Wheat and teff grain mineral micronutrient concentration and field data from GeoNutrition on-farm field experiments in Western Amhara region, Ethiopia

Experimental design The experiments were undertaken in the main cropping seasons of both 2018 and 2019. There were two sets of experiments in each season, one with wheat as the experimental crop, and the second with teff. The same treatments established on farmers’ fields, were applied to both crops. The fields were on farms in specified communities (“sites”). Wheat experiments were performed in Debre Mewi and Markuma, and experiments with teff were performed in Debre Mewi and Aba Gerima. Each farmer provided a single field for the experiment, and one replicate of each treatment was established in that field. Treatments were thus laid out in randomized complete block design, replicated across farms. Farm fields were selected at random from among those in each of three landscape positions: foot slope, mid-slope, and hillslope (Amede et al., 2022, Desta et al., 2022). There were five farms per landscape position in each of the three districts, resulting in a total of 45 farms. Each treatment plot had 5 × 5m dimensions. In 2019, the site locations were revisited, but the treatments were established on different farms from those used in 2018 following similar patterns of landscape positions. In some cases, the same farmer was involved in both seasons, but with a different parcel of land. This was performed to avoid adding residual effects of fertilizer treatment into the factors to be considered when interpreting the results for the second season. There were 300 observations for teff (2 sites × 2 years × 3 landscapes × 5 farmers’ fields × 5 treatments) and for wheat (2 sites × 2 years × 3 landscapes × 5 farmers’ fields × 5 treatments), respectively. We also established farm history before selecting target plots and excluded farms where high fertilizer rates (>50 kg ha-1) were applied in the previous year. The replication over seasons provided information on the effect of adding fertilizer to soil which mainly contains the background concentration of those nutrients attributable to organic sources and minerals in its parent material. Similarly, a single replication of each treatment was established in each farm during the second season. The treatment was allocated to a plot independently and at random. The randomization was performed on the R platform (R Core Team, 2021), and the R code produced a plot showing the randomization which could be used in the field to record landmarks and other features to facilitate orientation on visits to the plots. The five treatments fell into three broad categories: (1) nitrogen (N) fertilizer rate; (2) micronutrient fertilizer application method; (3) sole or co-application of Zn and Se fertilizer. Sample characteristics Type of sample: Grain Sample collection: Grain samples were collected from representative heads of each treatment at physiological maturity, and a composite sample was considered for nutrient analysis. Grain samples were oven dried, cleaned and milled. Sample milling was done in a domestic stainless-steel coffee grinder, which was wiped clean before use and after each sample with a non-abrasive cloth. All preparation was done away from sources of contamination by soil or by dust. Analytical methods Analytical method name: Microwave digestion Analytical method description: Approximately 0.2 g (dry weight, dw) of each finely ground plant sample was weighed and microwave digested in 6 mL trace analysis grade HNO3 in perfluoroalkoxy (PFA) vessels (Multiwave; Anton Paar GmbH, St. Albans, UK). The digested samples were diluted 1-in-10 with Milli-Q water immediately prior to multi-element analysis by inductively coupled plasma mass spectrometry (ICP-MS). Each digestion batch included a minimum of 6 blanks and a certified wheat flour standard (NIST 1567a) for QA purposes; recoveries were 94% for Zn and 95% for Se. Zinc and Se elemental analysis of diluted solutions was undertaken by ICP-MS (Thermo-Fisher Scientific iCAP-; Thermo Fisher Scientific, Bremen, Germany). Samples were introduced (flow rate 1.2 mL min−1) from an autosampler (Cetac ASX-520 Teledyne CETAC Technologies, Omaha, NE, USA) incorporating an ASXpress™ rapid uptake module through a perfluoroalkoxy (PFA) Microflow PFA-ST nebulizer (Thermo Fisher Scientific, Bremen, Germany). Sample processing was undertaken using Qtegra™ software (Thermo-Fisher Scientific) utilizing external cross-calibration between pulse-counting and analogue detector modes when required. The iCAP-Q employed in-sample switching between two modes using a collision cell (i) charged with He gas with kinetic energy discrimination (KED) to remove polyatomic interferences and (ii) using H2 gas as the cell gas. The latter was used only for Se determination. Internal standards Sc, Ge, Rh, and Ir, to correct for instrumental drift, were introduced to the sample stream on a separate line. Calibration standards included a multi-element solution including Zn and Se, in the range 0–100 µg L−1 (Claritas-PPT grade CLMS-2 from SPEX Certiprep Inc., Metuchen, NJ, USA), a bespoke external multi-element calibration solution (PlasmaCAL, SCP Sci-ence , Courtaboeuf, France) with Ca, Mg, Na and K in the range 0–30 mg L−1 and, a mixed phosphorus, boron and sulfur standard made in-house from salt solutions (KH2PO4, K2SO4 and H3BO3). The matrices used for internal standards, calibration standards and sample diluents were 2% Primar grade HNO3 (Fisher Scientific, Lough-borough, UK) with 4% methanol (to enhance ionization of Se). [Datasets have been safeguarded because they contain some sensitive information]