Soil Compaction and Corn Roots

N.B. MCLAUGHLIN, D.R. LAPEN, D. KROETSCH, X. WANG, E.G. GREGORICH, B.L. MA AND Y.X. LI
Agriculture and Agri-Food Canada, Ottawa, Ontario

Good soil management creates a favorable environment for healthy root growth. High soil strength can inhibit the penetration of roots, resulting in poor plant growth. Excess soil strength can occur naturally as soil with high clay or low organic matter hardens in droughty conditions, a process called ‘age-hardening’ or ‘hard-setting’. Soil compaction also increases soil strength and impedes root penetration.

Soil compaction is the consolidation of soil particles into close proximity with each other. This not only increases soil strength but also reduces pore volume, soil aeration and natural drainage. Wheel traffic on fields compacts the soil increasing soil strength and reducing yields by 20 to 30% depending on the severity of compaction, soil texture, soil conditions at planting, and weather during the growing season. Full season crops such as corn require field operations in early spring and late fall when the soil is often wet and vulnerable to compaction so producers often face difficult management decisions.

Fine textured soils with high clay and silt content and with low organic matter are especially prone to compaction. Water films around soil particles act like a lubricant allowing the soil particles to slide by each other into a compacted state when loaded with a heavy wheel. There is an intermediate range of water content where the greatest compaction damage occurs. Road builders intentionally wet the road surface to achieve the water content where greatest degree of aggregate compaction can be achieved. Wheel traffic in very wet conditions can surface seal (crust) the soil which is an added problem.

Surface Compaction

Soil compaction is sometimes classified as surface and subsurface. Surface compaction is usually caused by high contact pressure between the wheel and the soil surface, and is related to the tire inflation pressure. Surface compaction is visually evident as a smooth shiny surface of wheel tracks due to reduced pore space, and water laying in the wheel tracks due to reduced natural drainage (Fig. 1). Large hard clods on the soil surface following tillage also indicates surface compaction. Tillage can help break up surface compaction in a conventional tillage system, but this is not an option in a no-till system. Freeze-thaw cycles in spring and fall and use of forage crops in a rotation naturally loosen compacted soil. Surface compaction can be reduced by reducing traffic, using lighter machines, using dual wheels and larger tire sizes and reducing tire inflation pressure. Modern radial tires can usually be run at lower inflation pressures than bias ply tires. Manufacturers’ recommendations should be consulted for the correct inflation pressure for a given axle weight.

Figure 1. Soil trafficked after heavy rain; despite minimal rutting, infiltration has been reduced, with some water lying in the wheel tracks. The shiny soil surface indicates reduced pore space.

Subsurface Compaction

Subsurface compaction, often called a plough pan, is usually more problematic than surface compaction since it is not easily detected and is not corrected by natural freezethaw cycles. It is caused both by tillage and by traffic with heavy field machinery. A tillage tool passing through the soil compresses the soil in a roughly spherical zone above and immediately below the leading edge of the tillage tool. This compression causes the soil to break up or fragment, which is the desired effect of tillage. However, there is little opportunity for fracture planes to develop in the zone immediately below the tillage tool, so the soil remains in a compressed or compacted state. This compressed zone is called a “plough pan” because it is located immediately below the tillage depth, and because it was first identified in mouldboard ploughed fields. Repeated tillage operations over several years can contribute to both the formation and downward extension of a plough pan.

Compressive stresses from heavy axle loads can compact the soil to depths well below normal tillage depth. While reducing tire inflation may help to reduce surface compaction, reducing subsurface compaction must take into account total axle weight. Very heavy equipment such as grain buggies and liquid manure spreaders are especially problematic; hence confining grain buggies to headlands and applying liquid manure with drag hoses can help reduce compaction. A Swedish study found that the costs of soil compaction from manure spreading may be as large as the value of the plant nutrients in the manure.

Identifying Subsurface Compaction

Subsurface soil compaction is more difficult to detect than surface compaction. Poor crop performance and water ponding due to poor drainage may indicate underlying compaction. Plant roots can be exposed and the pattern of root growth observed by carefully removing soil from the walls of a pit. Roots take the path of least resistance, traveling horizontally along the periphery of compacted zones (Fig. 2 left) compared to the vertical movement for non-compacted soil (Fig. 2 right). Sometimes, roots will follow a crack in the soil fanning out within the crack (Fig. 3), because they are unable to penetrate compacted soil and reach water and nutrients beyond the crack.

