Nimin andMentha spicataoil as nitrification inhibitors for optimum yield of Japanese mint

In order to improve nitrogen utilization efficiency of crops, which seldom exceeds 50%, numerous synthetic chemicals have been examined by several workers, for inhibition of either urea hydrolysis or nitrification in soils. However, for nitrification inhibitors to be popular in developing countries it must be cost effective and biodegradable. Field experiments were conducted for two years (1997–98) to evaluate the performance of two natural products, Mentha spicata oil and nimin (tetranortriterpinoids), an alcohol extract of Neem (Azadirachta indica Juss) as nitrification inhibitors. Prilled urea was coated with essential oil of Mentha spicata, nimin and a synthetic inhibitor dicyandiamide (control) at the rate of 1% on w/w basis and their effects on herb and essential oil yield and nutrient accumulation in Japanese mint (Mentha arvensis L.) was studied. The natural products significantly increased the herb and essential oil yield of mint compared to prilled urea applied without any coating material. The natural products were also found as effective as DCD. As expected, nitrogen (N) accumulation in the plant also significantly increased with the coating of urea. The average apparent N recovery was estimated to be about 50%, 42%, and 34% at 100, 200, and 300 kg ha− 1 rate of N application, respectively. Average NO3–N formation was low in coated urea treated soils. The pattern of inhibition of nitrification and increase in fertilizer N use efficiency by nimin and M. spicata oil suggest that these materials may be used for retardation of nitrification in soil.