Response of azolla to phosphorus, potassium, and zinc in different paddy soils [Philippines]
1987
Sikandi Ali | Watanabe, I. (Nuclear Inst. of Agriculture and Biology, Faisalabad (Pakistan))
Eleven Philippine soils with various capacities to support azolla growth in the field were examined for their capacities to support azolla growth with the addition of P, K, and Zn. Soil samples were placed in 10-cm-diam brown bottles and flooded for 1 wk. Azolla, previously depleted of nutrients, was inoculated with five treatments (0 additional nutrients; complete P, K, and Zn; and -P, -K, and -Zn) and grown for 3 wk. P, K, and Zn contents in floodwater and the harvested azolla were determined. Azolla dry wt ranged from 10 to 50 mg/10 cm2 and N content ranged from 1.9 to 5.1% dry wt. Because P content in floodwater decreased sharply after P application, and azolla growth was determined largely by the indigenous level of P in floodwater. Azolla biomass correlated highly with its N% (r = 0.954), and N% correlated highly with P content in floodwater (r = 0.797). P-deficiency threshold values appear to be 0.1 mg/liter in floodwater and 0.1% in azolla drymatter. Available P (Olsen P) correlated lower with P content in azolla (r = 0.861) than in floodwater (r = 0.903). In some soils high in floodwater P, elimination of K (in two soils) and Zn (in one soil) reduced azolla growth, but it was difficult to draw any conclusion on the threshold values for K and Zn deficiency. It was concluded, however, that P availability was the most limiting growth factor for azolla in Philippine soils
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