Abstract: Phosphorus fertilizer is readily absorbed and fixed by soil particles after it is applied to soil, resulting in low bioavailability. Therefore, it is an important goal to improve the bioavailability of phosphorus fertilizer by modifying crop cultivation systems. The aim of this experiment was to compare the ability of different forages to activate and absorb insoluble phosphorus. The forage growth and phosphorus absorption characteristics of Sorghum dochna, Sorghum hybrid Sudan grass, and Dolichos lablab were evaluated under four treatments in hydroponic culture. The four treatments were water-soluble phosphorus (SP, 1/4SP, 1/8SP) and insoluble phosphorus (IP) supplied in the hydroponic growth medium. The responses of forage to insoluble phosphorus and low phosphorus varied among forage species and phosphorus levels. In the IP, 1/4SP, and 1/8SP treatments, the plant roots showed restricted growth, and forage growth was inhibited. The maximum inhibition of root length, stem length, and biomass were 2.25%-55.92%, 3.10%-63.35%, and 4.06%-57.72%, respectively, in the low phosphorus and IP treatments compared with the SP treatment. The nutrient contents and nutrient accumulation in forage were also significantly lower in the IP, 1/4SP, and 1/8SP treatments than in the SP treatment. In contrast, compared with the SP treatment, the IP, 1/4SP, and 1/8SP treatments showed increased root to crown ratios and increased organic acid secretion (maximum increases of 162.9%-337.1%, 84.4%-428.9%, and 354.6%-1027.3% in IP, 1/4SP, and 1/8SP, respectively). The H₂PO4- contents in the nutrient solutions of S. dochna and Sorghum hybrid Sudan grass were significantly higher under IP treatment than under the 1/4SP and 1/8SP treatments, but did not differ significantly among the treatments with D. lablab. These results indicated that the roots of the three forage plants can secrete organic acids to dissolve insoluble phosphorus in the nutrient solution and turn it into available phosphorus for crop absorption and utilization. The three species could be ranked, from highest phosphate solubilizing ability under low phosphorus stress to lowest, as follows: S. dochna>Sorghum hybrid Sudan grass>D. lablab. Therefore, adding a forage grass that can efficiently activate and utilize insoluble phosphorus as a green manure in a rotation system can reduce the application of phosphorus fertilizer and promote the soil nutrient cycle.

Key words: hydroponics, insoluble phosphorus, Sorghum dochna, Sorghum hybrid sudan grass, Dolichos lablab