Suitability of azolla(azolla pinnata) for biogas slurry enrichment

Abstract

A study on “Suitability of azolla (Azolla pinnata) for biogas slurry enrichment” was conducted during 2010-11 at College of Horticulture, Vellanikkara to evaluate the optimum ratio of cow dung and azolla for biogas slurry enrichment, to identify the best material to prepare quality organic manure from the enriched slurry and also to adjudge the crop response to enriched organic manure. To attain the objectives, three separate experiments were conducted in a phased manner.

In order to determine the optimum ratio of cow dung and azolla for biogas slurry enrichment, the floating drum biogas digesters of 0.5 m3 capacity were used. The different proportion of the substrates tried were cow dung alone and cow dung and azolla in 1: 0.25, 1: 0.5, 1: 0.75 and 1: 1 ratio. The results indicated that mixing of cow dung and azolla in a proportion of 1: 0.5 produced the highest volume of gas (0.29 m3 kg-1 TS) in 20 days of Hydraulic Retention Time (HRT). But the proportion of 1: 1 favored in terms of N (3.44 per cent) content in slurry followed by 3.23 per cent in 1: 0.75 ratio. These treatments were designated as enriched slurry I (3.44 per cent N, 0.74 per cent P and 2.93 per cent K) and enriched slurry II (3.23 per cent N, 0.77 per cent P and 2.91 per cent K). Addition of azolla increased pH from 7.1 to 7.8 but decreased the total solids of slurry from 5.40 per cent in cow dung alone to 2.68 per cent in the ratio of 1: 1.

For identifying the best material for preparing quality manure from the enriched slurry, it was mixed with saw dust, coir dust and powdered coconut leaves which varied in the nutrient content and C/N ratio. Based on the nutrient content, powdered coconut leaves were identified as the best material and the requirement was 612 g per litre in enriched slurry I and 607 g per litre for enriched slurry II.

Crop response to the enriched manure I and II was assessed in pot culture with rice (variety- Jyothy) as the test crop. The treatments were control (T1), NPK+FYM (T2), FYM alone (T3), enriched manure I (T4), enriched manure II (T5), enriched manure I+NPK (T6) and enriched manure II+ NPK (T7). The grain (44.97 g pot-1) and straw (72.73 g pot-1) yield was maximum when the crop was nourished with enriched manure II in association with the recommended dose of chemical fertilizers. Crop uptake of N (4260 mg pot-1) and K (4263 mg pot-1) was also the highest in this treatment.

Soil physico-chemical properties were also favorably influenced by the manure application. A shift in pH from 4.7 to 5.4 was obtained consequent to applying enriched manure I. Soil status of available N (1536 mg kg-1) and available K2O (471 mg kg-1) was the highest when enriched manure II was applied along with the recommended dose of chemical fertilizers. The content of available P2O5 in soil was the maximum (52.7 mg kg-1) when chemical fertilizers were excluded from this treatment.