In long-term no-till, sod or forest soil the penetration resistance may exceed 300 psi and yet there may not be a compaction problem. The reason is that in these soils, firm aggregates are surrounded by a network of pores that allow root growth, aeration, and water percolation to take place.

So in addition to using the penetrometer, you need a shovel and dig to a depth of 12-18 inches to assess soil structure. 

You need to determine if the soil is massive or crumb. If the clods fall apart easily or are held together by dense root networks, that is a first sign compaction may not be problematic.

Look for severe platy structure in the surface soil. Then you need to search for evidence of root growth restrictions. Look where there is living vegetation in the field (for example, weeds or cover crop), and determine if root growth is limited — in compacted soil roots typically follow cracks without being able to grow into the massive clods, or the roots crowd in the horizontal voids between massive plates.

Do you see evidence of shiny surfaces created by tillage tools working in wet soil?

If there is a hard pan, roots may make an abrupt turn or many fine roots crowd above this layer. If roots grow downward without a problem, compaction is not likely to be severe.

Also look for organic matter content — does the soil show evidence of organic matter accumulations, which lead to soil becoming better aggregated and crumb?

Finally, look for biological activity, such as that of earthworms.

 At the surface of the soil, you can look for middens (click here to see the Earthworm Factsheet) — underneath each burrow there is a 4-5 foot deep open nightcrawler channel so if you have many middens, there is not likely to be a compaction issue.

Other worms dwell in the surface of the soil and fill their channels with casts as they go. If you have many earthworms that is more evidence that compaction is not problematic.

Good times to look for earthworm activity is in spring and fall when the soil is moist.

(If you find the need for breaking up compaction, find tips on how best to proceed at What can you do to alleviate soil compaction?)

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