Värdering av kvävet i organiska gödselmedel

Abstract: Within agriculture it is important to optimize the use of nitrogen, not only from an economic point of view but also to minimize damage to the environment. This can sometimes be difficult, especially within organic farming. Organic farmers are using increasing amounts of waste products from the food industry to cover their nitrogen needs. These different organic fertilizers release nitrogen at different rates and under specific soil conditions. Because of this, it would be beneficial to have a standardized method to measure the amount and rate of nitrogen mineralization from organic fertilizers. This project contributes to this knowledge by investigating whether relatively simple and repeatable laboratory methods can be used to estimate the nitrogen fertilizer value of organic fertilizers. Incubation in a climate chamber was used to measure the mineralization rate, while water extraction was used to investigate possible relationships with mineralization in the incubation for four organic fertilisers: chicken manure, cattle slurry, Biofer (bonemeal) and Vinasse (waste product from the yeast industry). The objective was to evaluate whether these methods could reliably predict mineralization under field conditions. Incubation results were therefore compared with those of prior incubations under natural temperatures in field conditions. The four fertilizers were mixed with soil and incubated in a climate chamber (15°C) for 56 days. Samples were removed on six occasions and the amount of mineral nitrogen was measured. The results showed that there was rather rapid mineralization from Biofer and Vinasse during the first two weeks (45-50 % of total N). After this period, no net nitrogen mineralization could be seen. Both chicken manure and slurry contained rather high amountsof mineralized nitrogen at the start (35-40 % of total N), but during the first week they showed net nitrogen immobilization, which is common with manure. In the second week the chicken manure showed rapid nitrogen mineralization (mineral N increased 10 percentage units to 50 % of total N), probably due to mineralization of urea, after which time net mineralization stopped. However, the slurry continued to show slow net mineralization (only a few percent) to the end of the 56-day period. Between 35-50% of the total nitrogen in all four fertilizers appeared to be in mineral form two weeks after application. To measure true mineralization, we would need more information on immobilization. The nitrogen appeared to be present in two different pools, one readily degradable and one more resistant to degradation. Incubation in the laboratory showed approximately the same mineralization potential (40-50 % of total N) as incubation at field temperature, although variation between fertilizers was larger in the field. Attempts were made to predict net N mineralization in terms of daydegrees, to allow laboratory values to be converted to field conditions. However, this proved difficult, probably because there are too many processes involved. Nitrification, on the other hand, seemed easier to predict in terms of day-degrees. Comparisons of incubation results with nitrogen and carbon in water extracts revealed a weak relationship between amount of total nitrogen extracted in warm water and amount of total mineralized nitrogen extracted in the incubation.