Sylvia Kratz

Rock phosphates and P fertilizers as sources of U contamination in agricultural soils

U concentrations were analyzed in a set of mineral fertilizers with and without P and compared to U concentrations in various organic fertilizers. Mean concentrations between 6 and 149 mg/kg U were found in P containing mineral fertilizers, while mean concentrations in mineral fertilizers without P were below 1.3 mg/kg U. Mean U concentrations in farmyard manures did not exceed 2.6 mg/kg U. As a consequence, an average P dressing of 22 kg/ha P would charge the soil with up to 17-61 g/ha U when added as mineral fertilizer but less than 10 g/ha when given as farmyard manure or slurry. Expected U uptake by crops is less than 1 g/ha U.

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.

Heavy Metal Loads to Agricultural Soils in Germany from the Application of Commercial Phosphorus Fertilizers and Their Contribution to Background Concentration in Soils

Statistical data on the annual sale of phosphorus (P) containing mineral fertilizers in Germany combined with data on toxic elements in mineral fertilizers allow an estimation of heavy metal loads to agricultural land. For the time period from 1950/51 to 2009/2010, the mean annual loads of the elements As, Cd, Cu, Ni, Pb, U and Zn to agricultural land in Germany exclusively from the application of P fertilizers amounted to: As 40, Cd 22, Cu 95, Ni 54, Pb 11, Zn 431 and U 114 t/yr, while maximum values reached: As 73, Cd 42, Cu 146, Ni 90, Pb 20, Zn 764 and U 228 t/yr. Depending on the soil group looked at, the contribution over the last sixty years of mineral fertilizer bound heavy metal inputs to average background concentrations of agricultural in Germany ranges between 0.4–1.4% for As, 3.5–12.3% for Cd, 0.2–1.1% for Cu, 0.03–1.5% for Ni, 0.03–0.1% for Pb, 0.3–1.8% for Zn and 4.4–13.7% for U. It can be concluded that there is an important urgent need to limit the concentration of these elements in mineral fertilizers given that these heavy metals are toxic, causing harm to the environment and public health and for example the preferable pathway for Cd into the food chain is the soil-plant system, and for U intake by drinking water.

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.

Trace Element Contaminants and Radioactivity from Phosphate Fertiliser

A risk ranking model was developed to provide a systematic evaluation of the range and quantity of 28 elemental contaminants applied to land in New Zealand and applied to mineral P fertilisers. The methodology is transparent, flexible and robust and allows contamination issues to be ranked according to their real or potential impact. The quantitative ranking model is based on the relative importance of each element in relation to accumulation in soil, transfer to water or uptake by plants, toxicity to soil organisms, plants and people, and the contribution of any radioactive isotopes. The highest risk score for potential environmental significance of P fertiliser borne trace element contaminants was found for uranium, followed (in decreasing order) by cadmium, mercury, boron, fluoride, selenium, arsenic, silver and rare earth elements. The lowest score (rank 28) was attributed to strontium.