Azolla biofertilization to increase rice production with emphasis on dual cropping

Summary

The response of Azolla dual cropping with three rice varieties Annapurna, Ratna and Jaya of 105, 120 and 140 days duration respectively with and without application of nitrogen fertilizer was studied under field conditions. Inoculation of 500 kg fresh Azolla ha⁻¹ after a week of transplanting of rice seedlings, covered the field and formed mat in 20 days producing on the average biomass of 10.1 to 14.6 t ha⁻¹ containing 28.0 to 32.9 kg N ha⁻¹ and decomposed after further incubation for a period of 15–20 days. Growth and N-uptake of rice showed that one layer of Azolla was equivalent to 30 kg N ha⁻ supplied as urea. Growth of Azolla, N-contribution and yield of rice varieties were also similar in both line and randomized planting methods. The same three varieties were also tried with two layers of Azolla, where Azolla basal + dual cropping gave better growth and yield of rice than twice dual cropping.

Growth, nitrogen content of Azolla and rice yield were found to be similar when treated separately with 4.4 kg P kg ha⁻¹ and 1000 kg ha⁻¹ fresh animal dung. Super phosphate (8.8 kg P) when applied in three splits (10 + 5 + 5) increased Azolla production 50% more than 8.8 kg P applied as basal.

In a separate trial square (20 × 20 cm) and rectangular (40 × 10 cm) rice plant spacings were used where Azolla production, nitrogen contribution and grain yield were similar in both spacings, although east-west direction of planting showed more Azolla production than north-south direction. Five cm of standing water supported better Azolla growth than 10 cm in the planted field.

Azolla growth, N₂ -fixation and its response to 85 days duration of rice variety culture-1 was tested in direct seeded crop. The growth and N2-fixation of basal + dual crop of Azolla was 39.20 t ha ⁻¹ and 67.58 kg N ha ^{− 1} which were almost comparable to 130 days rice variety IR-36 under transplanted condition where those values were respectively 46.7 t ha ⁻¹ and 80.94 kg N ha^{− 1}. The grain yield obtained by Azolla basal + dual incorporation was 70.08 and 83% more than the control in transplanted and direct seeded rice respectively whereas Azolla compost, Sesbania green manuring, Eichornia compost and BGA dual increased grain yield to 24.4%, 23.5%, 29.29%, 42.98% and 35.26%, 61.12%, 35.8%, 40.69% over control respectively in transplanted and direct seeded rice. Among the four A. pinnata isolates from Bangkok, Vietnam, Bangladesh and India when tested as basal and dual culture with rice IR-36, Bangkok and Vietnam isolates were superior in growth, N₂ -fixation and response to rice crop.

Effect of four herbicides namely, butachlor, benthiocarb, 2,4-DEE and 2,4-DNa was studied at the field recommended doses on fresh weight, dry weight, chlorophyll and N content of Azolla pinnata in unplanted and planted field, where inhibitory effects were observed to be in decreasing order and their effects were nullified after 28 days. Application of field doses of the pesticide carbofuran was found to encourage Azolla growth and N₂ -fixation.

Azolla dual cropping was found to be economical. The expenditure involved in production of inoculum + Azolla dual cropping, supplying almost 30 kg N ha⁻¹ in rice field was Rupees 38.12 ($4.08), whereas the cost of 30 kg N ha^{− 1} as urea was Rupees 140.00 ($14.99). The increase in grain yield at Azolla dual cropping unincorporated and 30 kg N ha^{− 1} urea was 0.41 and 0.4 t ha^{− 1} over control respectively.