The readings taken with the penetrometer are called the cone index. The readings should be taken when the whole profile is at field capacity (approximately 24 hours after a soaking rain). The best time of the year for the compaction measurement is the spring because the whole profile has usually been thoroughly moistened during the winter. If the soil is too wet (muddy), compaction could be underestimated because the soil acts as a liquid. If the soil is too dry, compaction could be overestimated because roots will be able to penetrate the soil when it dampens. The idea behind using the penetrometer at field capacity is that this is the best-case scenario for roots. Hopefully, the soil will be at field capacity at various times during the growing season. During these periods, roots will be able to penetrate soil that has low penetration resistance. Penetration resistance will increase when the soil dries out, and root growth can then be expected to be limited. However, when the moisture content of the soil increases again, penetration resistance will decrease, and root growth will resume.
The penetrometer rod should be driven in the soil at a rate of approximately 1 inch per second. As you push the penetrometer into the soil, record the depth at which the 300 psi level is exceeded, using the gradients on the penetrometer rod. This level is the top of the compacted zone. Continue pressing the penetrometer down. Record the depth at which the penetration falls below 300 psi. This is the bottom of the compacted zone. For each measuring point, there are two numbers: the top of the compaction zone and the bottom of the compaction zone. If penetration resistance never increases above 300 psi, you will have blanks in both spaces, indicating no severe root-limiting compaction. If the penetration resistance increases above 300 psi, but never falls below 300 psi, there is no bottom to the compaction zone.
Cone index should be measured respective to tillage relief, wheel tracks, plant rows, and other recognizable patterns in the field. For example, if you know the areas of wheel traffic, take transects in and out of the track, and report them separately. If there are subsoiled zones in the field, measure penetration resistance in and out of the subsoiled zone. If there are planted rows, take measurements in and between the rows, and report them separately. Take separate readings for trafficked and non-trafficked areas.
The number of readings in a field depends on the accuracy you desire. As a first approximation, take some preliminary readings at a few places in the field to develop a sampling strategy. The cone index values are likely to be quite variable, so multiple readings are required per field. It is
Recommended to take one reading every 100 to 150 feet, or three to four readings per acre to develop a solid recommendation. This is a useful spacing if no recognizable patterns are present.
The penetrometer rod should be driven in the soil at a rate of approximately 1 inch per second. As you push the penetrometer into the soil, record the depth at which the 300 psi level is exceeded, using the gradients on the penetrometer rod. This level is the top of the compacted zone. Continue pressing the penetrometer down. Record the depth at which the penetration falls below 300 psi. This is the bottom of the compacted zone. For each measuring point, there are two numbers: the top of the compaction zone and the bottom of the compaction zone. If penetration resistance never increases above 300 psi, you will have blanks in both spaces, indicating no severe root-limiting compaction. If the penetration resistance increases above 300 psi, but never falls below 300 psi, there is no bottom to the compaction zone.
The number of readings in a field depends on the accuracy you desire. As a first approximation, take some preliminary readings at a few places in the field to develop a sampling strategy. The cone index values are likely to be quite variable, so multiple readings are required per field. It is
Recommended to take one reading every 100 to 150 feet, or three to four readings per acre to develop a solid recommendation. This is a useful spacing if no recognizable patterns are present.
Interpretation of penetration resistance measurements.
Percentage of measuring points having cone index > 300 psi in top 15 inches
|
Compaction rating
|
Subsoiling recommended
|
<30
|
Little-none
|
No
|
30-50
|
Slight
|
No
|
50-75
|
Moderate
|
Yes
|
>75
|
Severe
|
Yes
|
