Soil penetrometer resistance spatial variability in a Ferralsol at several soil moisture conditions

The penetrometer resistance (PR) spatial variability in a Ferralsol under different soil moisture conditions was characterized through statistical and geostatistical methods. PR measurements were made in a 10-row, 10-columns, 3-m spacing grid at 20–30 cm depth using a hand-pushing penetrometer. Measurements were made for dry soil conditions (before irrigation) as well as 2 and 24 h after irrigation. The soil bulk density (BD) and the relative topographical altitudes (RT) were measured at the same locations. The PR spatial variability is normally distributed for dry soil conditions and after irrigation-water redistribution. A normal distribution fits also to the BD spatial variability. PR mean and coefficients of variations are lower for wet soil conditions and vary notably according to the soil moisture condition. The PR semivariance values are much higher for dry soil conditions than those found for wet soil conditions. However, the semivariogram of PR before irrigation shows almost a pure nugget effect. Irrigation yields a spatial structure in PR measurements. Despite the differences in the shown spatial structure, the range of the PR semivariograms before and 24 h after irrigation is about 8–10 m, which is very similar to the range of the BD semivariogram. The correlation coefficients between PR and the other measured variables are very small before and after irrigation, which could lead to the conclusion that those variables are not related at all. However, according to the co-dispersion coefficients plotted as a function of the separation lag, the correlation between those variables changes according to the separation distance. Particularly, PR and the RT show correlation 24 h after irrigation only for lags between 5 and 12 m. The results pointed out that PR spatial variability depends on soil moisture condition, soil BD and on several local features as microtopography.