Azolla is an aquatic fern that easily grows in rice fields. It has a symbiont N2-fixing blue-green algae, Anabaena azollae, that lives in its cavity. Azolla is generally applied as a dual crop with rice and, on average, contributes 30–40 kg N ha–1 (Singh 1977). It is an essential nutrient for Azolla growth. Phosphorus is also applied to the rice crop in the form of single superphospate (SSP). Dawar and Singh (2002) observed soil-based cultures of Azolla and noted that a double dose of P (300 ppm) given to Azolla proved toxic to the algal partner. A few days’ to week’s incubation in high-P soil medium led to stunted Azolla growth. The fronds became small and, when examined for nitrogenase activity, no acetylene reduction activity was found. Light microscopy confirmed the disappearance of Anabaena azollae from the leaf cavities of the fronds.