Measuring soil compaction using a penetrometer (soil compaction tester)
Pennsylvania State University,Department of Crop and Soil Sciences, Agronomy Facts 63, 2002

Soil compaction can easily reduce crop yields by 10 percent, and can lead to water and soil quality degradation due to increased runoff and soil structure destruction.

Penetrometer: A diagnostic tool to measure the extent and depth of subsurface compaction is a penetrometer, or soil compaction tester.

There are two forms of compaction: surface and subsurface. While surface compaction can be partly alleviated with normal tillage operations, subsurface compaction below the normal tillage depth will remain. Fracturing or cutting subsurface compacted soil has, in some cases, resulted in remarkable yield increases.

Several companies sell penetrometers that are all based on the same technical specifications of the American Society of Agricultural Engineers. A penetrometer will cost around $200.

A penetrometer consists of a 30-degree circular stainless steel cone with a driving shaft and a pressure gauge (Figures 1a and 1b). The penetrometer usually comes with two cones, one with a base diameter of 0.798 (3/4) inches for soft soils and the other with a base diameter of 0.505 (1/2) inches for hard soils. The tip is slightly wider than the driving shaft to limit friction of the shaft with the soil. The driving shaft is usually graduated every 3 inches to allow the determination of depth of compaction. The pressure gauge indicates pressure in pounds per square inch (be sure to use the appropriate scale for the tip you are using).

The penetrometer is designed to mimic a plant root. Of course, a plant root is living, and much smaller than a penetrometer, so the penetrometer can be expected to have some shortcomings. In studies conducted at the United States Department of Agriculture's Agriculture Research Service (USDA-ARS), root penetration into soil cores packed to different densities was measured and compared to penetrometer readings. Root penetration decreases linearly with penetration resistance, until almost no roots penetrate into soil with a penetration resistance of 300 psi (Figure 2). Much of this research was done with cotton, but it also appears to hold true for other crops. Although the limit of zero root growth may not be exactly at 300 psi, it is certain that root growth will be greatly inhibited at higher penetrometer readings. This is true in both wet and dry soils, and is independent of soil texture. Unfortunately, the penetrometer does not capture pores created by physical or biological forces such as freezing/ thawing, wetting/ drying, earthworm burrowing, and root channeling. Plant roots will find and grow through these spaces in the soil if they are present.

From:

Agronomy Facts 63
Diagnosing Soil Compaction Using a Penetrometer (soil compaction tester) Department of Crop and Soil Sciences - Cooperative Extension, Pennsylvania State University

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