K Trials in Lodi Vineyards
Wanless introduced as new IPM coordinator at Lodi Grape Day
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Dr. Stuart Pettygrove of UC Davis is researching K nutrition in Lodi vineyards. |
"Worldwide, potassium is the most difficult nutrient to manage in winegrape production," Pettygrove said. It is the most abundant inorganic element in grapes, and the main cation in must and wine, with concentrations far above those for calcium, magnesium, and sodium. In grapevines, potassium plays an important role in enzyme activity and the uptake of other nutrients and sugars by cells, and is important in controlling plant water relations, cell turgor, and growth.
"Not only are K deficiencies a problem, but excesses are also a problem in relation to wine quality," Pettygrove said. High juice K increases pH, decreases free tartaric acid, decreases color quality, and can increase susceptibility to oxidation and spoilage. Thus, the need to understand and properly manage potassium is important for both the grapegrower and the winemaker.
Pettygrove listed several facts regarding potassium uptake by vines and fruit:
• Rapid vine uptake of K is high after veraison and through to harvest.
• Grape skins have high potassium concentration.
• High fruit loads can lead to potassium deficiencies, but a link between crop load and berry K concentration is not established.
• Rootstocks differ in their ability to supply, or translocate, potassium to the scion. Freedom, St. George, 1616C, 039-16 and SO4 can provide high K to scion vines, whereas, 420A, 110R, 5BB, 5C, 1103P and 140R have lower capacity to supply potassium, and are sensitive to K deficiency.
Several fertilizer materials are available for making potassium additions to vineyards that include K chloride, K sulfate, K thiosulfate, and K nitrate. However, there is also a wide range of recommended application rates, from 30 pounds to 1,500 pounds per acre. One problem with effective potassium applications is that it is a relatively immobile nutrient and will move only a very short distance to vine roots.
However, Pettygrove said, "Drip line fertigation is an efficient way to apply K to grapevines, enabling the use of much lower application rates." One caution is that late season deficit irrigation strategies with drip can reduce root uptake of potassium, at the time when demand by the vine is high.
Pettygrove also discussed potassium status diagnosis and test methods used in vineyards, noting that observing visual symptoms, and leaf petiole sample analysis are the most commonly used. Soil sampling is also used, especially during preparation for planting. Although these methods have limitations, petiole analysis is currently the standard method for evaluating vine potassium status.
The researchers conducted a survey of Lodi district growers in 2006 that showed 85% of vineyards had received potassium fertilizer additions during the previous three years. About 28% of vineyard blocks in the survey reported potassium deficiency based on petiole K samples, soil testing, or low yields that were corrected by potassium applications. Only a few growers reported any problem with excessive potassium levels in fruit.
Soil studies were done in 36 district vineyards over the past four years, with the digging of more than 120 soil pits, and the collection of more than 700 soil samples. Pettygrove and UC Davis research colleague Toby O'Geen prepared a map of the Lodi district showing six major regions based on soil types.
Soils in the Lodi district are diverse, but differ somewhat systematically along an east to west gradient from the Sierra Foothills to the Delta. Soils types and locations have been characterized in relation to potassium supply. Some soils have plant-available potassium found in soil solution or retained on mineral particles and on organic matter. In other soils, it will be leached over time and must be replaced by fertilization.
Some soils have the property for K fixation, in which potassium is removed from solution and trapped within mineral layers. A portion of the potassium fixed in this manner can be a slow-release source for plants, but most will not be available fast enough during times of high demand.
Soil samples collected by the researchers were used for fixation analysis in the lab. This data was used to characterize the soils in the six major soil regions by their ability to supply potassium, and their K fixation potential. Although, the soil map may not be refined enough to be accurate to the scale of a block or a vineyard in all cases, in many locations, it could provide guidance to aid growers in rootstock selection and selecting adequate rates of potassium fertilization.
Current field trials
There is little data and field research regarding whether winegrapes grown on K-fixing soils and non-fixing soils require different potassium fertilization rates and regimes. Potassium fertilization rate field trials were started in 2009 in two established commercial Syrah vineyard blocks with different soil test potassium levels and different fixation potential. Potassium application rates of 0, 30, 60 and 90 pounds/acre were applied in different experimental plots. As expected after just one year, little response to treatments was observed, although some increase in petiole potassium levels at veraison was observed.
A third experimental trial site will be established this year on another regional soil type. Pettygrove said field experiments will need to continue for several years to obtain sufficient data to know whether potassium nutrit ion must be managed differently in K-fixing and non- fixing soils.