IDENTIFYING AND ADDRESSING SOIL COMPACTION

Soil compaction can be a serious yield-limiting factor in crop production, and indications are that this problem is more widespread than is generally perceived. Since compaction is below ground and, thus, not visible, its role in yield decline often goes unnoticed.

Soil types

Soils vary in their susceptibility to compaction. Research indicates that soils with higher sand and silt contents are more vulnerable to being compacted than loam and clay soils. In addition, the higher levels of organic matter in humic soils reduce their susceptibility to compaction.

Causes

Management practices that promote compaction include the following:

• Intensive tillage. Tillage may temporarily loosen the soil; however, in the long-term regular tillage increases the bulk density of soils through the weakening of soil structure and the depletion of soil organic matter. In addition, implements such as the mouldboard plough and disk harrow compact the soil beneath their working depth. Repeated use of these implements can result in the development of plough pans (compacted zones immediately below the ploughed layer (Figure 1, left).
• Wheel traffic. The wheels of vehicles used to apply fertilizers, lime and other products, harvest crops and pull implements, result in decreases in soil porosity and yield-limiting compaction (Figure 1, right). In the production of annual crops, 60% or more of the soil surface can be trafficked through cultivating, planting, fertilization, herbicide and other chemical applications (Mitchell & Berry, 2001). The in-field traffic associated with maize silage operations and the production of sugarcane frequently results in severe compaction problems (Figures 2 and 3) and associated yield losses.
• Animal hooves. The role of animals, particularly in intensive grazing systems, in promoting soil compaction is often overlooked. However, although animals are not as heavy as machinery, their weight is applied to the soil in a relatively small hoof-print, and so can cause significant compaction and damage to pastures, thereby decreasing forage yields and animal performance (Figure 4).

Importantly, soils are most susceptible to being compacted when they are wet. Unfortunately, timing of farming operations, such as harvesting, fertilizing and rotating livestock, often makes it difficult to exclude machines and animals from soils when wet.

Effects of compaction

Detrimental impacts of compaction on soil health and plant growth are due largely to the decrease in the proportions of macro-pores in compacted soils. Effects associated with this include the following:

1. Root penetration into compacted soil layers is restricted, and root and biological soil health compromised through water logging and anaerobic conditions in soils.
2. Water infiltration as well as water-holding capacity of soils are reduced, and effective rainfall is diminished through increased runoff. Ponding on the soil surface after rain or irrigation is usually an indication of compaction.
3. Nutrient uptake by roots is poor in compacted soils. This is due to, firstly, limited root development, and secondly, restrictions on the movement of nutrients to roots by mass flow. Yellowing of plants due to nitrogen deficiency is often associated with poor nitrogen recovery by roots in compacted soils. The picture in Figure 2 (left) provides clear evidence of disturbed nutrition as a result of soil compaction.

Identifying compaction in the field

Compacted layers may be identified in two ways:

1. Use of a penetrometer or a steel probe. Penetrometer readings, as well as assessing the degree of difficulty in pushing a probe into the soil, are widely used to detect compacted layers. It is important to bear in mind, however, that these measurements can be highly subjective in that they are dependent on soil moisture content.
2. Examining the profiles in soil pits. Soil pits are particularly useful for identifying compacted layers and scrutinizing root health and proliferation. For observing compacted layers, pits generally need to be only about 50 cm deep. In the case of row crops, the pits should be across the row to enable root growth to be examined. The procedure is then to gently chip away at the exposed face of a pit with a pocket knife or trowel so as to expose a natural soil surface. Important characteristics to look for include the following:
   • Evidence of crusting at the immediate soil surface.
   • Soil health in the plough layer: is it well-structured with porous crumb structure, or poorly structured with large clods having smooth dense faces? (these conditions are clearly illustrated in Figure 3).
   • Is there evidence of a hardened, compacted ‘plough-pan’ immediately below the plough layer (Figure 1, left)? When plough-pans are well-developed, there is often horizontal growth of roots above the pan (Figure 2, right), with little or no roots occurring below the pan. Furthermore, there may be ‘perching’ of water above the pan, with the soil showing signs of greying or mottling due to the resultant anaerobic conditions.
   • Is there evidence of compaction further down in the soil profile, or in certain areas due to wheel traffic?

Correcting soil compaction

Where compacted layers are identified, it is important that corrective action be undertaken as soon as possible in order to minimize yield losses. In the short-term, this usually involves ripping; however, as detailed below, a number of options that may complement ripping are also available.

