Characterization of Nitrogen Release Properties of Slow-Release Fertilizers.

Slow-release fertilizers marketed to the public usually include a claim that nutrient release will last for a specific time period, e.g. 6, 9, or 12 months. However, no official method exists that can verify these claims. A slow-release fertilizer taskforce was established in 1994 by the Association of American Plant Food Control Officials in order to address issues regarding the effective regulation and analysis of slow-release fertilizer materials. As a response to this taskforce, a long-term (180-d) soil incubation method has been developed to characterize nutrient release rates as a function of time. In addition, a relatively short-term (74 hr) extraction method has been developed to assess nutrient release under accelerated laboratory conditions. Over the last three years, both methodologies have been optimized by evaluating the effect of critical variables on nitrogen release characteristics. Different variables of the soil incubation (soil sample size, soil type and incubation temperature) and the laboratory extraction methods (sample size, extraction temperature and extraction time) have been investigated. The development and optimization results of these methodologies will be discussed in detail. Ultimately, it is intended that these methodologies will be accepted as an official method to verify nitrogen release claims placed on slow-release fertilizers.