Changes in crop yields and available phosphate and exchangeable potassium content of open-culture vegetable field soils by continuous use of L-type fertilizers (N:P2O5 :K2O=14:8:8)

L-type fertilizers with reduced phosphoric acid and potassium components (N: P2O5: K2O=14:8:8) were used for 13 works for 6 years in an open-culture vegetable field, and the yield of Komatsuna, spinach, and lettuce and its impact on soil were investigated. In addition, the effect of L-type fertilizer and organic matter (cow manure compost) was investigated. Crop yields were almost the same as those in the control zone where L-type fertilizers were used in an accumulated soil by phosphoric acid and potassium, but in standard soils without accumulation of phosphoric acid and potassium, they tended to decrease if L-type fertilizers continued to be used. The amount of available phosphate in the soil decreased during the cultivation period of 6 years in the phosphate accumulation soil, but the decrease was not observed in the standard soil of about the soil diagnostic reference value. The exchangeable potassium content in the soil decreased regardless of the amount of potassium accumulation in the soil, and it decreased to the vicinity of the lower limit of the soil diagnostic reference value in two years (5 works), and the degree of decrease was larger for L-type fertilizer continuous. When organic matter (cow manure compost) was used in combination, the content of potassium in the soil increased rapidly, and in particular, the soil diagnostic reference value was exceeded in combination with high-analysis compound fertilizers, but with the L-type fertilizers, it was within the soil diagnostic reference value. These results suggest that L-type fertilizers are applied in an open-field vegetable field without applying compost or other organic matter for about two years, and then used in combination with organic matters.