# Portable X-ray Fluorescence Analysis of Organic Amendments: A Review
## 4 Conclusions  
The mentioned studies show that pXRF can be a suitable technique for the elemental analysis of organic amendments. The speed and ease of performing the analysis, its competitive price, and the absence of polluting residues from the processing of the samples are evident. However, the studies undertaken so far in which this technique has been applied to organic amendments, such as compost, sewage sludge, manure, growing media, or organic fertilizers, have been very few. These studies have shown:1. The technique generally correctly measures elements such as Fe, Pb, Zn, Mn, Ca, and K; offers contradictory results for elements such as Cr, Ni, and As; and is not suitable due to its high LOD for Cd and Hg.
2. The reasons for the poor performance of the technique with certain elements or under certain measurement conditions are varied: * A mismatch between the total amounts measured with pXRF and those soluble in _aqua regia_ used as the reference concentrations in many studies. * The different internal calibration procedures used in different models of instruments. * The unknown effect of measurement conditions such as scan time, sample size, and sample thickness.
3. A not well-known effect of the influence of the high content of organic matter that characterizes this type of sample on the fluorescence signals.
4. The effect of the influence of moisture content on the fluorescence signals, and if there could be some kind of interaction with organic matter, is not known. At the moment, this represents a great challenge for measurements in the field.
5. Despite the poor performance, the prediction using chemometric procedures of other physicochemical properties of organic amendments (such as organic matter, electrical conductivity, cation exchange capacity, moisture content) has given encouraging results. Among these properties would be the contents of elements such as N, Cd, or Hg not directly measurable by pXRF instruments or below their detection limits.  
Probably, the achievements obtained by the technique favor its future use to be more common in the characterization of organic amendments. It would be necessary to develop a standardized protocol for this type of matrix. As the first requirement for this, it is necessary to have certified reference materials (CRM) of organic amendments that serve as calibration/verification checks, since soils or sediments are mostly used. Descriptions of methodology in pXRF studies need to be more complete, both in terms of the actual implementation of pXRF methods and contrasting analytical methodology. It is common for studies to omit the sample digestion procedures used, mentioning only the analytical instrument for the final determination (AAS, ICP-OES, etc.).  
The complementarity of the pXRF technique with other fast and portable spectroscopic techniques such as those based on MIR or vis-NIR, which are already being explored in other fields, can offer important advantages in the case of organic amendments. NIR or MIR spectroscopic techniques offer information on the organic matrix while fluorescence does it on the inorganic constituents, so their association could pave the way to a green laboratory, free of toxic laboratory waste and able to carry out in situ measurements aimed at decision making.  
This research was funded by European Union's Horizon 2020 research and innovation programme: Grant agreement No 862695, Project EJP Soil ProbeField.  
Not applicable.  
The author belongs to the Interdisciplinary Platform SosEcoCir, CSIC.  
The author declares no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.