, seeds and stems leftover from wine processing and pressing, can indeed be returned to the field as a soil amendment. You deposit it in a thin layer in the vine rows where it can act as effective mulch against weeds and help keep moisture in the soil. When it is tilled back in, the pomace provides additional organic material and nutrients to the vineyard. However, as you suggest, it’s not that simple. Spent grape skins and stems have to be composted and ideally mixed with other amendments before they are ready for use and may need quite a lot of help from you to become an ideal part of your viticultural regimen. Read on for more dirt (ahem) on how you can turn your winery waste to your advantage.
Using pomace for compost in the vineyard has many benefits. It can increase the number and variety of beneficial soil microbes, which better protects your vineyard from the ravages of pests and diseases. It increases the level of organic matter in the soil, adds to the soil nutrient profile (N-P-K-Ca is around 2.0-0.5-2.0-2.0), loosens heavy soils and improves drainage. Compost is typically applied at a rate of 1–5 tons per acre, typically in the summertime around June or July, which can return anywhere from 1⁄2 to 1⁄3 of the nutrients back to the soil.
As beneficial as compost from pomace can be, it’s important to not over-apply it as too much nitrogen can cause overly vigorous canopies, can increase the amount of hours needed to tend those big canopies and can increase susceptibility to rot and mold. Once compost is applied, its effects can be felt for 5–7 years afterwards so it’s important to go gradually and not to over-add. Figure that about three tons of grapes will yield around 1 ton of compost.
The most important thing to remember is that you really shouldn’t just dump pressed grapes or pressed, fermented must directly in the vineyard. Especially if you are using pressed white grapes, because doing so you risk attracting fruit flies and encouraging a high microbial population of undesirable organisms in the vineyard. You need to actually compost the pomace and turn it two or three times every week to keep it aerated (the aerobic breakdown is what helps develop the beneficial microbes). Like cooking meat, time and temperature are important. Keep your pomace at about 130–140 °F (54–60 °C) for at least a week (never let the temperature spike above 160 °F/71 °C) to kill pathogens. Keep moisture levels somewhat humid but never moist or wet, which will breed undesirable molds and can actually ferment and produce acetic acid. A typical compost windrow or pile, when managed well, will take about 6 months to “ripen,” which is why it’s typically applied in the summertime.
Pomace, which has a pretty high pH (not ideal for soils) also works best for your vineyard’s health if you can mix it with some other things before you compost it and apply it. Try a mixture of 50% composted grape pomace, 25% straw, 24% manure and 1% lime. The lime will help to lower the pH back to a more soil-friendly level. If you don’t have straw or manure, try working in yard clippings, turkey manure or even purchasing some of your community’s yard waste recycling compost. Just be sure that whatever you apply to the vineyard has been kept at 130-140 °F (54–60 °C) for at least one week to kill weed seeds and pathogens. Otherwise, you may not know what you may be introducing to your vineyards.
So to answer your question, yes, by all means re-use your winery’s pomace in your vineyards. It’s a wonderful way to also “complete the circle of life” from vineyard to winery and back again. Just remember that it’s not as simple as depositing your freshly pressed Chardonnay back into the vineyard. It takes a little work to make proper compost from pomace, but the rewards can be well worth it!