Potassium - From the Soil to the Wine
Potassium (K) is the most abundant cation in plant tissues, grapes, juices, and wines. Several factors affect K availability in the soil, K uptake by vines, concentration of K in the fruit, and ultimately the concentration of K in juices and wines. As with anything in viticulture, one size does not fit all in regard to K requirements and recommendations. However, understanding the grand scope of K relations from soils to wines can assist with making the best K management decisions specific to a site and situation.
Function of K in Vines
Unlike most other nutrients, K is not metabolized to become part of the structural components of vines. It remains in its molecular ionic form, and plant membranes are highly permeable to it. The following are the roles of K in all plants:
- Enzyme activation
- Cell membrane transport and translocation of assimilates (sugars, etc.)
- Anion neutralization (important for maintaining membrane potential)
- Osmotic potential regulation, critical for water relations and turgor pressure, the drivers of plant growth and stomatal control
K is primarily phloem mobile, which means that it can be redistributed to different parts of the vine as needed (phloem flows bi-directionally). While a lot of K leaves the vineyard in the fruit every year, it also accumulates throughout the growing season and during post-harvest into storage in permanent woody structures.
Problems Associated with Poor K Management
K is a major driver of plant growth – in the event of a K deficiency, yield and vigor will be compromised. Every year, K is removed from the vineyard in high quantities in the fruit (Mullins et al. 1992 makes a general estimate of 5 lbs of K removed per ton of fruit per acre), making it an important nutrient to monitor for deficiency and replace regularly. However, there are problems associated with overabundance of K in the vineyard; it is competitive with calcium and magnesium for adsorption to soil particles, so an overabundance of one can lead to deficiencies of the others. In some regions such as Australia, Virginia, and Pennsylvania, excess K is a routine problem and juice and wine quality can be negatively impacted by high concentrations of K in the fruit. In such instances, tartaric acid adjustments are required to remediate juice and wine pH (which can be expensive on a large scale), and may not even resolve the problem entirely because the K concentration is unchanged and can later result in the precipitation of acid as potassium bitartrate. Color, mouthfeel, and perception of quality can also be negatively affected. An example of high K concentration in juice and wine would be 27-71 mmol/L in Australia, compared to 22-32 mmol/L in Bordeaux (Somers 1977). However, high K concentrations can happen anywhere if fertilizer is indiscriminately applied and/or if soil and tissue tests are interpreted wrongly. The best way to address problems associated with K in the juice and wine is to address it first in the vineyard.
K in the Soil
Soil pH strongly influences K availability – as soils decrease in pH below 6, K availability and uptake can be inhibited. In New York, soils are generally naturally low in K (less than 200 lbs/acre). Soil texture is another factor: sandy and gravelly soils have relatively low cation exchange capacities, hence low K adsorption. Other factors are organic matter content and soil moisture – low levels of both reduce K availability and uptake. Irrigation can facilitate uptake in drought conditions. Also, calcium and magnesium compete with K for exchange sites on soil particles, and vice-versa. Chances of magnesium deficiency go up when the K:Mg ration is greater than 5:1 (Wolf 2008). Applications of gypsum (calcium sulfate), dolomitic lime, and magnesium sulfate can all reduce K availability to the plant (Wolf 2016).
K in Berries, Juice, and Wine
A number of factors affect K uptake and accumulation in the berries. First and foremost is its availability in the soil. Other factors affecting uptake and accumulation are scion/rootstock combinations, canopy microclimate, and irrigation (Moss 2016). Lastly, K accumulation is differentiated within the berry anatomy.
The literature sometimes conflicts on the effect of rootstocks, but generally speaking, rootstocks with V. berlandieri parentage have been shown to take up less K compared to V. champinii rootstocks (Wolpert et al. 2005). However, even rootstocks that share a V. berlandieri parent may have significantly different K uptake and translocation abilities: 1103 Paulsen has significantly higher K uptake compared to 110 Ruggeri, although both share a V. berlandieri x V. rupestris parentage (Kodur et al. 2009).
Irrigation is known to increase K availability in solution in the soil, and consequent vine uptake and translocation to the berries. The final result is increased K and pH in the fruit in irrigated vines compared to unirrigated (Freeman and Kliewer 1983). The effects of irrigation and canopy microclimate are coupled, since high water availability increases K transport, as well as contributes to canopy density and shading of leaves (Moss 2016). It has been observed that leaf shading (not fruit shading) due to relatively dense canopies contributes to significantly higher K in Cabernet sauvignon fruit (Morrsion and Noble 1990).
Shoots and leaves require the most K between post-bloom and verasion, during their period of intense active growth. In the berries, K demand and concentration rises sharply after verasion, the period of berry cell expansion. At this point, berries are the strongest sink for K. Within berries, the skin has the highest concentration of K, followed by the seeds, followed by the flesh. This is a factor in some winemakers’ decisions when it comes to pressing the grapes – higher press pressure will extract more K, resulting in an increase in must pH. This may be undesirable if you are making base wine for sparkling, for example.
