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).

摘要
目前学界尚未就精准农田土壤贯入阻力（Soil Penetration Resistance, SPR）制图所需的子样本数量达成共识。为此，本研究旨在明确每个采样点量化土壤贯入阻力所需的子样本数量。
本试验在4.7公顷的区域内开展，采用50×50米的网格采样方案，共设置18个采样点。研究于两个不同年份开展，针对每个采样点分别设置1、2、3、4、5、6、9、12、15共9组不同子样本数量，使用数字贯入仪测定土壤贯入阻力。以15个子样本生成的土壤贯入阻力图作为参考图，采用相对偏差系数与皮尔逊线性相关系数，对不同子样本数量下生成的土壤贯入阻力图进行对比分析。
子样本数量的减少会导致土壤贯入阻力数据的离散程度上升。综合本研究结果，若要生成相对偏差系数小于10%（即各采样点相对于均值的最大误差不超过30%）且与参考图（15个子样本）具有显著相关性的土壤贯入阻力图，每个采样点至少需采用4个子样本。