Dr. Camberato: We have here is severe potassium deficiency on the edge of field and then moderate potassium deficiency in the field. Potassium deficiency is pretty characteristic as a marginal necrosis along the edges of the leaf. This begins at the base of the plant first and then moves up the plant as the deficiency becomes progressively worse.
The symptoms express themselves this way because the plant is able to mobilize the potassium from the older leaves and send it to the newer leaves in order to continue to grow. Potassium is especially important in the plant during drought because potassium is very important in controlling stomatal opening and closing which allows CO2 in for photosynthesis but also controls the loss of moisture from the plant or transpiration and so the [stomates 00:01:08] react very slowly when potassium is low and the plant ends up losing more water than it would if it had adequate potassium.
Potassium also is responsible for the plant being able to take up water. It provides the driving force for root uptake of water and so again during drought when accumulation of water is most important, a potassium deficient plant is less able to do that.
We’re here in the soybean field looking at potassium deficiency symptoms. They’re very prevalent this year because we have a drought or low rainfall conditions throughout Indiana. Potassium is important in the stomatal opening and closing which controls photosynthesis and transpiration, so when potassium is deficient the [stomates 00:02:05] open and close more slowly so you have a reduction in photosynthesis and you end up transpiring more water.
Not only is the uptake of potassium reduced by the drought but the plant’s requirement for potassium is actually higher, so in a period where they need the potassium the most it’s harder for the plant to accumulate it. Often we see factors like compaction, nematodes, herbicide injury, things that affect rooting, oftentimes result in potassium deficiency symptoms because the plant root system is damaged and it’s not able to adequately take up the potassium.
There can be quite a bit of variability within and between rows and the amount of symptoms. I think the drought and the inability for the plant to acquire the potassium from the soil is actually exposing low soil test potassium areas in fields. Not only is the drought making it hard in those fields for the plants to take up potassium but those fields are already low in potassium. This year maybe a good time to note those areas and take soil samples from them in the future to confirm their potassium status.