Nitrogen release characteristics of commercial organic fertilizers in turfgrass

Abstract

Although organic fertilizer sources are widely marketed for application to turfgrasses, there is limited research that examines their nitrogen (N) release characteristics. For this research project, ‘organic’ refers to commercial fertilizers which are manufactured from wastes such as sewage sludge (bio-solid), poultry by-products (manure, litter, feather-meal), or other by-products (whey, meals, oils). The process of mineralization, which is the microbially-mediated conversion of organic N to plant available ammonium-N (NH4) and nitrate-N (NO3) forms, is the controlling factor of N release from organic fertilizers. Thus, organic fertilizers behave as a slow release N fertilizer, with a delayed greening response, low burn potential, and varying N release rates in different environments. Of the environmental variables, temperature has perhaps the greatest effect on microbial activity, which in turn controls inorganic N release from the organic sources. Although N mineralization from raw wastes such as manure or bio-solids has been widely studied, N release from processed, commercially-blended organic fertilizers has been less studied, especially in turfgrass systems. Two separate three-month studies evaluated the effects of organic fertilizers on hybrid bermudagrass growth and performance. In general, application of sources with large portions (or 100%) of soluble N (urea and Scotts Turf Builder) produced turf with a darker green color, higher shoot density and greater clipping yield than plots fertilized with large portions of organic N. Although organic fertilizers are often touted as maintaining color for longer periods of time, it was not the case in this study, as urea provided consistent high quality color for the same 8-10 week period as the organic sources. There were few differences in soil N, C, or microbial biomass due to N source. Six individual ten-week incubation studies (two repetitions per temperature) were performed at 15, 25, and 35°C. Mineralization of organic N was measured and it was significantly affected by treatment and a treatment x temperature interaction. Optimum inorganic N release was observed at 25°C, with the least amount of N mineralization occurring at 15°C. Ammonium-N was largely produced during the first two weeks after application, and then converting to mostly NO3-N.