Figure 2. Corn roots in compacted (left) and non-compacted (right) heavy clay soil from an experiment in Australia. Roots were not able to penetrate the compacted layer so growth was stunted.

Figure 3. Corn roots following a crack in compacted soil; the roots have fanned out in two dimensions within the crack, but have not penetrated the hard soil on either side of the crack.

Preventing and Remediating Soil Compaction

  • Soil compaction can be best prevented by staying off wet fields. Subsurface drains and contouring promote drainage helping the soil dry out.
  • Tire inflation pressure may be lowered as this spreads the axle load over a larger surface area. Tire manufacturers literature should be consulted to determine the proper tire inflation pressure.
  • Always driving over the same tracks (tramlines) reduces overall field compaction. This is especially effective when all field implements have the same working widths (preferably large). Because the suppressed growth strips are narrow, plants growing adjacent to the compacted tramlines will have access to additional light, water and nutrients, hence produce compensatory growth.

Deep tillage is often suggested as a method of breaking up plough pans. However, this operation is expensive, not always successful and may even cause additional damage in wet soils. Deep tillage is likely to be most effective if performed when the soil is very dry in late summer after harvest of a cereal or forage crop. It may be best to try deep tillage in a small test area known to have a subsurface compaction problem.

Compaction Demonstration Experiment

A compaction experiment was conducted in 2002 and 2003 at the Central Experimental Farm (CEF) in Ottawa to examine the effects of untimely field traffic. The objective was to determine the impact of wheel traffic compaction, such as might occur as a result of early spring spreading of liquid manure on wet soil, on the germination, growth and yield of corn. After compaction, the field plots were cultivated, fertilized and planted using normal procedures for the region.

Compaction reduced corn plant establishment and plant height in both years. In 2002, compacted plots yielded 8.0 t/ ha (127 bu/ac) compared to 8.4 t/ha (134 bu/ac) for the noncompacted plots. In 2003, compaction reduced yields by 3.1 t/ha (49 bu/ac). Field plots compacted in 2002 continued to reduce yields in 2003. The non-compacted plots also responded better to fertilizer. These results clearly demonstrate the damage that can be done by driving on fields when they are too wet.

By excavating the soil and carefully exposing roots we found that roots in the compacted strip (Fig. 5 right) exhibited much less branching than those in the non-compacted strip (Fig. 5 left). As in Fig. 3 above, roots in the compacted areas tended to follow cracks, fanning out in two dimensions within the crack.

Figure 5. Corn roots in non-compacted (left) and compacted (right) soil;
note the difference in branching of the corn roots.

Testing Soil Strength

Soil strength can be measured with a cone penetrometer which measures the force required to push into the soil, a probe fitted with a standard 12.7 mm (0.5 inch) diameter, 30 degree conical tip. In a sense, the penetrometer mimics a root pushing its way into the soil. Compaction testers consisting of a probe and a dial gage to indicate penetration force are commercially available for on-farm use. A simple manual probe can be made from a one meter (40") length of 9.5 mm (3/8") stainless steel rod, fitted with a conical tip, and a T-handle (Fig. 4). The tip can be made by first building up the surface of the rod end with a weld bead, and then machining or grinding it to a conical shape. By comparing the effort required to push the probe into known compacted areas (such as headlands) to un-compacted areas (such as fence lines), one can quickly develop a “feel” for degree and extent of soil compaction.

Soil strength is measured in units of pressure: 1 Mega Pascal (MPa) = 145 lb per square in (psi). Root growth is reduced by about half at a penetration resistance of 2.0 MPa (290psi) and severely limited at 3.0 MPa (435 psi). The 2.0 MPa threshold is equivalent to a force of about 26 kg (57 lb) to push the 0.5 inch diameter probe into the soil; penetration resistance in compacted soils can be two to four times this value. Higher soil water content typically results in lower penetrometer values so assessments should be carried out at consistent soil water contents.

Figure 4. Simple probe with conical tip and T-handle for
investigating subsurface soil compaction.