Type of experiment
In this research, pot experiment was conducted to evaluate the effect of BPB on the growth of Ipomoea aquatica. Ipomoea aquatica was taken as the test crop, because this plant is very common and popular. Moreover, it is convenient to grow in this area and very fruitful to observe the growth performance under different treatments.
Collection of soil and banana peel waste
Soil used in the experiment as the growth medium was collected from the surface of the agricultural field inside the Khulna University campus, Khulna, Bangladesh by following the procedure as suggested by USDA (1951). Banana peel wastes were collected from nearby tea stalls inside the Khulna University campus.
Soil sample preparation
The collected soil sample was processed in the laboratory of Soil, Water and Environment Discipline of Khulna University. The sample was opened in the laboratory and air-dried by spreading on a sheet of plastic paper. After air drying, the larger aggregates were gently broken by a wooden hammer, then the soil sample was passed through a 0.5 mm sieve and some basic properties were analyzed (Table 1).
Production of BPB
Banana peel biochar was produced through slow pyrolysis process. Collected raw banana peels were passed through a very low oxygen-containing decomposer container heating through a gas stove for 2 h at about 400 °C temperature. Then, the charred materials were grinded and passed through a 0.5 mm sieve. The chemical properties (Table 2) of sieved BPB was analyzed in the INARS laboratory under Bangladesh Council of Scientific and Industrial Council (BCSIR) and applied into the soil to evaluate the growth of Ipomoea aquatica.
Experimental layout
Fifteen earthen pots (2-L) were collected for this experiment. One kg soil was poured into each pot. Three rates (1, 2 and 3% w/w) of BPB were applied in soil as treatment along with control (no BPB) and three replications for each treatment. After mixing of BPB into the soil, the pots were kept in field capacity for 7 days. Then, the seeds of Ipomoea aquatica, collected from local market, were sown in each pot. Only tap water was used for irrigation when it was needed and same volume of water was applied for all the pots. The plants were thinned when it was grown enough. No pest damage was observed during the growth period. Forty-two days after seed sowing the plants were harvested by cutting the stems about 2-cm above the ground. Different growth parameters were measured and recorded.
Biochar from banana peel waste was highly rich with potassium (K) content which can be used as a source of K for plants in field. Another separate pot experiment was also conducted to evaluate the efficiency of BPB to recover K. Nine earthen pots (2-L) were collected for this experiment. One kg fresh sand after washing with distilled water was poured into each pot. Banana peel biochar and KCl were applied for K source in sand medium as treatment along with control (neither BPB nor KCl) and three replications for each treatment. Potassium was applied as rated 10 t ha−1 and BPB was also applied as required amount by adjusting with K fertilizer considering the K content in BPB. Then, seeds of Cucurbita moschata as an experimental plant were collected and sown in each pot to run the experiment. Cucurbita moschata is a high K demanding crop that is why it was selected for this experiment to observe the K deficiency symptoms clearly. Seedlings were irrigated in every day by same volume of water at same time. Finally, seedlings were allowed to grow to observe the K deficiency symptoms in the growing plants.
Laboratory analyses
Soil textural class was determined by hydrometer method using Marshal Triangle (Gee and Bauder 1986). Soil pH and EC was measured as soil:water ratio 1:2.5. Total nitrogen of the soil and BPB were determined by the Micro-Kjeldahl method following H2SO4 acid digestion as suggested by Jackson (1973). Soil organic matter was determined by wet oxidation with potassium dichromate and sulfuric acid system. Available phosphorus was extracted from the soil with 0.5 M NaHCO3 at pH 8.5 (Jackson 1973) and p was determined by the ascorbic acid blue color method (Murphy and Riley 1962). Total Na, K, Ca, Mg, Mn, Zn, Fe concentration of BPB was measured through digestion with concentrated HNO3 and HCl (1:3) acid system, by Atomic Absorption Spectrophotometer (Imamul Huq and Alam 2005).
Statistical analyses
Statistical analyses were performed using SPSS 13.0 version and MS Excel 2010 software. Graphs were drawn using OriginPro8 (Origin Lab, Northampton, MA, USA). Analysis of variance (ANOVA) was used to test the BPB treatment effects on plant growth parameters as height, leaf number, fresh weight and dry weight. The least significant difference (LSD at p = 0.05) test was applied to assess the differences between treatment means (n = 3).