ReviewSoil compaction in cropping systems: A review of the nature, causes and possible solutions
Introduction
Intensive farming of crops and animals has spread all over the world and involves shorter crop rotations and heavier machinery that lead to an increase in soil compaction (Poesse, 1992). The extent of compacted soil is estimated worldwide at 68 million hectares of land from vehicular traffic alone (Flowers and Lal, 1998). Soil compaction is estimated to be responsible for the degradation of an area of 33 million ha in Europe (Akker and Canarache, 2001) and about 30% (about 4 million ha) of the wheat belt in Western Australia (Carder and Grasby, 1986). Similar problems related to soil compaction have been reported in almost every continent (Hamza and Anderson, 2003 (Australia); Aliev, 2001 (Azerbaijan); Ohtomo and Tan, 2001 (Japan); Bondarev and Kuznetsova, 1999 (Russia); Tardieu, 1994 (France); Suhayda et al., 1997 (China); Mwendera and Saleem, 1997 (Ethiopia); Russell et al., 2001 (New Zealand)).
Although farming systems have improved significantly to cope with the new pressures associated with intensive agriculture, the structure of many otherwise healthy soils has deteriorated to the extent that crop yields have been reduced. Soil compaction is defined as: “the process by which the soil grains are rearranged to decrease void space and bring them into closer contact with one another, thereby increasing the bulk density” (Soil Science Society of America, 1996) and is related to soil aggregates because it alters the spatial arrangement, size and shape of clods and aggregates and consequently the pore spaces both inside and between these units (Defossez and Richard, 2002).
The nature and extent of this degradation, which can be exaggerated by the lack of organic matter, has been recognised worldwide. Compaction also affects the mineralization of soil organic carbon and nitrogen (Neve and Hofman, 2000) as well as the concentration of carbon dioxide in the soil (Conlin and Driessche, 2000).
Although compaction is regarded as the most serious environmental problem caused by conventional agriculture (McGarry, 2001), it is the most difficult type of degradation to locate and rationalize, principally as it may show no evident marks on the soil surface (Fig. 1). Unlike erosion and salting that give strong surface evidence of the presence of land degradation, degradation of soil structure requires physical monitoring and examination before it is uncovered and its extent, nature and cause resolved. The hidden nature of soil structural degradation (SSD) leads to specific problems such as poor crop growth or water infiltration that may be blamed on other causes. In addition, SSD is often blamed for poor crop performance when it is actually not present. Farmers rarely link their land management practices to the causes of SSD and remain unaware that many deep-ripping exercises worsen SSD (McGarry and Sharp, 2001). Because subsoil compaction is very persistent and possibilities of natural or artificial loosening have been disappointing, it has been acknowledged by the European Union (EU) as a serious form of soil degradation (Akker and Canarache, 2001).
The effects of soil compaction on crops and soil properties are complex (Batey, 1990) and since the state of compactness is an important soil structural attribute, there is a need to find a parameter for its characterization, such as relative bulk density, that gives directly comparable values for all soils (Håkansson and Lipiec, 2000). Since soil bulk density is the mass of dry soil per unit volume, then the relationship between soil compaction and its capacity to store and transport water or air is obvious. For this reason the dry soil bulk density is the most frequently used parameter to characterise the state of soil compactness (Panayiotopoulos et al., 1994). However, in swelling/shrinking soils the bulk density should be determined at standardised moisture contents, to prevent problems caused by water content variations (Håkansson and Lipiec, 2000).
Soil strength is also used as a measure of soil compaction because it reflects soil resistance to root penetration (Taylor, 1971, Mason et al., 1988, Panayiotopoulos et al., 1994, Hamza and Anderson, 2001, Hamza and Anderson, 2003). Soil water infiltration rate also can be used to monitor soil compaction status, especially of the topsoil. Water infiltrates uncompacted soils that have well-aggregated soil particles much faster than massive, structure-less soils (Hamza and Anderson, 2002a, Hamza and Anderson, 2003).
Interestingly a slight degree of topsoil compaction may prove beneficial for some soil types (Bouwman and Arts, 2000) indicating that there is an optimum level of compaction for crop growth. The concept of optimum level of compaction is important, especially in controlled traffic system where any external source of compaction is avoided because it might cause a sub-optimal level of compaction and yield depressions. Also if compaction is confined to the sub-surface only, roots may grow more laterally or coil upward toward the less compacted layers with no significant decrease in yield (Rosolem and Takahashi, 1998).
This review concentrates mainly, though not exclusively, on crop/livestock systems in the rainfed areas. It mainly considers research published in the period since the major reviews on soil compaction by Soane et al. (1982); Soane (1990); and Soane and Van Ouwerkerk (1994).
Section snippets
Factors effecting soil compaction
In modern agriculture, farm animals and machines cause most of the soil compaction. Working the soil at the wrong soil water content exacerbates the compaction process. Accordingly, the influence of soil water content and compaction induced by farm animals and machines will be reviewed here.
Solutions to soil compaction problems
Since soil compaction mainly decreases soil porosity (or increases soil bulk density), then increasing soil porosity (or decreasing bulk density) is a clear way of reducing or eliminating soil compaction. Managing soil compaction, especially in arid and semi-arid regions, can be achieved through appropriate application of some or all of the following techniques: (a) addition of organic matter; (b) controlled traffic; (c) mechanical loosening such as deep ripping; (d) selecting a rotation which
Conclusion
The ever-increasing population of the world necessitates the intensification of farming and cropping systems to cope with the demand for more food. As a result, more and heavier farm machinery and/or animals per land surface area have become common all over the world. This intensification of the farming system has led to soil compaction and deterioration in soil physical fertility particularly in dryland areas. Soil compaction adversely affects soil physical fertility, particularly storage and
Acknowledgments
This study was supported by the Grains Research and Development Corporation and the Department of Agriculture Western Australia. Helpful comments during preparation of the manuscript were received from Mr. P. Price, Dr. R. Belford and Dr. D. Carter.
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