Is your alfalfa lacking persistence? Have you experienced a high frequency of winterkill in the past? Are your stands susceptible to insect damage and disease pressure? If you answered “yes” to any of these questions, you may be missing the mark on potassium (K).
In last week’s "Field Crops Virtual Breakfast" webinar from Michigan State University Extension, Kim Cassida explained why potassium is the golden ticket to alfalfa persistence. Without enough of it, plants are subject to environmental stress and are less likely to survive the winter. But alfalfa is a heavy potassium consumer compared to other field crops.
In hay systems, alfalfa can require 40 to 60 pounds of potassium as K2O or 70 to 100 pounds of potash fertilizer (0-0-60/62) for every ton of dry matter harvested. For example, Cassida said a high-yielding hayfield that produces 6 tons of forage per acre would need more than 340 of K2O per acre compared to roughly 270 pounds and 250 pounds for perennial grasses and corn silage, respectively.
Therefore, the forage and field crop specialist encouraged farmers to test soil and apply potash this fall to set up another successful hay season. You’ve got to bring your K-game so alfalfa can bring its A-game.
It combats plant stress
Potassium serves several plant functions, including its role in osmatic adjustment. “That basically means it is helping the plant control water pressure and the movement of water through the plant,” Cassida said. Potassium is a critical enzyme co-factor for photosynthesis, respiration, and protein synthesis; it is involved in nutrient transport; and it bolsters root growth. It is also the most important nutrient for stress tolerance — especially cold-stress tolerance.
“Potassium is very much involved in cold-hardening in the plants at this time of year when we transition from summer into winter,” Cassida said. “One of the big problems we have with the persistence of alfalfa is winterkill. We lose a lot of plants over the winter, or they are damaged, and having enough potassium available is essential for the plant’s ability to resist that winterkill damage.”
Potassium also helps plants fight drought, soil salinity, and waterlogging, which Cassida said is rather intriguing. “Waterlogging is one of the things that contributes to the winterkill that we see in alfalfa because it is often getting a double-whammy of cold temperatures along with saturated soils and ice sheeting,” she said. “Potassium may also be helping with that.”
In regard to stand persistence, potassium is involved in disease resistance thanks to its ability to strengthen plant stems and cell walls, blocking the entrance of bacteria, fungi, viruses, and nematodes. Cassida added that there is also evidence that potassium’s role in stomatal control can prevent pathogen entry into plants.
Spot the signs
The classic sign of potassium deficiency is yellow spots along alfalfa leaf edges; however, this is easily confused with potato leafhopper damage. In some cases, both issues may occur. “If you don’t have enough potassium, you’re likely to have more potato leafhopper damage than you wouldn’t have if your plants were in good status,” Cassida asserted.
Potassium deficiencies don’t always cause obvious yield drag, though — it can be more subtle, Cassida said. A higher frequency of winterkill, poor drought tolerance, and greater susceptibility to insects and root diseases can all eat away at total yield over time. Sometimes, the first sign of potassium deficiency is weed invasion.
“If the alfalfa is dying out and disadvantaged, the weeds are going to be happy to come in,” she said. “One of the very classic things that we see when we start to investigate one of those situations is that the field is too low in potassium. It’s not a weed problem — it’s a fertility problem.”
There is typically only a small pool of available potassium in the soil at a given time. This is because potassium is slowly mobilized from soil parent material, and plants release relatively small amounts of the nutrient through root exudates and residue. Therefore, adding potassium to hayfields is critical.
“It is nearly impossible to maintain adequate soil test potassium levels in a hayfield if you are not making some kind of addition,” Cassida said. “That might be fertilizer, that might be other soil amendments like manure, but it is not possible for even the healthiest soil to replace all of the potassium in the time frame we are asking it to. Hay just removes it too fast.”
Optimal soil potassium levels vary with soil texture. To determine fertilizer application rates, Cassida recommended testing soil in the fall since this is when nutrient levels are lowest. Testing soil and applying potassium in the fall can also support alfalfa winter survival. On the contrary, don’t base potassium applications on first-cut alfalfa tissue tests in the spring because these can be misleadingly high.
Potassium pointers
So, why don’t farmers add enough potassium? It can be expensive, Cassida said. Moreover, without an obvious yield bump from potassium applications, it’s easy to write off the nutrient as unimportant. With that said, she provided a few tips for potassium management.
• Optimize soil test potassium before environmental stress occurs.
• Soil test hayfields at least every three years.
• Do not apply more than 300 pounds of potash per acre at one time to avoid salt buildup and luxury consumption.
• Aim to improve soil organic matter to improve soil cation exchange capacity, which ultimately improves soil potassium holding capacity.
• Balance phosphorus and potassium to realize optimal alfalfa production.
