Combined Waste Resources as NPK Fertiliser: Results from a Pot Experiment
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- Master's theses (IPM) 
Returning waste to agricultural land is a holistic systems approach to meet the challenging task of future food supply. As nutrient contents in organic material often are unbalanced in comparison to the plants’ needs, the aim of this paper was to study the fertilisation effect of waste-based NPK compound fertiliser products and their potential to substitute conventional fertilisers in agricultural plant production. A pot experiment with Italian ryegrass (Lolium multiflorum var. italicum) as experimental crop was conducted where the four N-rich waste resources meat and bone meal (MBM), composted fish sludge (CFS) and two types of industrial compost (Dynea 2009 and Dynea 2004) were tested alone and in combination with K-rich bottom wood ash (BWA). Fertilisation levels (150 kg N ha-1 + 120 kg K ha-1; 300 kg N ha-1 + 240 kg K ha-1) were based on total N and K content in Nrich waste and BWA, respectively. Treatments with BWA, artificial compound fertiliser (minNPK) and calcium nitrate (minN) only, as well as an unfertilised control were used as references. Availability of mineral N was the key limiting factor to plant growth. Mineral fertiliser treatments resulted in the highest total yields being significantly different from all waste combinations Plants that received MBM or CFS fertilisation had good and even biomass production throughout the season, but fertilisation effects were limited by mineralised N. Mineral fertiliser equivalents of MBM and CFS treatments were between 48-73%. MBM treatments increased the amount of soluble P in plant-soil systems to amounts that were higher than total P applied with the fertiliser product. MBM might therefore be a more valuable alternative P-fertiliser than one assumed so far. CFS seems to be more appropriate as an ingredient in alternative NPK fertiliser products than MBM because of reduced effects on residual P in the soil, good availability of P in the material, as well as a wider N:P ratio and relatively high initial amounts of mineral N. Dynea composts had poor fertilisation effects and can therefore rather be classified as soil conditioners than as fertilisers. K fertilisation effect of BWA was hidden by sufficient K supply from the soil but K-AL values of soils that were fertilised with BWA were significantly higher than soils of unfertilised control treatments. MinN + BWA treatments had poor establishment due to local pH increase and initial P deficiency, but leaching or denitrification of NO3-N was avoided so that the treatments still resulted in vigorous plant growth towards the end of the season. Before waste-based NPK fertiliser products are ready for commercial production, further studies have to be done on the optimisation of their N fertilisation value.