Effect of addition of phosphorus on the growth of Azolla.
The possibility of using A. pinnata as a form of organic manure in paddy fields was studied at Peradeniya, Sri Lanka in relation to the effect of P fertilizer on the growth, spread and nutrient content of the mature aquatic fern. Strains were obtained from (a) Bangkok and (b) an irrigation reservoir at Debokkawa Wewa. Increasing P rates increased N, P and K content of both A. pinnata strains although no extra gains occurred in the 3rd crop of strain (b) after increasing weekly P rates from 8.33 to 16.66 kg P2O5/ha. P utilization decreased with increasing P rates. FW increased exponentially and P and N content linearly in response to increasing P rates with overall higher FW yields and P concn in strain (a). A marked reddening of A. pinnata was noted in treatments with low levels or no P. The colour changes took place within 4 to 7 days after transfer to the experimental plots and spread from the centre of a frond outwards. The percentage red colour in the A. pinnata cover decreased with increasing P rates but remained at a high level in the 3rd crop. The possibility of growing A. pinnata prior to a rice crop as a green manure is discussed in relation to competition for K.