The Effect of Some Thai Medicinal Herb Extracts on Nitrification Inhibition

This study aimed to determine the effect on nitrification inhibition of some Thai medicinal herb extracts. The experimental design was completely randomized design, consisted 33 treatments with 4 replications. The experiments were performed in a laboratory, using surface soil of Yang Talat soil series as media. In each treatment, soil sample (100 g) was mixed thoroughly with 1 ml of herbal extract and 50 mg kg-1 of ammonium sulphate. The mixture was then incubated at ambient temperature. Ammonium oxidizing bacteria (AOB), nitrite oxidizing bacteria (NOB), ammonium-N (NH4+-N), nitrate-N (NO3--N), and soil pH were determined at week 1 to 4 after incubation. The results showed that the numbers of AOB in soil sample mixed with galanga stem decreased during the first 2 weeks. It was obvious that after the first week of incubation, the activity of NOB in soil samples mixed with extracts of ringworm bush leaf, heart- leaved moonseed stem, mangosteen fruit, kariyat leaf and galanga rhizome was suppressed. Soil samples containing herbal extracts had higher concentraions of NH4+-N and lower concentraions of NO3--N than those of control. The highest amount of NH4+-N was found in clove flower treated soil during the first 2 weeks of incubation. Kariyat leaf treated and cinnamon bark treated soil samples contained the highest amount of NH4+-N in weeks 3 and 4, respectively. Samples with extracts of ringworm bush leaf, mangosteen fruit, Thai copper pod leaf, Indian mulberry leaf, lemon grass leaf, bitter cucumber fruit, egg woman stem, fingerroot stem, fingerroot rhizome and hog plum leaf contained the lowest amount of NO3--N during the first 3 weeks. The concentraion of NO3--N in heart- leaved moonseed stem treated soil was the lowest in the last 3 weeks. The highest ratio of NH4+-N (100 %) and the lowest ratio of NO3--N (0.0 %) to inorganic N were found in samples with extracts of ringworm bush leaf, mangosteen fruit, Thai copper pod leaf, Indian mulberry leaf, lemon grass leaf, bitter cucumber fruit, egg woman stem, fingerroot stem, fingerroot rhizome, hog plum leaf and heart-leaved moonseed stem for 3 weeks. At early stages of incubation, low pH of herbal soil samples were observed. The pH of those samples, however, increased over time.