Back in June I posted a blog about the relationship between high soil temperatures and leaf scorch/shedding of pecan leaves. Since that time not much has changed. The heat has been relentless and it has remained hot and dry throughout the pecan belt in Georgia.
Almost every leaf sample I have looked at from all corners of the state so far this year has been low in potassium (K). Because this is so widespread, the simplest explanation (which is usually the correct one) is that this is an environmental issue. The one common denominator we all have this summer is heat. Since May 30, we’ve had no more than 5 days at less than 90 degrees F and we’ve had 25 days over 95 degrees here in South Georgia. Our soil temperatures have been running at 95-105 degrees down to 4″ and 90-95 degrees down to the 8″ depth. At these temperatures, with or without adequate soil moisture, heat stress can be highly limiting to plant growth and development.
Pecans are adapted to maintain photosynthesis at high air temperatures (above 95 degrees) as long as they maintain good soil moisture. However, extreme heat stress can reduce plant photosynthetic and transpiration rates and negatively impact plant root development, which can limit the uptake of important nutrients like K. Our cotton and soils folks are telling me they are seeing the same thing in cotton right now. In our case, I believe soil temperature is having a negative effect on root survival and growth in the shallow depths, which has limited K uptake. For pecan, K is critically important to kernel filling, a growth stage we are on the brink of. The nuts pull K from the leaves as the kernels fill. Therefore its important to have proper K concentrations in the foliage at this time.
The combined effect of both heat and water stress on yield of many crops is much stronger than the effects of individual stress alone. Heat stress is a complex function of intensity, duration, and the rate of the increase in air temperature. In addition, the effect of an increase in soil temperature may be even stronger when accompanied by a decline in soil water content. Therefore, during extreme heat periods soil water content must be kept at a CONSISTENTLY adequate level to provide for plant uptake and to minimize the impact of higher soil temperature caused by higher air temperature.
When soil water distribution in the plant root zone is not even, which is the case in most orchards, plant roots can compensate for drier soil areas by taking more water from wetter areas. However, when there is a heat wave and soil temperatures increase, plant roots are less able to compensate for varying levels of soil moisture. When soil temperature rises above an optimum level, plant water and nutrient uptake can be affected.
What can be done at this point if you have low K in your leaves? Ideally, you want leaf K to be about 1.25%. However, under this year’s conditions I would not be alarmed unless you see levels at less than 1.10%. Many I am seeing are from 0.47%-0.95%. In most of these orchards the soil K levels are good. Its uptake that’s causing the problem, thanks to the heat. To be honest, I don’t know that you will be able to get K into the leaf and then to the nut in time for kernel fill unless you can get a good rain pretty soon after application. A narrow concentrated band or injection of K through the irrigation system would be the quickest way to move it in. A soil application certainly won’t hurt but I don’t know if we can get it there in time to help this year’s crop. With or without a soil application you will likely be ok next year anyway if you have good soil levels of K and the heat subsides during next year’s growing season.
I know the inevitable question will be asked, “What about foliar K sprays”? I have never conducted a study or seen a study in which foliar K had any measureable effect on pecan yield or quality. That being said, your best shot is to band K or inject K, however, foliar applications won’t hurt anything but your pocketbook.
The other side of this is soil pH. Soil pH levels are running low as well and this can contribute to low K uptake. However, I am seeing low leaf K in orchards with perfect soil pH as well. Liming at this point would boost the levels of Ca and Mg in the soil, both of which compete with K for uptake. K, of course, is not taken up at low pH but because Ca and Mg are already too high in most commercial orchards I recommend addressing the low pH in the winter to avoid further limiting this season’s K uptake.