Optimization of nutrient supply in drip-application (fertigation) culture, 1: Phosphorus application adjusted with the soil phosphorus status

In the drip-application (fertigation) culture, plant nutrients are supplied both from nutrient solution and from soil media. In this study, the effect of phosphorus contents in the soil or in the nutrient solution on the growth of melon (Cucumis melo) and tomato (Lycopersicon esculentum) was examined. The soil used in this study was allophanic andosol which had high phosphorus fixation capacity. 1) When the Truog-P content was approximately 200 mg P2O5 kg(-1) or more in the soil, the yield of melon (one fruit per plant) and tomato (in the fifth cluster stage culture) was almost the same regardless of the phosphorus contents in the nutrient solution. 2) When the Truog-P content was approximately 1000 mg P2O5 kg(-1) or more in the soil, the absorption of phosphorus in melon and tomato was almost the same regardless of the phosphorus contents in the nutrient solution. 3) In the soil highly enriched with the phosphorus as frequently observed in farmers' greenhouse soils, there is no need to supply any further phosphorus in the nutrient solution because its recovery is very low. Curtailment of phosphorus is certainly helps to make the soil phosphorus status more appropriate and to reduce the fertilizer cost for the farmers. 4) As to the nutritional diagnostic criteria for the phosphorus concentration, we propose 150 mg P L(-1) in the sap of melon petioles collected at the thinning in the spring-summer season, and 200 mg P L(-1) in the sap of tomato petioles just under the first cluster fruits of 2-3 cm diameter in the fifth cluster stage culture in autumn-winter season. 5) Phosphorus concentration of the petiole sap reflected the phosphorus nutritional status very well. Therefore, by adjusting the nutrient composition of the solution with the soil testing before planting and with petiole sap diagnosis during the plant growth periods, we can achieve the optimized fertilization in the drip-application culture.