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Hydraulic Penetrometer


This simple tool allows you to observe the degree of soil compaction at depths from 0-18 inches (0-45 centimeters). The penetrometer consists of a removable tapered-tip cone, a rod with depth marks at 3” intervals, a machined precision-bore pressure chamber, and a 0-1000 psi pressure gauge. The diameters of the bore/piston and tip cone base are essentially equal. This means that the pressure reading of the meter is approximately the same as the pressure required at the tip to force the rod through the soil.

REMINDER: Soil must be moist in order to obtain meaningful readings.


Compaction is the measure of resistance to further penetration into the soil of a tip of a certain shape and base area, in units of pressure.

Pressure is the force per area, expressed as pounds per square inch (psi) or Newton per square centimetre (Newton/cm2).

The AREA of 1 square centimetre corresponds to a diameter of 0.44”.

The FORCE exerted per 100 psi pressure is equal to (100 x 0.222 x 3.1416), or 15.5 pounds.

Therefore, for example, a pressure of 600 psi would require a force of 93 pounds (6 x 15.5).

Care & Handling:

The penetrometer must be handled with care. If properly cared for, it should operate without problems for many years. The Allen set screw located on the side of the aluminium body opposite the base of the gauge must only be removed to fill the pressure chamber, if the need ever arises.

The probe tip may be removed by unscrewing it at the 3” mark. There are longer rods and larger diameter probe tips available for other applications, such as testing compost during the early stages where aeration is so important.

CAUTION: Never push on the penetrometer hard enough to cause the gauge needle to go over the 1000 psi full scale mark!

To check the penetrometer for satisfactory operation using bathroom scales:

  1. Place a small steel block between the penetrometer tip and the platform of the bathroom scales.
  2. Push the penetrometer handles until the bathroom scales read 93 pounds.
  3. The penetrometer gauge should read 600 psi (within the accuracy limits of the meter movement).
PSI (meter) Force/cm2(lbs/cm2) Newton/cm2
0 0 0
100 15.5 69
200 31 138
300 46.5 207
400 62 276
600 93 414
1000 155 690

Guidelines & Interpretation:

Highly compacted soils are void of air spaces. Aerobic, oxygen-loving microbes are unable to make nutrients available to plant rootlets under those conditions. Soil air management is very important to low input farming practices.

Plant roots will have good growth patterns when soil penetrometer readings are less than 200 psi. When a pressure greater than 300 psi is required to force the tip through the soil, very poor growth conditions will exist for most plant species.

The 0-1000 psi gauge allows you to monitor highly compacted soils and then measure the improvements as you modify farming practices. Tilling techniques and timing may be altered to see if relative improvement occurs. Soil wetting agents may also be evaluated for their effectiveness.

The depth marks along the rod allow you to determine the penetration resistance at various depths. By watching the gauge as you pass each mark, you can see the relative compaction at each depth.