Phosphorus and nitrogen contents of azolla grown in the Philippines

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Abstract
The distribution of azolla, its growth, and P-deficiency were studied in the Philippines by collecting 232 azolla samples from ponds (33%) and rice fields (67%: 40% with rice and 27% without rice) in 11 regions. The dominant species, coverage, color, healthiness, fertilizer treatment, and use by farmers were recorded. The N and P concentrations in azolla were expressed on an ash-free dry matter basis. The average N concentration was 4.5% and the median 4.5%. The average P concentration was 0.385% and the median 0.332%. Region, species, color, and visual judgment of healthiness were correlated with P concentration. Red Azolla pinnata var. imbricata samples had a lower average P content (n = 42, P = 0.245%) than green samples (n = 41, P = 0.46%). A. microphylla was always green and had significantly higher N and P concentrations than A. pinnata var. imbricata. Nitrogen and P contents were highly correlated (simple correlation coefficient = 0.64, ranking correlation coefficient = 0.73). As the content of P increased, the content of N was likely to approach a plateau. The N concentration at the plateau and the P concentration required to reach this plateau were higher in the A. microphylla-dominated samples than in A. pinnata var. imbricata-dominated samples. When the P concentration corresponding to 90% of the N% plateau was set as the critical concentration for P deficiency, 53% of azolla plants was considered to be P deficient. Soil samples were taken from 66 out of the total number of azolla sampling sites, and their chemical properties were analyzed. Average available soil P content of the soils, where the azolla samples were taken, was higher than the average for the Philippine soils. There was a distinct difference in the content of available soil P (Olsen P) between the soils where A. pinnata var. imbricata was dominant (n = 51) and those where A. microphylla was dominant (n = 8). The average content of available soil P of the former was 28 ppm and the median 12.5 ppm, whereas the average of the latter was 54 ppm and the median 47 ppm. No statistically significant differences were observed in other chemical properties. In soils where A. pinnata var. imbricata was dominant, the simple correlation coefficient between plant P concentration and soil available P content was 0.31 and the ranking correlation coefficient 0.34. The ranking correlation coefficient excluding samples from ponds was 0.54 (n = 42).