Agricultural Soil Compaction

Abstract

The soil type, crops grown, and weather conditions determine the effects of soil compaction on plant growth and yield. Soil compaction can significantly reduce yields due to N losses, decreased K availability, and lowered root respiration because of reduced soil aeration, under wet conditions. Water infiltration and storage, root growth, and soil volume explored by roots and crop yields are reduced due to excessive soil compaction. Axle load and soil moisture content are directly correlated with soil compaction. Soil compaction can be reduced through crop rotations and management, direct seeding, drainage improvement, and introduction of deep rooting plants, using in-row or bent-leg subsoilers that minimally disturb soil surface and using smallest vehicle possible for farm operations.

1 Introduction

The reduction of soil volume (a simple reduction in pore space) due to external factors is called soil compaction. Soil compaction is defined as increase in soil bulk density or decrease in soil volume and porosity (Fig. 3.1) due to mechanical stress on soil (e.g., from traffic of farm machinery). It can also occur due to natural reconsolidation of soil. There are two types of compaction, namely, surface compaction and subsoil compaction. The compaction that occurs in the surface “plow layer” is called surface compaction, while the compaction that occurs as a result of a surface load below the plow layer is called subsoil compaction.

Fig. 3.1

Effect of soil compaction on pore space

2 Causes

At present, increase in the size of farm equipment used to carry out various farm operations increases the risk of soil compaction. The agricultural soil compaction can take place due to frequent movement of farm machinery. Factors responsible for compaction due to vehicular traffic include weak soil (soil density and moisture content effect) and excessive loads (size of vehicles, tire size, and number of passes are directly related to compaction). Soil tillage operations are also responsible for soil compaction.

Contact pressure of tires (expressed in psi) causes surface soil compaction. This can be reduced by increasing the tire width and height. The narrower the tires, the more the surface soil compaction, while the wider the floatation tires, the lesser the surface soil compaction (Fig. 3.2).

Fig. 3.2

Effect of contact pressure of tires on surface soil compaction

Axle load (expressed in tons) is responsible for subsoil compaction; the higher the axle (or wheel) load, the deeper the stress transmitted into the soil (Fig. 3.3). The subsoil compaction is reduced when axle load is reduced.

Fig. 3.3

Effect of axle load on subsoil compaction

The soil compaction due to natural soil reconsolidation is more in well-graded soils compared to poorly graded soils. The raindrop impact is certainly a natural cause of soil compaction which causes soil crust (usually less than 1.25 cm thick at the soil surface) that may prevent seedling emergence.

The plow pans (hard pans caused by severe soil compaction) are caused by moldboard plow and disk plow and harrow just below the tillage level.

Soil moisture content also affects soil compaction (Fig. 3.4). Maximum soil compaction occurs at a moisture content approximating field capacity.

Fig. 3.4

Effects of soil moisture on soil compaction (left) and crop yield (right) (Soane and van Ouwerkerk 1994)

Significant livestock trampling resulting from livestock grazing in agricultural land is also a major cause of soil compaction. This is not affected whether the grazing is continuous or short term; however it is affected by the intensity of grazing.

3 Symptoms

The symptoms of soil compaction include ponding of water, degradation of soil structure (clod formation) (Fig. 3.5), more runoff, and resistance to tillage implements. The symptoms on plants include deficiency of nutrients (nitrogen and phosphorus) (Fig. 3.6), uneven and stunted growth (Fig. 3.7), reduced root growth (Fig. 3.8), and decrease in yield.

Fig. 3.5

Clod formation due to soil compaction

Fig. 3.6

Nitrogen (left) and phosphorus (right) deficiency symptoms on maize due to soil compaction

Fig. 3.7

Uneven crop stands and stunting of growth of maize plants due to soil compaction

Fig. 3.8

Pancake root mass due to soil compaction

4 Effects

The following effects are observed due to soil compaction:

• Affects water-holding capacity of soil, water infiltration, water redistribution over landscapes, and roots ability for extraction of water (slow root growth).

• Enhanced erosion of soil.

• Poor soil drainage.

• Reduction in uptake of nitrogen and other nutrients. Results in increased denitrification rates because of limited aeration, increase in ammonia volatilization losses in surface-applied liquid manure (due to reduced infiltration), and reduced uptake of phosphorus and potassium (due to inhibition of root growth).

• Resistance to root penetration. Reduced root and shoot growth (Fig. 3.9).

  Fig. 3.9

  

  Effect of soil compaction on root and shoot development

• Soil is dense with low porosity.

• More energy is required for tilling soil.

• Decreased soil oxygen (oxygen is required for root respiration and active uptake).

