I have an old research problem that I have often thought of but never been able to answer. It is from a geostatistical study we made a long time ago in an agricultural soil in Tunisia. We found what is often called a "hole" effect for some major soil metals looking at the different scales (~10 cm - 60 m) (see enclosed figure). This means that for specific major soil metals (Al, Ca, Fe, and Mg) the variation increases with decreasing scale (~20 m to 4 m). Thus, there are two peaks in the variograms for scales (4 m and 40 m). I have often asked myself if this is an artifact of the sampling strategy or a real physical fact, and in this case, what the physical explanation would be? I would appreciate any thought on this and possible explanations.