A stiff wire (surveyor’s flag), a welding rod, or a piece of rebar will work just as well. Those fortunate enough to have a soil sampling tube can use this to measure soil compaction.

The key is to do it when the soil is wet or at least moist. All dry soils have a relatively higher soil strength than wet soils. Roots do not grow into dry soils anyway so it makes sense to measure compaction when the soil is suitable for root growth i.e., when it is moist or near field capacity. 

Don’t try this in mid-summer after a 3-week drought. Late fall, winter, and early spring after a good, soaking rain is the best.

Take a stiff wire (surveyer's  flag) or welding rod or small rebar and try to push it into a very moist soil. Do this in several spots, especially if you have rocky or cherty soils. You should be able to push the wire or rod into the first few inches with little difficulty. If you cannot push a wire or rod into a moist soil, roots cannot grow into it.

Grass roots should naturally keep the first few inches of topsoil relatively permeable. If you don't have any vegetation, then it too, can get very compacted. If you cannot push it into the soil 3 or 4 inches, you definitely have surface soil compaction. This is a serious problem, because neither rainfall nor roots can get through this.  A good, deep sod is the best solution to surface soil compaction.

Another problem arises when raindrops beat the surface of a bare soil into a thin crust such that seedlings cannot emerge through. This increases  runoff and is a sure sign of poor soil quality. Crusting should be obvious without the wire or rod test.

In field crops, you can usually push the wire or rod down about 4 to 6 inches with no difficulty because this is the plow layer. However, if you encounter a stiff layer between 4 and 8 inches deep, you may have discovered a “hardpan” or “traffic pan” created by tillage equipment and tractors

With extra effort you should be able to break through this hard pan and then it will get easier to push the wire or rod on into the subsoil. A traffic pan is usually only an inch or two thick so we use subsoiling or paratilling to break up traffic pans. 

Some soils have a natural "fragipan" or compacted subsoil about 18 to 24 inches or so deep. There is nothing you can do about this. An example is a Savannah fine sandy loam which is a lot more common in the Upper Coastal Plain of Alabama. 

A Dickson soil has a pronounced fragipan and is found in north Alabama and Tennessee. These soils make pretty good pastures, but they tend to be wet in winter and very dry in summer.

What can I do about soil compaction?

First, find out why the soil is compacted and where the compaction is occurring and try to eliminate whatever caused it. Perhaps it was over-grazing a pasture or plowing a field when it was too wet. 

Establishing a good root system is the best way to reduce soil compaction.  That is one reason so many row crop farmers have switched to some form of  high-residue, conservation-tillage. This system leaves old root channels in the soil to reduce soil compaction and increases surface organic matter. 

Raindrops on a bare soil result in crusting, another form of soil compaction on the surface. 

Rototillers, disk harrows, and turning plows are the worse machinery for creating subsoil compaction. This is another reason for the popularity of conservation-tillage or no tillage. 

Subsoiling under the row and paratilling are very energy intensive operations, but these will break up traffic pans or hardpans and allow for deeper rooting of row crops.