Phosphorus Determination in Forage and Manure using Portable X-ray Fluorescence Spectroscopy to Support Comprehensive Nutrient Management Planning

Abstract

Phosphorus Determination in Forage and Manure using Portable X-ray Fluorescence Spectroscopy to Support Comprehensive Nutrient Management Planning Yadav Sapkota Phosphorus (P) runoff from agricultural fields is a major cause of water quality degradation problems. A P mass balance across the farm could maintain profitability and sustainability of animal-based farms and minimize water quality problems. Comprehensive Nutrient Management Plans (CNMPs) require mineral composition, especially P, data on soil, forage, and manure samples for accurate planning. However, traditional wet chemical methods of P determination are costly, time-consuming, and generate hazardous waste. X-ray fluorescence (XRF) could overcome many of these disadvantages and allow rapid determination of P concentrations. Portable XRF (PXRF) units are Energy Dispersive (ED) systems with low power Xray tubes (10-40W) in comparison to benchtop units (50-300W). They are light and convenient to use either in benchtop or field-analysis modes. When a sample is scanned, the resulting spectrum identifies the element (peak position or energy); area under the peak (intensity) is proportional to concentration. A few studies have indicated the possible use of PXRF for the analysis of plant tissue and compost samples. However, there is a lack of information for analysis of heterogeneous forage and manure samples, including optimal sample preparation (particle size and moisture content) and instrumental parameters (scan time). The objective of this study was to evaluate the effect of manure moisture content and forage particle size on elemental concentrations determination using PXRF in benchtop mode. Manure samples (n=40) were oven dried at 50 and ground to 0.5 mm size and adjusted to four gravimetric moisture ranges: 10-20%, 20-30%, 40-50%, and 60-70%. Dry hay samples (n=42) were oven dried (60 0 C for 3 days) and ground into two particle sizes (0.25-0.5 mm and 1-2 mm). Prepared samples were scanned by PXRF using a vacuum (<10 torr) and without a filter. Samples were placed in a sample cup over a thin proline X-ray film and scanned for 180s. Some forage samples (n=29) were also scanned for 60s and 120s for scan time analysis. Spectra for each analysis, photon counts (intensity) and P concentration were collected using the S1PXRF software. Reference standards were prepared by microwave digesting forage and manure samples in triplicate followed by elemental quantification through Inductively Coupled Plasma Optical Emission Spectrometry (ICP). Regression analysis, two sample t-test, matched paired t-test, and repeated measures ANOVA were used for data analysis. ICP measured P, K, Ca, Mg, Fe and Cu were in close agreement with West Virginia Department of Agriculture-measured concentration in manure samples. Dried and ground manure samples produced a stronger relationship with ICP-determined P, K, Ca, Fe, Cu, Zn (r 2 > 0.90) and Mg (r 2 =0.59). Presence of moisture negatively affected elemental determination in manure samples. The strength of the relationship decrea...