Fertilizer components of the bottom mud of agricultural ponds on the Shioda-daira plain in Ueda, Nagano Prefecture.

Abstract

Bottom mud deposited in agricultural ponds has potential as a source of fertilizer for plants. I investigated associations of the vegetation area of the pond bank and water surface, the area of dry pond, and the river system with the hydrogen ion concentration (pH) and fertilizer components (nitrate nitrogen, water-soluble phosphate, and water-soluble potassium) of the bottom mud of 10 agricultural ponds by using simplified soil diagnosis kit. The study area was Shioda-daira plain, Ueda, Nagano Prefecture, central Japan, which has many agricultural ponds. The best generalized linear mixed models (GLMMs) revealed a significant associations of the river system with pH, a significant association of vegetation area with water-soluble phosphate content, a significant negative association of vegetation area with water-soluble potassium content, and no significant association of any factor with the nitrate nitrogen content. These results suggest that vegetation on the pond bank acts as a source of water-soluble phosphate, and as a sink absorbing water-soluble potassium. However, relative errors compared to conventional soil diagnosis methods were high for water-soluble phosphate and water-soluble potassium. Although the fertilizer component concentrations of bottom mud varied among the 10 ponds, they exceeded values suitable for plant growth (3 mg/L) in most of the ponds. The pH values were slightly lower than those suitable for plant growth (6.0-6.5) in all 10 ponds. Thus, the bottom mud deposited in agricultural ponds was a rich source of fertilizer characterized by weakly acidic soil. Adjustment of the relative proportions and concentrations of the fertilizer components would be required for use of the bottom mud as fertilizer.