Response ofAzollato P, K, and Zn in Different Wetland Rice Soils in Relation to Chemistry of Floodwater

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Abstract
A pot culture test in the greenhouse investigated the supply of P, K, and Zn to Azolla from flooded soils and the kinetics of P, K, and Zn in floodwater. Eleven Philippine rice soils, ranging from loamy sand to clay and with a pH range of 4.7 to 7.7, were used. In each soil, no addition, minus P, minus K, minus Zn, and PKZn treatments were designed. Three elements were added just before Azolla inoculation, and Azolla was harvested 3 weeks after inoculation. The concentration of phosphate in floodwater decreased sharply to a level of minus P treatment within a week after its application. Phosphorus content in Azolla at harvest did not change by phosphate application. The decrease of K and Zn concentrations in floodwater was slower than that of P. Hence, K and Zn content of Azolla at harvest increased with K and Zn application. Phosphate concentration in floodwater at 1 week after Azolla inoculation and a high correlation coefficient (R=0.83) with Olsen P content in soils except for one soil. Azolla biomass ranged from 9 (dry weight) to 53 mg/10cm2 and its N content ranged from 2.1 to 5.2%. The N percent in Azolla had a high correlation coefficient with dry weight (R=0.953). All dry weight, N%, and total N of Azolla had a high correlation (R=0.63–0.55) with P content of Azolla, but not with either K or Zn, indicating that P supply limited the growth of Azolla. Critical level of P content in Azolla was about 0.15%. An alkaline soil gave lower biomass than expected from its P content. Among the P concentrations in floodwater measured at 1, 2, 3 weeks after Azolla inoculation, the measurement at 1 week gave the highest correlation with P content in Azolla (R=0.739). When floodwater P at 1 week was less than 0.1 ppm, Azolla was certainly deficient in P. When floodwater P was higher than 0.4 ppm P, the supply of P to Azolla was sufficient. In only two soils, minus K or minus Zn treatment gave a lower biomass than the PKZn treatment. But it was difficult to confirm the presence of K or Zn deficiency based on the K and Zn contents in Azolla.