Last week I got two calls regarding unusual symptoms starting to appear on ripening rice plants. In both cases, the symptoms were described as medium sized round patches turning reddish or orange. Close inspection of plants showed typical symptoms of K deficiency. 

Leaf tips turned reddish and yellow and started to dry out. Stems looked thin and soft. At first, the symptoms were only in small patches, but as plants matured the symptoms extended to the whole field, with some areas showing more symptoms than others.

The critical flag-leaf K value in mature plants is 1.2% (optimal is between 1.5-2%). Leaf samples from asymptomatic parts of the field were at 1.22%, while samples from areas where symptoms were clear were around 1.18%.

I've been seeing K deficiency for several years now. The take-up of K by plants is similar to N, around 150 lbs/a. Most of the K (80%) goes to the straw, while a smaller fraction (20%) goes to the grain. This means that even in fields where straw is incorporated, 20-30 lbs/a of K are removed each year. Over time, K levels in fields thought to be high in K could be reduced substantially. Currently, Bruce Linquist is examining soil K levels and his research shows that K fertilizers may be needed when soil K is below 120 ppm.