Keywords: Ash, animal manure, biochar, phosphorus cycling, phosphorus recovery, resource use efficiency, sewage sludge, soil phosphorus, waste, waste treatment
This paper addresses the question ‘Can fertilisers from processed by-products and wastes become a competitive alternative for conventional mineral phosphorus fertilisers?’ In answering this question, we take a global perspective. Firstly, we briefly summarise the global phosphorus (P) stocks and flows. We then describe the main by-products and wastes available in the world that may be used as P fertiliser. Next, we discuss the possible processes and treatments used before by-products and wastes are used as fertilisers, and consider the P availability of these fertilisers to crops. We argue that fertilisers from (processed) by-products and wastes may become a competitive alternative for ordinary mineral P fertilisers only regionally. We proceed by addressing the question ‘How to use ordinary P fertilisers from rock phosphate and P fertilisers from by-products and wastes in an optimal way, from the viewpoints of food security, resources use efficiency, equity, environmental sustainability and feasibility?’
Animal manure is by far the largest by-product and source of P. Estimates suggest that the total amount of P in manure produced per year (20-30 Mt P) is rather similar to the annual amount of ordinary P fertiliser consumed per year (15-20 Mt). However, only a fraction of the manure is collected and applied to crop land, while the remainder is dropped in pastures or wasted. Sewage sludge (3-5 Mt) is a much smaller source. Slags, ashes, chars and bone meal are other relatively small and uncertain sources. The availability of P from by-products and wastes is highly variable, but our compilation of literature results indicate that the availability is as large as that of ordinary P fertiliser.
The major advantage of increasing the use of P fertilisers from by-products and wastes is the contribution to increasing resource use efficiency, to decreasing surface water eutrophication and associated biodiversity loss, and to decreasing the rate of P rock depletion. The major disadvantage of using P fertilisers from by-products and wastes is their variable nutrient composition and P availability, potential presence of contaminants, unknown status and low acceptance by farmers, and the lack of a proper marketing and distribution infrastructure.
In the end, society will need the P fertilisers from by-products and wastes, because of the likely future depletion of P rock deposits. Strategies should be developed for optimal, long-term use of the various possible P sources. It is argued that an international agreement and institution might help here.
Oene Oenema, Wim Chardon, Phillip Ehlert, Kimo van Dijk, Oscar Schoumans and Wim Rulkens, Wageningen University, Alterra, P.O. Box 47, NL-6700 AA Wageningen, The Netherlands.
40 pages, 6 figures, 8 tables, 96 references.