Have you ever wondered why you see such varying vegetation on the landscape depending on use? One big reason is soil compaction.
Land allowed to stay in permanent vegetation is easy to plant into and dig. Forests generally have low levels of compaction. This is due to low access by heavy livestock, machinery and vehicles. Instead, the leaf and needle litter remain light and undisturbed, building soils over time. Settlement and various land uses (pasture, perennial and annual crops, etc.) have removed much of the forest. Now, the more developed land is easily accessible and compaction and erosion have generally contributed to declining soil health. Almost all fields west of the Cascades that have deteriorated enough to need reseeding also have soil compaction problems in the crop rooting zone.
What is healthy soil?
Soil is made up of air, water, decayed plant residue, organic matter from living and dead organisms, and minerals, such as sand, silt and clay. Increasing soil organic matter typically improves soil health since organic matter affects several critical soil functions. Healthy soils are also porous, which allows air and water to move freely through them. This balance ensures a suitable habitat for the many soil organisms that support growing plants.
Uncompacted soils are fairly easy to push a soil probe into. If you are curious about compaction layers in your field, get a soil probe! The best way to test is to probe the soil with a compaction tester when your soil is at field capacity. This means the soil is moist, but not oversaturated. Push down 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. When the pressure exceeds 300 psi, plant roots have trouble penetrating the soil. Often penetration abruptly stops at a fairly uniform depth across a field or landscape area. This is referred to as “plowpan.”
Compacted Soils Are Less Healthy and Productive
Land that is intensively cropped, grazed, or harvested is highly susceptible to compaction. The pores in the soil close up, reducing available air and water access. When water cannot easily move into the soil, it begins to run across the surface, resulting in erosion. Wet soils and those with high clay contents are especially vulnerable to compaction.
When the soil has become so compacted that it loses its structure, the soil pores are miniscule and plant roots have difficulty penetrating the soil. Because water can’t infiltrate, more ponding can occur in winter, and less soil moisture will be available in summer as the soils will dry out more quickly. Soil fertility is lost because the soil can no longer supply the needed water, nutrients, and air to the roots. Once this happens, the vegetation that persists will typically be weeds with low prostrate growth form, which can cover the compacted soil, or weeds with a large taproot that are able to penetrate the compacted soil layer.
What Causes Soil Compaction?
Farm fields can have varying levels of compaction. This compaction is based on how much tillage is done and the type of implements or machinery used. Other considerations include how heavy and frequently machinery is used, and whether it has low pressure tires or tracks that spread the weight, or hard tires that concentrate it. The time of year the work is done can also affect compaction. Any machinery use on wet soil will result in more compaction.
How Can I Reduce Soil Compaction?
Ways to reduce compaction include:
- timing to minimize wet soil impacts
- dedicating wheel traffic routes
- minimizing tillage
- lowering tire pressures
- using tracked vehicles
- keeping living roots in the soil, etc.
Compacted soils can also be improved or restored with deep ripping, aeration, top dressing with mulch or compost, and other mechanical methods. These methods, however, have costs. Restoring the soils to a healthy texture still takes the work of several growing seasons of plant roots and their associated biota (e.g. soil microbes, mycorrhiza, earthworms, etc.). By minimizing soil disturbance and maximizing plant cover and diversity, soil health, fertility, and water holding capacity can all be improved.
Livestock Can Contribute to Soil Compaction
Livestock pastures and range have a wide variety of compaction, depending on the number and size of livestock per acre and the timing they are on the pastures. The wet season is the least desirable time to concentrate animals on a pasture. Also, fields that have been heavily grazed in the winter will likely have a compaction problem. As you get closer to areas of concentration like barnyards and sacrifice lots, you will notice the soil becomes harder from the constant heavy wear. More compaction-tolerant weeds will appear, such as broadleaf dock, tansy ragwort, wild carrot, redroot pigweed, annual bluegrass, bermudagrass, chickweed, prostrate knotweed, moss, prostrate spurge, etc.
You Can Reduce Livestock Soil Compaction
One way to reduce compacted soils in barnyards is to construct fenced all-weather Heavy Use Areas with geotextile cloth and compacted gravel. This will provide a footing for livestock and a surface that can be cleaned of accumulated manure. By keeping livestock contained to Heavy Use Areas (or sacrifice lots) during the wet season, compaction can be avoided or minimized on the remaining pastures. Rotational grazing can also greatly benefit soil health by giving forage grasses rest periods to grow bigger, stronger roots and leaves, which also works to reduce weed pressure.
We’re Here to Help!
If you have questions about pasture management, please contact the Clackamas Soil and Water Conservation District at 503-210-6000 or e-mail your questions to [email protected].