How to evaluate cropping management practices: the cook book
Soil compaction is a form of physical soil degradation where an increase in soil bulk density, and a decline in percentage and stability of aggregates as well as porosity and pore continuity, is verified (Kooistra and Tovey, 1994). Possible problems associated with soil compaction are: decreased aer...
|Subjects:||AGRICULTURAL SCIENCES AND BIOTECHNOLOGY, CROP MANAGEMENT, FARMING SYSTEMS, TRAINING COURSES, TRAINING PROGRAMMES,|
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|Summary:||Soil compaction is a form of physical soil degradation where an increase in soil bulk density, and a decline in percentage and stability of aggregates as well as porosity and pore continuity, is verified (Kooistra and Tovey, 1994). Possible problems associated with soil compaction are: decreased aeration (increased proportion of soil pores filled with water) (Stepniewski el aL, 1994), decreased water infiltration, increased surface runoff and erosion, as well as poor crop establishment (seed germination and early root growth) and root development (Lal and Shukla, 2004; Logsdon and Karlen, 2004). Compaction can occur when heavy farm machinery circulates over the field, especially under wet conditions (Hom el al., 2006). Tillage has also been found to cause soil compaction through plough pan formation. Because of its importance, soil bulk density (Blake and Hartge, 1986) is very frequently included in analysis of soil quality (e.g. Govaerts el al., 2005). There have even been attempts to establish soil bulk density (SBD) threshold values, to indicate when compaction is occurring (e.g. 1.55 Mg m3 in silt and silt loam soils). This is based on the rationale that if bulk density is higher than a critical level, considering variations caused by soil texture, compaction would be present (USDA-NRCS, 1996). However, increased SBD values are not necessarily related to compaction since this parameter is dependent on a wide array of factors such as type of parent material, the crop being grown, soil organic matter content and type of present and past management (Logsdon and Karlen, 2004). Management, for example, has an overarching effect on soil physical properties, including soil packing density. Under conservation agriculture (CA), soil is not tilled and a protective residue cover is left over the soil's surface. Recent studies (Logsdon and Karlen, 2004; Osunbitan el al., 2005; Mati and Kotorová, 2007) found that soil bulk density values were higher under CA when compared to conventional systems. Soils treated under CA are denser but characterized by stable macroporosity (formed by soil macrofauna and decayed plant roots) with significant effects on soil hydraulic conductivity. Greater total porosity in tilled soils is not related to greater water infiltration given their lack of connectivity, lower proportion of macropores and temporary character (Osunbitan et al., 2005). This illustrates how soil bulk density is not necessarily related to compaction and threshold values designed for conventional tillage (CI) are not necessarily applicable to CA. In conclusion, measures on aggregation, water infiltration and crop performance have to complement SBD values. But also temporal variations in soil bulk density values due to changing conditions, particularly at the topsoil where agricultural management and environmental conditions have their greatest impact, makes repeated measures throughout the season highly recommended. For example, Logsdon et al. (1999) and Logsdon and Cambardella (2000) showed significant temporal changes in near-surface incremental bulk density for tillage systems in a subhumid climate.|