Changes in nutrient levels over time in major farmland soil types in Chiba Prefecture, and analysis of related factors

We determined the current condition of available N, available P2O5 and exchangeable K2O content in soil, based on the results of seven rounds of surveying farmland soil in Chiba Prefecture, and analyzed the effect of nutrient input by fertilizer and manure on soil nutrient change. 1. Available N tended to decrease after the 4th survey round in the paddy fields, after the 5th round in the Andosol, after the 6th round in the brown lowland soil of vegetable fields, and after the 6th round in the Andosol of vegetable facilities. These results were presumed to reflect the amount of fertilization. Available P2O5 tended to increase after the 6th round in the Andosol and brown lowland soil of the vegetable fields and vegetable facilities. 2. The amount of N, P2O5 and K2O fertilizer in the paddy fields gradually decreased after the 2nd survey round, and by the 7th round was less than 50% of the 1st round. 3. Typically, in paddy fields and vegetable facilities, the lesser the amount of N fertilization (vs. the standard amount), the lower the available N. Therefore, it was concluded that the standard amount of N of fertilizer application was necessary for maintaining nitrogen fertility. 4. Many areas of brown lowland soil in the vegetable fields and vegetable facilities in Chiba Prefecture exceeded the soil diagnostic criteria of 100 mg/100 g of available P2O5. In the brown lowland soil of the vegetable facilities, the greater the amount of P2O5 fertilization (vs. the standard amount), the greater the available P2O5. Therefore, in order to suppress the excessive accumulation of P2O5 in the soil, it is considered necessary to apply less than the fertilization standard amount and/or reduce the amount of applied manure.