Plant growth and crop yields are greatly affected by root sensitivity to soil compaction. The most common cause of soil compaction in greenhouses, gardens and farms is heavy equipment traffic on wet soils.

Compaction is a frequent problem and is yield-limiting, as roots lose the ability to spread out and search for nutrients. Plants growing in tightly packed soils are shallow-rooted and cluster together in narrow linear zones that lack breadth and depth, resulting in poor plant growth, lowered crop yield and reduced vigor.

Compaction also reduces water infiltration into the soil and reduces drainage by slowing percolation rates. Compacted soils are prone to increased water logging, ponding, excessive runoff and soil erosion, again hampering yields. Common cultivation practices that further advance compaction include:

  • Consistently plowing or disking to the same depth
  • Shallow tilling, resulting in stratification and layering of soil
  • Driving heavy equipment or allowing foot traffic on soil, especially wet soil
  • Poor crop rotation without variability in root structure or rooting depth

Soil compaction usually occurs on the surface as crusting of the top layer or in the sub-soil below the depth of maximum tillage. Determining if a soil compaction problem exists and to what extent it affects growth helps the gardener develop management practices to alleviate the present conditions and prevent further damage.

Compaction can be quickly determined by using a shovel to remove and shear away the surface soil to reveal the root zone. Horizontal root growth along or above a hardened layer is a good indicator. Lower into the soil profile is the compacted layer called hardpan—the layer of soil immediately under the depth of tillage. If the hardpan is under 18-in. in depth, this could seriously limit yields.

Soil health or quality is the soil’s ability to support growing a large, vibrant plant to maturity while achieving maximum growth. Greenhouse soil quality can change rapidly in modern, heavily amended plots. Soil health deteriorates mainly through the loss of organic matter and the breakdown of the sub-structure through compaction in the lower soil layers. Soils are particularly vulnerable to structural degradation when they are fine-textured, clay based, wet and low in organic matter content.

Years of adding organic matter to soil, combined with shallow tilling, often results in stratified upper layers, with the upper layers consisting of richly amended organic matter, while the lower substructures contain clay.

Lower level stratified soils that have small particles such as clay allow only slow movement of water and contaminants through the soil. Soil compaction occurs when soil particles are pressed together, reducing the pore space between them and increasing the soil’s bulk density.

The loss of large pores decreases oxygen supply to the plant’s roots, restricts growth and increases the probability of anaerobic root diseases. Water in compacted soil accumulates because it cannot drain away and will eventually build up high amounts of nutrients and salts that become toxic to roots.

The size, shape and arrangement of the pore spaces and sand, silt and clay are key factors in soil quality. Organic matter and other binding agents stabilize the arrangement and maintenance of pore spaces and particle separation.

Soils with good breath and structure allow air, water and nutrients to move freely through the spaces within and between the soil layers. These soils also retain their ability to resist compressive forces when exposed to the stresses of cultivation, harvesting, rainfall and foot traffic.

Retaining good sub-soil structure is as essential to sustaining long-term plant productivity as is amending the upper soil areas with organic matter. Good sub-structure means that restricted aeration and drainage do not limit a plant’s total root mass. Opening up sub-soil pore space provides a plant that is deeper-rooted, better able to withstand climate fluctuations and lowers harmful anaerobic conditions in the root zone.

The Solution to Soil Compaction

To reduce compaction in outdoor gardens, soils should be tilled only when dry and as deeply as possible. Spiked broad forks can be used to penetrate the hardpan layers, causing fractures and helping to break up the soil below. Sowing of deep, tap-rooted, fibrous crops can also help penetrate the soil layers. These crops enhance the aeration and drainage through the hardpan boundary layer, foster good movement of water and nutrients and result in:

  • Deeper aeration for roots and soil microbes
  • Increased resistance to erosion by wind and water
  • Increased resistance to compaction and crusting
  • Enhanced storage of water and nutrients in soil and their availability to crops
  • Ensuring adequate drainage and leaching of water and nutrients in a storm event

Most planting areas will eventually suffer from soil compaction issues. There are several ways to change or modify soil structure and density. Removing compacted soil and importing new soil or soil amended with supplements and organic matter such as compost and mixed from 24- to 36-in. deep is one way. Alternatively, the hardpan layer can be top-filled with soil with less bulk density. Often reducing density is as simple as limiting surface disturbances such as foot and machinery traffic on the soil.

Methods to reduce the bulk density of indoor and greenhouse soils include:

  • Eliminate soil disturbance and movement on wet soils
  • Use designated walking lanes or boards for equipment and foot traffic
  • Double-dig soil with long, pronged, broad forks to break up compacted layers
  • Deep tilling such as sub-soiling to loosen the lower boundary layers
  • Installing weeping tile to remove sub-surface water accumulations
  • Trenching to guide surface water and reduce pounding

Best Amendments to Improve Soil Structure

Amendments that increase the amount of organic matter and aid in drainage are beneficial. Generally, the more organic matter content added to a soil will translate in greater pore space and lower bulk density. Amendments that improve soil structure include:

Soils that are deep, open and able to freely exchange water and air enhance a plant’s growth in the rhizosphere, or root zone. Reducing soil compaction allows the soil to drink and breathe freely, ensuring a thriving community of flora and fauna and increased yields.