Abstract
With the increasing popularity of local blending of fertilisers, the fertiliser industry faces issues regarding quality control and fertiliser adulteration. Another problem is the contamination of fertilisers with trace elements that have been shown to subsequently accumulate in the soil and be taken up by plants, posing a danger to the environment and human health. Conventional characterisation methods necessary to ensure the quality of fertilisers and to comply with local regulations are costly, time consuming and sometimes not even accessible. Alternatively, using a wide range of unamended and intentionally amended fertilisers this study developed empirical calibrations for a portable handheld X-ray fluorescence (pXRF) spectrometer, determined the reliability for estimating the macro and micro nutrients and evaluated the use of the pXRF for the high-throughput detection of trace element contaminants in fertilisers. The models developed using pXRF for Mg, P, S, K, Ca, Mn, Fe, Zn and Mo had R2 values greater or equal to 0.97. These models also performed well on validation, with R2 values greater or equal to 0.97 (except for Fe, R2val = 0.55) and slope values ranging from 0.81 to 1.44. A second set of models were developed with a focus on trace elements in amended fertilisers. The R2 values of calibration for Co, Ni, As, Se, Cd and Pb were greater than or equal to 0.80. At concentrations up to 1000 mg kg-1, good validation statistics were also obtained; R2 values ranged from 0.97–0.99, except in one instance. The regression coefficients of the validation also had good prediction in the range of 0–100 mg kg-1 (R2 values were from 0.78–0.99), but not as well at lower concentrations up to 20 mg kg-1 (R2 values ranged from 0.10–0.99), especially for Cd. This study has demonstrated that pXRF can measure several major (P, Ca) and micro (Mn, Fe, Cu) nutrients, as well as trace elements and potential contaminants (Cr, Ni, As) in fertilisers with high accuracy and precision. The results obtained in this study is good, especially considering that loose powders were scanned for a maximum of 90 seconds without the use of a vacuum pump.
Highlights
Fertilisers are an important input used in modern agriculture worldwide
PXRF can be used as a rapid screening tool to give an indication of the elemental composition of fertilisers
Poor calibration statistics were obtained for Na, Al, Ni, Se, Cd and Pb. This could be attributed to the fact that, the “Fertiliser” calibration targeted macro and secondary elements and the fertilisers used did not have a uniform distribution of portable handheld X-ray fluorescence (pXRF) analysis of nutrients and trace element contaminants in fertilisers
Summary
Fertilisers are an important input used in modern agriculture worldwide. According to the FAO, the Fertiliser Outlook Expert Group forecasts that the world will need 201 million tonnes of N, P and K for fertiliser use in 2022 [1]. Straight fertilisers that contain only one nutrient (for example urea, triple superphosphate or potassium chloride), binary fertilisers containing two macro nutrients (for example diammonium phosphate), compound fertilisers (mostly containing NPK in variable concentrations but each granule contains all elements), and blended fertilisers are commonly used [2]. The fraudulent practice of fertiliser adulteration involves adding extraneous material to a standard fertiliser to lower its nutrient concentration [2]. This is common in Africa and Asia even with commercial blends, which get re-packaged by stockists and sold in smaller quantities. The sales of substandard fertiliser products reduce farmers’ income, undermine their trust in fertilisers and might even slow down technology adoption [5]
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