Soil compaction causes serious damage and can only be reversed very slowly and at significant cost. Compaction restricts root growth and reduces infiltration of water into soil. It can increase runoff, which may lead to greater flooding, increased erosion, and the transfer of potential pollutants (including nutrients and pesticides) to surface waters. As the air getting into the soil is also restricted, the biological activity and root growth is affected. This reduces the fertility of the soil and, more specifically, the availability of plant nutrients. It is important to minimize all forms of soil compaction. (Point 52 of The Soil Code, 1998).
Compaction reduces yields:
Compaction is a significant factor influencing crop yield. Barriers to root growth restrict access to water, air and nutrients, reducing yield by more than 50% in many crops, and causing a total write-off in severe cases. Excessive runoff and erosion can also result.
Compaction, as measured by cone resistance or by soil bulk density, directly affects root growth and yield (graph, above; Russell, 1977).
Compaction restricts water and nutrients:
In addition to creating a physical barrier to roots, compaction also affects the transmission of nutrients and trace elements through the soil, again restricting growth and yield. In most cases, nutrient uptake is restricted by compaction, and N losses downward to groundwater and upward to the atmosphere are generally found to be greater in compacted soil (Lipiec, 1991).
Compaction assists the production of N2O in soil due to its negative effect on soil aeration. N2O is formed both naturally by nitrification during breakdown of crop residue, and also during denitrification under compact, anaerobic conditions. Denitrification therefore reduces beneficial soil nitrate levels. Where high mineral N levels are present (either naturally or by fertilization), such emissions can quadruple (Sitaula, 2000), leading to increased environmental concerns (N2O is a greenhouse gas having a global warming potential many times that of CO2) and increased fertilizer cost implications through its inefficient use.
• Water movement (1) through changes in bulk density or aggregate size by capillary action is severely restricted—this being the case unless the soil is saturated, where gravity can then take over.
• Aggregates (2) with smaller or larger pore size are often influenced by cultivation. Layers 1 to 4 represent a traditionally ploughed and power-harrowed profile on many soil types.
• Water movement (3) from small pores to large pores is very slow in an unsaturated state (Eagleman, 1962), as the capillary pressure holding the water is higher in aggregates with small pores.
• Zones (4) below such layers therefore can have restricted water (a perched water table is one example).
• Compacted layers (5) of high bulk density also restrict water, air and nutrient movement.
• Zones (6) below compacted layers are not easily available to roots. Therefore, in dry summers, pod or grain filling can be restricted, which leads to early maturing and loss of crop potential and yield.
• Upward water (or air) movement (7) is also restricted by pans or changes in aggregate size. This can have a significant effect in dry seasons where upward water movement by capillary action can benefit establishing crops with shallow root systems. Tillage should therefore reflect the need to create the correct soil structure, and avoid sharp changes in aggregate size. Vertical tillage creates uniform density through which air, water, nutrients, and water can move freely.