• Unfavorable environment for soil organisms (especially earthworms).

• The raindrop impact is certainly a natural cause of soil compaction which causes soil crusting that may delay seedling emergence.

• Yield reduction (Table 3.1) (Fig. 3.10). The average yield loss of 15 % was noticed in tilled soils due to compaction during the first year.

  Table 3.1 Effect of soil compaction on crop yield

  Fig. 3.10

  

  Effect of soil compaction on yield (a) yield losses due to compaction of the topsoil, (b) yield losses due to compaction of the upper subsoil, (c) yield losses due to compaction of the lower subsoil) (Hakansson and Reeder 1994)

5 Measurement

The agricultural soil compaction is directly affected by change in bulk density, porosity, and soil volume. Soil cone penetrometer can be used as a diagnostic tool to measure soil compaction (Fig. 3.11). The compaction categories based on penetrometer readings in field surveys are presented in Table 3.2.

Fig. 3.11

Soil cone penetrometer used for measuring soil compaction

Table 3.2 Compaction categories based on field survey

6 Management Strategies

The management strategies (avoiding or remediation) can be developed by understanding the causes of soil compaction. Avoiding soil compaction is easier, while remediation strategies are costly and may not correct the problem entirely.

6.1 Avoiding Compaction

Limiting the surface soil compaction as far as possible and avoiding subsoil compaction altogether should be the aim of compaction management.

6.1.1 Avoiding Surface Soil Compaction

The surface compaction can be avoided by reducing the contact pressure of tires to less than 35 psi. This can also be achieved by using floatation tires, reducing tire air pressure to minimum allowable, using radial-ply instead of bias-ply tires, using multiple tires (Fig. 3.12), reducing contact pressure by replacing singles by doubles or tracks, reducing number of passes in field, increasing footprint length by using larger diameter tires, limiting the area of the field that is impacted by providing permanent wheel tracks for movement of farm machinery traffic, and proper ballasting tractor for each field operation.

Fig. 3.12

Multiple tires reducing contact pressure

6.1.2 Avoiding Subsurface Soil Compaction

The subsurface compaction can be avoided by increasing the number of axles and reducing the axle load below 10 tons (Fig. 3.13).

Fig. 3.13

Effect of axle load on soil compaction

6.2 Avoiding Plow Pans

The plow pans can be avoided by using no-tillage, not driving a tractor wheel in the furrow, using a field cultivator instead of disk harrow, and using a chisel instead of moldboard plow.

6.3 Enhancing Soil Resistance

The soil resistance to compaction can be increased by building a soil ecosystem that has a permanent macropore system, increasing its organic matter content, and using cover crops with deep root systems that serve to reduce or remediate the effects of soil compaction (Fig. 3.14).

Fig. 3.14

Effect of taproot of canola (left) versus fibrous root of pea (right) on soil compaction (Photo: R Dunn)

6.4 Remediation Through Farm Machinery

The soil compaction can be remediated through tillage by subsoilers and chisel plow and recommending more than 30 % residue cover after planting which reduces erosion of soil and increases the soil quality. Tillage implements used should not reduce the soil residue covers below 30 %. Hence, subsoilers which have narrow shanks, coulters cut through residue in front of the shank, leave the soil in a condition that is ready to be planted, leave most residues at the soil surface, and do not create much surface disturbance (Fig. 3.15).

Fig. 3.15

The modern subsoiler for remediating subsoil compaction and retaining surface residue cover

In conclusion, the soil compaction can be managed by using the following recommendations:

• Increase soil organic matter content (Fig. 3.16) and soil biota.

  Fig. 3.16

  

  Effect of soil organic matter on soil compaction

• Allow traffic only when soil moisture is low.

• Adopt conservation tillage system including cover crops.

• Use controlled traffic systems (research shows 10–15 % yield increase from controlled traffic).

• Use smallest vehicle possible for the job.

• Direct seeding.

• Crop rotations and management (alternating maize and soybean with fibrous and tap root systems, respectively).

• Introduction of deep rooting plants.

• Use in-row or bent-leg subsoilers that minimally disturb soil surface.

• Drainage improvement.

7 Conclusions

The soil structure, soil quality, and productivity everywhere in the world are threatened by soil compaction. The strength of the soil and soil compaction are directly correlated. The soil strength is reduced by higher soil moisture content. The conservation agricultural practices include controlling wheel/track loads, using low tire inflation pressure, avoiding unnecessary field traffic, and subsoiling/chiseling to remove hardpan developed due to traffic and puddling. The interaction effects of landscape position and land management practices on soil compaction are needed to avoid site-specific compaction and to optimize soil management practices using precision agriculture principles and technologies.