Comparative efficiency of Azolla, blue-green algae and other organic manures in relation to N and P availability in a flooded rice soil

Summary

Pot incubation study with fresh Azolla (Azolla pinnata-India and Vietnam isolates,A. mexicana andA. filiculoides), blue-green algaAulosira sp., green manureSesbania cannabina, Azolla compost, farm yard manure and ammonium sulphate was conducted under flooded condition at CRRI, Cuttack keeping an equivalent amount of 25 ppm N through all the amendments where changes in availability of N and P, C∶N ratio and pH were recorded. Application of chemical N-fertilizer recorded a release of about 87% NH₄ ⁺−N at 10th day of flooding which gradually decreased and reached the minimum of 6% at 50 days. The C∶N ratio of the organic manures ranged between 9–10 favouring release of NH₄ ⁺−N and among theAzolla speciesA. pinnata isolates released NH₄ ⁺−N more efficiently thanA. filiculoides andA. mexicana. A. pinnata Indian isolate released maximum of 88% NH₄ ⁺−N, whereas Vietnam isolate recorded 77% at 40 days of flooding; green manure on the other hand, reached a maximum of 50% release at 50 days. The blue-green alga recorded a gradual increase and attained the maximum release of 38% at 40 days of flooding. Farmyard manure recorded a highest NH₄ ⁺−N release of 69% at 20 days of flooding and gradual decrease thereafter, whereas Azolla compost released 41% at 40 days of flooding. Soil amended with fresh organic matter achieved reduction upto a pH of 7.2 after 50 days of flooding, whereas the pH in Azolla compost and farmyard manure amendment was <7. The P availability increased from 20th day onwards and reached the highest values of 26 and 18 ppm in the fresh organic matter and compost amended soil respectively after 40–50 days of flooding. The cumulative release of available P was found superior in fresh Azolla incorporation.