Using a penetrometer to measure soil compaction makes sense because it most closely resembles the way root caps are forced through soil. I use a measure of 300 psi to be the limit to successful root penetraion. Soil moisture is of critical importance here, so I measure it with a Kelway moisture/pH meter at the same time. In the dry summer (Pacific Northwest)with a clayey soil, psi is going to be greater than 300, and the roots are essentially dormant. But in winter, penetration is easy because soil is moist. So It is best to use the penetrometer at field capacity. The point of this is that those soil survey books from the county coop. extension are very useful. I just measured soil strength with a penetrometer in a soil that was cobbles and pebbles- an old river area. Naturally, if you penetrate a few inches below the loam veneer, you hit a stone with the tip, and you might think the soil was so compacted the roots are all rotted or shallow. This was moist soil, with Douglas-fir growing. Actually, the roots threaded their way around the stones to a depth of at least 11 ft. I was lucky to be there when the track-hoe dug that deeply. The soil survey maps can make us aware of a soil's characteristics, so we can interpret penetrometer readings, or not even use it if inappropriate. Are there others with penetrometer experiences that I can learn from?
