<b>Soil penetration resistance mapping quality: effect of the number of subsamples

Abstract

There is no consensus in the literature regarding how many subsamples are needed to perform accurate on-farm soil penetration resistance (SPR) mapping. Therefore, the objective of this study was to define the number of subsamples per sampling point needed to quantify the SPR. The experiment was performed in a 4.7 ha area and employed a 50 × 50 m grid system (18 sampling points). The SPR was evaluated using a digital penetrometer in two different years with 1, 2, 3, 4, 5, 6, 9, 12, and 15 subsamples per sampling point. The SPR maps produced with increasing numbers of subsamples were compared to the reference maps (15 subsamples) using the relative deviation coefficient and Pearson´s linear correlation. A reduction in the number of subsamples promoted an increase in the variability of the SPR data. Generally, the results from this study suggest the use of at least four subsamples per sampling point to achieve SPR maps with a coefficient of relative deviation less than 10% (30% maximum error per point around the mean) and significant correlation with the reference maps (15 subsamples).

Spatial Variability of Soil Apparent Electrical Conductivity - Effect of the Number of Subsamples

“…The increase in the number of readings per sample point caused an increase in the proportion between the contribution (C) and plateau (C0+C) and a reduction in the number of errors based on the estimation done using kriging during cross-validation (Table 1). This implies greater accuracy of the maps generated, as verified by Pias et al (2018) in a study on the influence of the number of subsamples on the preparation of the maps of the soil resistance to penetration. Increasing the number of readings per sample point helps detail the representation of the local variability, which tends to smooth the variability between the sample points.…”

“…Increasing the number of readings per sample point helps detail the representation of the local variability, which tends to smooth the variability between the sample points. According to the methodology of Pias et al (2018), to obtain a 10% error based on the mean of ECa in the area where the present study was performed, it would be necessary to perform 30 readings per sample point. Therefore, additional 10 readings would be required.…”

“…The number of readings per sample point to determine the ECa with an error of up to 10% around the mean was obtained using the methodology presented by Pias et al (2018). The below equation was proposed by Petersen and Calvin (1965) (Equation 1).…”

“…The study conducted by Pias et al (2018) showed that, as well as the number of sample points, the number of sub-samples or readings/measurements per sample point should be considered in spatial data analysis. The results obtained indicate that the greater the number of subsamples per sample point, the greater the accuracy of the map generated.…”

Soil morphostructural characterization and coffee root distribution under agroforestry system with Hevea Brasiliensis

“…Similarly, Canarache (1990) stated that soil PR must be lower than 2.5 MPa. However, critical PR values may vary depending on soil structural conditions, tillage system, and soil moisture and texture (Pias et al, 2018). Conversely, Seidel et al (2018) stated that, in a system without annual plowing, PR values up to 3.0 MPa are tolerable and do not reduce soybean yield.…”