We assessed plant nutrient problems in rice paddy soils in southern Okayama and based on results, developed guidelines for potassium fertilizer application. (1) We investigated the soil in 135 parcels of rice paddies located around Kojima Bay in southern Okayama. The soil pH was <6.0 in 83% of the fields and the potassium saturation level (exchangeable K/total exchangeable cations × 100) was &ge;5% in 67% of the fields. (2) The potassium saturation level tended to be higher in double-cropped rice paddies in which rice and wheat were cultivated in rotation. Presumably, the soil pH was low in these rice paddies as a result of reduced base saturation caused by leaching of the exchangeable calcium, and this was exacerbated by an excess of potassium in the soil. (3) To prepare guidelines to reduce potassium fertilizer application based on the chemical conditions described above, we conducted a potassium fertilizer application test in rice paddies with various soil exchangeable potassium contents. The results show that the rice shoot sodium content was greater in the test group when the potassium saturation was &le;4%. We therefore established a target level for the improvement of potassium saturation level in the soil based on the increased sodium content resulting from potassium shortage. (4) Based on the target improvement level, we designed a fertilizer application guideline in which potassium fertilizer was applied with the target of 4% when the potassium saturation level before fertilizer application was &le;4%, whereas potassium was not applied when the level was >4%. This guideline would result in lower application rates of fertilizer than if a fixed rate for potassium fertilizer application was prescribed.