Compaction occurs when pores, or small open cavities, are "squeezed" out of the soil. Soil pores may be reduced in size or eliminated altogether, usually due to pressure applied to the soil surface by equipment, foot traffic or hard rain on bare soil.
Testing Soil Compaction
Why be concerned with compaction?
Without enough sizable pores, plant roots find it difficult or impossible to grow and penetrate into new soil. Air, water, fertilizers and pesticides fail to infiltrate through surface crusts or plowpan layers in compacted soils, robbing the roots of essential nutrients, water and oxygen. Plants are starved because roots are not able to function to capacity. Drought symptoms may appear, even with plenty of rain, watering, or standing water on the soil surface because water cannot get through the soil to the roots. Compacted soils are also harder to work, taking much more power to plow.
Tips to Avoid Compaction
Never work wet soil.
Vary cultivation depth each year.
Use flotation tires, dual tires, or tracks on equipment.
Always inflate tires to the lowest safe psi.
Always use the same limited number of drive rows through the field.
Use shrub, mulch or fence borders to keep foot traffic away from planted areas in landscapes.
Build temporary "roadways" for equipment, using a 6" to 10" bed of dry wood chip mulch over areas to be landscaped.
How do you test for soil compaction?
Soil compaction occurs below the surface, so it's hard to spot. The best way to test is to probe the soil with a compaction tester to a depth of 36" to 48". The testing rod should move down through the soil with steady, even pressure, Hard, compacted soils resist penetration with the rod.
Often penetration abruptly stops at a fairly uniform depth across a field or landscape area. This is referred to as "plowpan". For trees and shrubs, comparing root growth inside and outside the root ball is a quick way to tell if compaction is a problem. If you find evidence of compaction, dig to the depth indicated and check for abnormal root growth.
Tools for Testing Soil Compaction
Soil compaction tools range from manual soil compaction rods to digital recording compaction meters. With a manual compaction rod, you can tell when you hit a hard compacted layer. Dial compaction probes tell you how much pressure (in pounds per square inch) it takes to penetrate the soil via a needle moving across a scale on the display.
Digital compaction meters are similar to the dial probes, but they give you a digital reading and have the ability to record the depth and pressure for each test. No matter which tool you choose, make sure that the probe has depth markings so if you hit a compacted layer you will know how deep to go in order to correct the problem.
Tips to Correct Compaction
Cultivating, subsoiling or core aerating to the proper depth to break up the plowpan.
Radial trenching or drilling holes under the drip line of trees and backfilling with compost
or top soil.
Top-dressing repeatedly with mulch or compost to add organic matter and preserve
Limiting traffic in areas of the field with a compaction problem.
Do's and Don'ts of Soil Compaction
Know what causes compaction
Test for compaction
Take corrective action.
Work wet ground
Forget the importance of properly inflated tires.
Ignore compaction problems.