Assessment of bat guano as source of nutrients for rice production

Abstract

Rice production in Tanzania is generally low and continuing to decline partly due to low soil fertility. Smallholder farmers are continuing cultivating in the same area with little or without use of fertilizer due to limited access to costly industrial fertilizers. This trend results into continuous mining of plant nutrients such as phosphorous (P) and other essential nutrients elements which are needed by rice for growth and development; and consequently decline in soil fertility and hence rice production. The country has several bat guano deposits, but none of them are adequately exploited as alternative source of plant nutrients and soil amendment to improve the soil properties because of lack of information on their potential suitability. This study, therefore, aimed to evaluate the agronomic potential of selected bat guano for enhancing soil fertility and improving rice production. Selected bat guano in Tanzania were studied in a laboratory incubation experiment for 112 days to assess P release patterns and establish the pick periods of P mineralization in Department of Soil and Geological Sciences (DSGS) laboratory. Guanos used in this study were from Kisarawe cave A (BGK-A), Kisarawe cave B (BGK-B) and Sukumawera cave (BGS) with 8.55, 7.03 and 3.45 % total P, respectively. Guano from each deposit was mixed with soil at varying rates of 0, 10, 20, 40 and 80 mg P 200 g -1 soil. The incubation experiment was arranged as a 3x5 factorial experiment in randomized complete design (RCD) with three replications. In addition to incubation experiment pot experiment was setup to evaluate the response of rice to selected bat guano as a source of phosphorous. The experiment was carried out in a screen house at Sokoine University of Agriculture (SUA). The experiment was a 4 × 6 factorial in a Randomized Complete Block Design (RCBD) replicated three times. The treatments were three bat guano from Kisarawe cave A (BGK-A), cave B (BGK-B), and Sukumawera (BGS) and Triple Super Phosphate (TSP) as standard fertilizer all at six application rates (absolute control, 0, 10, 20, 40 and 80 mg P kg -1 soil). Bat guano was supplemented with urea (CO (NH 2 ) 2 ) which was applied at a rate of 400 mg N kg -1 and zinc sulphate (ZnSO 4 ) was applied at a rate of 2.5 mg Zn kg -1 . Rice SARO variety (TXD 306) was grown as a test crop. From the incubation experiment, results showed gradual increase of extractable/available P from 28 days to 84 days followed by a decrease in P release up to 112 days of incubation. In all days of incubation there was a highly significant interaction effect of P- sources and application rates (P < 0.05) on P released from guano applied at the rate of 80 mg kg -1 . The soil pH showed a very strong negative correlation with phosphorus released (r > 0.8) from the first day to 112 th days of incubation. The pot experiment results indicated that there was a significant (P < 0.05) effect of all three guano and standard P fertilizer (TSP) on dry matter accumulation at maturity in the order TSP > BGK-A > BGS > BGK-B. There was a significantly (P < 0.05) higher effect of P at 80 mg P kg -1 for all P sources on dry matter accumulation at maturity. Interaction effect of P sources and P rates was highly significant (P < 0.05) on dry matter accumulation at maturity. Plant physiological parameters (number of tillers, plant height, panicle height and number of panicles per plant), as well as rice grain yield were also significantly higher for plants receiving 80 mg P kg -1 for all P sources. Positive correlation between grain yield and other yield components was observed indicating better crop response to P sources and P rates applied. Both incubation and pot experiment results indicated guano to have similar effect to TSP in releasing P for rice; hence the potential for application as alterative P source. It was further observed that at application rate of 80 mg P kg -1 of guano, the studied guanos would release adequate phosphorous for plant growth. Because these results were obtained under controlled environment field trials are recommended to evaluate the response of rice and other crops to soil applied guano for meaningful recommendations to farmers.