• Ripping. Chisel ploughs (rippers) effectively fracture compacted layers, allowing roots to penetrate to depth. For row crops, ripping under the row before planting has been shown to be highly effective in compacted soils (Botha & Bennie, 1982). In the case of the ratoon growth of sugarcane, where compaction has been identified, ripping inter-rows is generally beneficial and often results in significant yield increases. The beneficial effects of the deep ripping of a compacted soil on the growth of soyabeans is shown in Figure 5 (left).
• Ripping is costly, since considerable power is required to pull chisel ploughs through the soil; therefore, it is important that ripper penetration into the soil is carefully controlled and does not extend to depths beyond the compacted layer (there is often no need to rip to depths of greater than about 40 cm). Soil moisture content also needs to be taken into account when ripping. If the soil is too wet, lifting and cracking will not take place, and if the soil is too dry, large blocks of soil may be lifted, but not cracked. Ripping is best carried out when the soil is moist and friable, as at this moisture content, a block of soil will crumble when pressed between the fingers (Haynes, 1995).
• Unfortunately, the effects of ripping are usually temporary, and, depending on field management practices, there is usually a need to repeat the ripping within a year or two.
• In the case of soils under pastures, the damage caused by trampling and ‘pugging’ can generally be addressed through ripping (‘aerating’) to a depth of 20 to 25 cm. However, here too, the operation often needs to be repeated annually.

 

• Deep-rooted plants. In many situations, the use of deep-rooted cover crops is an effective alternative to ripping. Species with thick taproots, such as forage radish, lucerne and lupins, are best suited for this purpose. It has been found that mixing forage radish with grasses such as stooling rye or oats provides a useful combination of both ‘bio-drilling’ and mulching benefits.
• Use of organic products and crop residues. Organic products such as crop residues, filtercake, bagasse, compost and manures are of immense value in maintaining soil structure and reducing soil susceptibility to compaction. Data presented in Figure 6 clearly reflect the decreasing compactability of soils with increasing organic carbon content.
• Green manure and cover crops. Green manure and cover crops, by adding organic matter to the soil, improve soil structure and reduce the compactability of soils. It should be noted, however, that there are differences between species in this respect. Thus, grasses such as oats, wheat, barley and stooling rye, have dense, ramified root systems that have very beneficial effects of soil structure. In addition, indications are that the crucial role of soil cover has long been neglected in conventional farming operations. Where the soil surface is covered with residues, the development of surface crusts is prevented, water infiltration is improved, earthworm activity encouraged and compaction minimized (Figure 5 right).
• Earthworms. Too frequently overlooked is the valuable role of earthworms in combating soil compaction, particularly under no-till cropping and ‘green-cane’ harvesting. When soil acidity problems are addressed through liming and residues are maintained on the surface as a food supply, earthworms proliferate and their burrows promote water infiltration and aeration, and provide channels for root growth and extension (Figure 6, right). Charles Darwin wrote that “it may be doubted whether there are many other animals which have played so important a part in the history of the world as have these lowly organised creatures”. Darwin referred to earthworms as “natures ploughs”.

Prevention of soil compaction

The following are important strategies for avoiding the development of compacted layers in fields:

• Limit tillage operations and passes of heavy equipment over fields, and use wide flotation tyres wherever possible.
• Do not allow soils to be trafficked when very wet.
• Controlled traffic: restrict all wheel traffic to previously designated lanes, and thereby keep 90% or more of the field free of compaction.
• Maintain crop residue cover on the soil surface.
• Avoid tilling soil when very wet, and if possible, avoid grazing pastures immediately after heavy rainfall and irrigation.
• Vary the depth of tillage and use different types of tillage implements to minimize the development of plough-pans.

 REFERENCES

Botha, FJP and Bennie, ATP. The influence of tillage on the water use efficiency of maize under irrigation. Crop Production 11: 26-29.

Haynes, RJ. 1995. Soil Structural Breakdown and Compaction in New Zealand Soils. MAF Policy Technical Paper 95/5. Wellington, New Zealand.

Mitchell, FJ and WAJ Berry. 2001. The effects and management of compaction in agricultural soils. Proceedings of the South African Sugar Technologists Association. 75:118-124.

Thomas, GW, Haszler, GR and Blevins, RL. 1995. The effect of organic matter on maximum compactability of soil. Soil Science News and Views. 122. https://uknowledge.uky.edu/pss_views/122