Judith Schick

Trace elements in rock phosphates and P containing mineral and organo-mineral fertilizers sold in Germany.

68 rock phosphates and 162 P containing (organo-)mineral fertilizers sold in Germany were evaluated with regard to trace element contents. While Al, As, B, Be, Cd, Cr, Mo, Ni, Pb, Sb, Se, Tl, U, and Zn were higher in sedimentary than in igneous rock phosphates, the opposite was true for Co, Cu, Sn, Mn, Ti, Fe, and Sr. Comparing element concentrations to the currently valid legal limit values defined by the German Fertilizer Ordinance, it was found that some PK and many straight P fertilizers (superphosphate, triple superphosphate, partly acidulated rock phosphates) exceeded the limit of 50 mg Cd/kg P2O5. Mean values for As, Ni, Pb, and Tl remained below legal limits in almost all cases. While no legal limit has been defined for U in Germany yet, the limit of 50 mg U/kg P2O5 for P containing fertilizers proposed by the German Commission for the Protection of Soils was clearly exceeded by mean values for all fertilizer types analyzed. A large share of the samples evaluated in this work contained essential trace elements at high concentrations, with many of them not being declared as such. Furthermore, trace elements supplied with these fertilizers at a fertilization rate leveling P uptake would exceed trace element uptake by crops. This may become most relevant for B and Fe, since many crops are sensitive to an oversupply of B, and Fe loads exceeding plant uptake may immobilize P supplies for the crops by forming Fe phosphate salts. The sample set included two products made from thermochemically treated sewage sludge ash. The products displayed very high concentrations of Fe and Mn and exceeded the legal limit for Ni, emphasizing the necessity to continue research on heavy metal removal from recycled raw materials and the development of environmentally friendly and agriculturally efficient fertilizer products.

Contamination of organic nutrient sources with potentially toxic elements, antibiotics and pathogen microorganisms in relation to P fertilizer potential and treatment options for the production of sustainable fertilizers: A review

Organic nutrient sources such as farmyard manure, sewage sludge, their biogas digestates or other animal by-products can be valuable fertilizers delivering organic matter to the soil. Currently, especially phosphorus (P) is in the focus of research since it is an essential plant nutrient with finite resources, estimated to last only for some more decades. Efficient utilization of organic P sources in agriculture will help to preserve P resources and thereby has the potential to close nutrient cycles and prevent unwanted P-losses to the environment, one of the major causes for eutrophication of water bodies. Unfortunately, organic P sources usually contain also various detrimental substances, such as potentially toxic elements or organic contaminants like pharmaceuticals as well as pathogenic microorganisms. Additionally, the utilization of some of these substrates such as sewage sludge or animal by-products is legally limited in agriculture because of the potential risk to contaminate sites with potentially toxic elements and organic contaminants. Thus, to close nutrient cycles it is important to develop solutions for the responsible use of organic nutrient sources. The aim of this review is to give an overview of the contamination of the most important organic nutrient sources with potentially toxic elements, antibiotics (as one important organic contaminant) and pathogenic microorganisms. Changes in manure and sewage sludge management as well as the increasing trend to use such substrates in biogas plants will be discussed with respect to potential risks posed to soils and water bodies. Some examples for abatement options by which contamination can be reduced to produce P fertilizers with high amounts of plant available P forms are presented.

P Solubility of Inorganic and Organic P Sources

P solubility of mineral and organic fertilizers can be estimated by a variety of different chemical extraction methods. In Europe, the characterization of P solubility in fertilizers is regulated in the European fertilizer regulation 2003/2003 for commercial fertilizers, assigning different methods to the various fertilizer types. Relationships between chemical solubility and agricultural performance/P availability for plants have been documented in numerous pot and field trials. Non-commercial fertilizers like farmyard manures and slurries, on the other hand, as well as “new products” based on recycling materials such as sewage sludge ash, are not included in this regulation yet, and a number of methods, designed for mineral fertilizers as well as for P solubility in soils, have been tested over the last couple of years to adequately characterize these products. This review gives a critical overview of the chemical extraction methods currently practiced and/or tested in the European countries, and their performance as estimates for plant availability of fertilizer P from inorganic and organic P sources.

Determination of Plant Available P in Soil

Fertilization should be based on a proper diagnosis of the plant nutritional status. Without, there is always the risk of under fertilization causing economic losses by not exploiting the yield potential of the site and its crop. On the opposite, over fertilization not only hampers fertilizer economy through inefficient nutrient rates, but also causes serious environmental impacts on neighboring ecosystems. There are four basic diagnostic methods each of which has its advantages and disadvantages: ceteris paribus fertilizer trials are complicated and time consuming delivering results far too late for fertilization planning. Visual assessment is fast but requires experts eyes and works only for a few nutrients and only when severe nutrient deficiency occurs. Plant analysis is accurate, however, bound to well defined growth stages, but shows only the actual stage of supply with little information about available reserves in the soil or growth media and is also too late with its results for practical fertilization. Last but not least: analyzing the soil can be independent of crop development, well in advance to contribute for the fertilizer design for the actual crop and offers insight into the reserves in the substrate. However, its value for fertilization planning is strongly depending on the quality of the calibration of the results. Of all essential plant nutrients phosphorus is the one for which the most and intensive research work on assessing soils has been conducted in the past. This chapter introduces the basic conceptions of soil analysis for plant available P, provides an overview on available methods and discusses their advantages and disadvantages.