Crop yields may be limited by soil compaction. Subsoiling is the most common method used to alleviate soil compaction, but is time consuming and costly. It is best to identify the areas in the field where it exists, what depth the compaction begins and what depth the compaction ceases.
The best method to document this information is with a soil penetrometer. Most county Extension offices have penetrometers that may be borrowed to make these measurements. A soil penetrometer is a tool with a sharpened point that is pushed into the soil profile. The amount of force required to insert the penetrometer into the soil is measured and indicated on the top of the instrument, usually in pounds per square inch. This measurement of force is referred to as penetrometer resistance or PR.
Penetrometer resistance values below 300 psi are most likely not limiting corn yield. If 50 percent or more of the samples are greater than 300 psi, then corrective action is expected to provide yield increases that result in an economic return above the cost of subsoiling.
Soil conditions should be moist enough that the force measured is a result of soil compaction, not differences in soil moisture within the soil profile. A good rule of thumb is to make the measurements when the soil is too wet to till. For this reason, it is recommended to test for soil compaction in the winter on unfrozen soil and spring when soil moisture levels are consistently “wet” throughout the soil profile.
Like soil sampling, the best results come from sufficient sampling intensity. A minimum 10 to 20 samples should be taken from small fields. Sample size should increase as field size increases. High-traffic areas within a field such as field entrances, “lanes” where grain carts operate, and end rows should be sampled separately.
For more information on determining soil compaction please see University of Kentucky Cooperative Extension publication ID-153, "Assessing and Preventing Soil Compaction in Kentucky."