Potassium is needed to regulate normal metabolic processes in the plant including photosynthesis and protein synthesis as well as maintaining plant osmotic pressure. Deficiencies of K in rice have been related to increased disease incidence, typically brown spot, stem rot and blast. Rice takes up K in the ionic (K+) form. Deficiencies of K in rice are more often found on coarser textured soils with a relatively low cation exchange capacity (CEC).
Potassium fertilizer rates should be determined from a recent soil test. Potassium fertilizer is most efficient in rice production when applied just before planting until permanent flood establishment (5th leaf to first tiller). Splitting K applications between preplant and permanent flood establishment has been shown to be beneficial in severely K deficient, coarser textured soils.
Late season K fertilizer applications applied after the onset of deficiency symptoms (“rescue” applications) are generally beneficial; however, yields will be reduced as compared to earlier K application timings. It is uncertain whether fertilizer K applications made after head emergence is beneficial to yield.
In the figure above, you can see the results of a K time of application study that was conducted in Mamou in 2013. In this trial, maximum yield was obtained when K fertilizer was applied at planting, and 12%, 32% and 51% of the yield potential was lost when K fertilization was delayed to a pre-flood application, applications between mid-tillering to green ring, and 50% heading, respectively.
Applications of K to deficient plants between midtillering and green ring “rescued” approximately 22% of the rice yield potential. However, applications at 50% heading only saved 3% of the yield potential. Therefore, K fertilization after heading is not generally recommended.
The one exception would be if the rice is going to be ratooned. In that case, you would want the rice stubble as healthy as possible heading into the ratoon crop. Therefore, this would justify a K application after heading.
Ratoon research with K has shown that an additional 30 pounds of K2O is needed for the rice ratoon crop above the current LSU AgCenter soil test recommendations when the soil falls into the medium, low, to very low soil test categories. The original soil test calibration for K in rice only accounted for the main crop.
Potassium is a mobile nutrient within the rice plant. When a plant becomes deficient in K, it is translocated from the older (lower) leaves into the newer (upper) leaves. Common symptoms of K deficiency in rice include:
- Stunted plants
- Interveinal chlorosis (yellowing) of oldest leaves beginning at leaf tips and moving backward down the leaf
- Necrosis (death) of leaf tissue following chlorosis beginning at leaf tips
- Increased incidence of brown spot
- Potential for increased disease pressure from stem rot and blast