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Potassium (K) fertilizer consumption in rice production has increased in developing countries where negative K balance was observed, but it has recently decreased in Japan. This situation raises a question of how K fertilization is managed in Japanese paddy fields and how it affects soil K balance and soil K status. Rice straw (RS) is a good source of K, and RS recycling after harvesting is a common practice in Japan. However, in mixed crop–livestock systems, RS is taken at harvesting time to use as the feed for cows and substituted with the application of cow dung compost (CDC) to the fields. We investigated soil K balance and soil K status in 8 (2017) and 10 (2018) pairs of adjacent RS- and CDC-treated fields in Mamurogawa, Yamagata, Japan. The K balance was calculated from K inputs (RS or CDC, fertilizer, and irrigation water) and K outputs (plant uptake and leaching). K fertilizer application varied widely in both treatments, with no significant difference between RS and CDC fields. K fertilizer was applied in amounts lower than those recommended for paddy rice in the study area in 56% of the fields in both treatments. The K balance was positive in most fields with RS recycling even if K fertilizer application was lower than recommended, but it was negative in half of the fields where RS was substituted with CDC. Most fields in the RS treatment had higher soil exchangeable K than the standard value for fertile soil. Therefore, K input through RS is sufficient for maintaining positive K balance, whereas K input in the CDC treatment from CDC or fertilizer may need to be increased to ensure positive K balance.

Potassium balance in paddy fields under conventional rice straw recycling versus cow dung compost application in mixed crop–livestock systems in Japan