Potassium (K) is an essential nutrient for plant development and for sustaining high-yield and fruit quality. There are many physiological processes in which K is involved including: assisting in the production of energy molecules (ATP) and maintaining photosynthesis levels; activation of enzymes, facilitates conversion of nitrogen intro amino acids, and improvement of water-use efficiency to list a few.  

The involvement of K in water movement in plants can impact fruit size since the accumulation of K in cells lead to an increase in water movement inside the cells which in turns increases cell turgor and cell expansion. Adequate K nutrition will result in bigger fruit, but this is not a linear relationship and increasing K fertilization beyond optimum will NOT result in bigger fruit. In fact, excessive K fertilization will result in lower yields and increased fruit rot based on studies done in Wisconsin in the early 90s. In addition, excessive K fertilization will reduce the uptake of Calcium which is a key nutrient for increasing berry firmness and reducing fruit rot.  

How to determine K requirements 

1. The best tool to determine cranberry vines nutritional status is the tissue analysis. We recommend growers do a tissue analysis every year to track what is happening to the vines overtime and to determine the need for adjustments to the fertilization plan. In the case of K, tissue levels should be between 0.4-0.75%. Higher level of K in tissue analysis do NOT correlate with higher yields, so if your K tissue levels are over 0.75% you are most likely over fertilizing. 

2. The next thing to consider when determining how much K fertilizer to use is yield. Higher yielding cultivars will require more K fertilizer because more berries are produced, and higher levels of K are being extracted from the soil. In general, beds that average between 250-350 BBL/ac will require 50-70 lb K/ac while those that have higher yield between 400-600 will require 70 to 150 lb K/ac.  

3. Soil K levels will provide additional information to refine K fertilization needs. Soil K levels are not as good of an indicator of the nutritional status of the vines as tissue analysis. If there is higher concentration of K in the soil, the vines will uptake as much as it is available in detriment of taking up Calcium and Magnesium. Soil K availability is higher in soil with adequate moisture compared to soil with low moisture levels. 

4. When should you apply K? When there’s the highest demand for it, which is during fruit development. You can use a blended fertilizer with N and P, or if you need additional K to over come a deficiency, potassium sulfate is a good alternative. 

The Myth of Potassium as a Promoter of Vine Cold-hardiness and Vine Growth Controller

There is no scientific evidence from any study that has proven that additional K fertilization help vines gain cold hardiness. Vines will acclimate and increase their cold-hardiness levels in the fall as a response to lower ambient temperature and the shortening of photoperiod. Additional K fertilization in the fall has no effect on plant’s perception of cold temperatures and thus no effect on cold hardiness. The same is true regarding K effect on setting buds or controlling vine vigor. Additional K fertilization will NOT increase bud set or control vine vigor, however reducing Nitrogen fertilization will do so.  

Final Note 

Fertilizer prices are through the roof, if there is ever a time to reduce unnecessary fertilizer applications it’s now. Make sure you are doing tissue analysis every year and collecting yield data on all your beds so that you can correctly estimate K fertilization needs. Over fertilizing with K has NO benefits to fruit size or yield, and can lead to lower yields over time, higher fruit rot, and softer fruit. 

This article was posted in Cranberry and tagged Allison Jonjak, Amaya Atucha, Cranberries, potassium fertilization.