Potassium Deficiency Can Compromise High Quality Juice And Wine

The roller-coaster weather patterns that so often have been severe the past couple of years have seriously stressed grape vines.

The result by veraison: stalled maturity and potassium deficiency that can threaten grape and juice quality. Weather gyrations have gone from polar vortex to long, cold and very wet spring seasons, to hot weather, then to record-setting mid-summer chills, and even drought in some areas. In the Midwest and East this year, the heavy and persistent rains and cooler than normal temperatures during a long spring meant high levels of nitrogen uptake from wet soils, reducing potassium and magnesium and throwing nutrition out of balance.

That is the finding of tissue analysis from many vineyards, says Larry Shafer, vice president, Agro-K Corporation in Minneapolis. Potassium deficiency in particular produces poor, unevenly set and ripened fruit, reduces vine vigor,compromises brix development, and reduces yields.As an essential part of chlorophyll, magnesium is another crucial nutrient, especially for photosynthesis, along with important roles in carbohydrate metabolism and protein synthesis.

“Nutrient balance across the board is so important to make quality fruit,” says Shafer. “You can get the nutrient balance out of whack sometimes, and the vineyard will still make grapes. But, they won’t be superior grapes for high-quality wine. That’s why balance is so important. You aren’t just looking for fruit to hang on there and sell. You want to turn those grapes into high-quality wine that you’ll be proud to display or enter into wine contests. If that’s the goal, you had better have everything up to snuff in terms of nutrients and then keep them in balance.”

Shafer says rain-soaked soil encourages nitrogen to come up in the vines, and excessively cool weather delays development.

“There is late vegetative growth and the vines get out of balance,” says Shafer. “We recommend starting KDL®(Potassium Dextro-Lac®) at first color because of the fruit load, nutrient imbalance, and the season likely being shorter to get the grapes to finish at the required brix for harvesting.” Vines that are well-balanced nutritionally and have adequate potassium levels will produce more and mature more uniformly for a more efficient harvest, Shafer says.

Depending on the year weather patterns can cause vinesto run two to three weeks behind where they normally would be. Even if the vines are on their normal schedule, periodic mid-summer temperature dips for several consecutive days this year will tend to cause maturity to stall.

“Foliar feeding is one way you can make up some of this by not leaving the crop to its own discretion and instead stimulating and re-stimulating the vines, pushing them on and keeping them going,” says Shafer. University of Virginia Extension research has demonstrated that foliar feeding is six to 30 times more efficient than applying nutrition amendments to the soil, and in mid-season every day is important.

Shafer says starting KDL at first color or about 20% to 30% color in the fruit will allow the grower to tweak the balance. If the heat persists and the weather cooperates, the berries may sugar well.

“If the grapes are not coming up to between 19 and 20 brix we may hit them with a quart of Sysstem™-Cal, some 3-18-18 Agrobest®, and as much as a gallon of KDL,” says Shafer. “Adding calcium helps with overall late-season vine health.” 3-18-18 is recommended especially if tissue tests show the vines are low in phosphorus. An initial spray to correct that might be two quarts of 3-18-18 and two quarts of KDL.

“At one time our thinking was that phosphorus is important for flower and fruiting, but once you get the berries on, any phosphorus is going to be for next year’s bud return and crop,” Shafer explains. “But in recent years our experience has been that where growers had extremely low phosphorus, the grapes failed to finish. There were fewer seeds in the fruit, and it was harder to get the grapes to have the uniform color and get the brix the growers wanted. While it’s clear that phosphorus is critical for early season vine growth and fruit development it is also an important ingredient for final fruit and seed development.”

Shafer says it’s this science-driven nutritive balance that enables grape growers to produce fine wines, or contract to provide grapes for others who produce high-quality wine. It means tailoring dynamic nutritional programs to each vineyard and changing the sprays during the season based on tissue analysis. Samples are best taken at two important times: post-set and pre-harvest. Additional tissue samples at pre-bloom and pre-veraison can be crucial when it is obvious that nutrient balance has been compromised.

KDL’s unique formulation links potassium to the proprietary Dextro-Lac sugar complex. The spray quickly penetrates plant tissue to encourage sugar development in the leaf and transport it into the fruit. KDL has been shown to increase anthocyanins, sugars and phenolics that improve juice quality and character without raising pH. Shafer notes that it is stress on the vines, whether from adverse weather or other causes, that raises pH, and KDL mitigates those stresses.

John Burns has used the Agro-K nutrient rebalancingprogram at his Barrel Head Winery near Dubuque, Iowa.

“We sample leaves from each variety to find out how the nutrients are doing,” says Burns. After application of even the first phase of the Agro-K nutrition program “we saw a big growth spurt, which provides more foliage for the follow-up KDL sprays.Plants seem to really like it. There is a fast sugar climb when we get KDL into the plants.We don’t want to wait until the threat of first frost to give the vines the nutrients they need. This way we get ahead in the game by getting the sugar balanced before we get cold weather.”

Burns agrees that managing plant nutrition with foliar sprays is the most efficient and timely way to make adjustments.

Shafer says keeping grape vines in nutritive balance in many areas of the country will always be a struggle.

“When it’s dry it’s difficult to get the nutrients in the soil to come up the root system into the vines. But when there is a lot of water in the soil, the vines will access too much nitrogen,” he says. “Heavy soils are wonderful for corn, soybeans and other crops, but for grapes this is a soil very rich in organic matter, and the result can be excessive vegetative growth. You’re always trying to bring nutrition back into balance. During the growing season if there is a heavy fruit load, unusual weather, or other stresses, the vines don’t get enough potassium. And if the vines go full steam, the grower is always trying to catch up the potassium.

“Nutritive balance is a management tool. Growers will still have to hedge the vines in most cases, but not as much. Most growers stay busy hedging and tying up shoots. And if there is a big fruit load, they need sun on that fruit. The Midwest isn’t like California or Texas where the grapes get lots of sun. So if growers expect the grapes to finish with top quality, they need to balance the nutrition in the vines and fruit throughout the growing season.”