Compaction caused by applied force
Compaction of soil is created by a combination of pressure and sliding forces as they are applied to the soil from a wide range of sources. These forces include driving and trailed wheels, plough soles, disc edges, rotary blades and grazing livestock. The way a soil reacts to application of pressure depends on the texture of the soil, how wet it is, how hard it is, its depth below the surface and also on the shape of the contact area.
The water content is the critical factor that influences the reaction of a soil to pressure. A hard, dry soil of whatever texture has considerable internal strength and may not react in any way to pressure until enough pressure is applied to fracture the soil, loosening rather than compacting it. In such a dry state, the force required to break it (i.e. cultivate it) may be large and beyond the capability of all but the most powerful equipment. As the soil rewets, its internal strength reduces and less force is required to break it up. Just before it contains enough water to become plastic and malleable, a soil reaches a state when lumps of it break off with minimum force. This is the ideal state for safe cultivation. If too wet (soft and plastic when handled) then compaction is inevitable from any significant pressure applied. Thus, compaction can usually be traced to pressure applied to the soil when it was wet and soft. Compaction is greatest when rainfall coincides with either seedbed preparation or harvest.
Apart from moisture content, soil organic matter content is an important factor influencing soil compactibility. Increases in organic matter may reduce compactibility by increasing the stability and strength of aggregates and, therefore, their resistance to deformation, and also by increasing the elasticity of the soil (i.e. the ability of the soil fabric to "rebound" back to its original state after initial compaction).
Effect of Various Crops on Soil Structure
For many generations, some crops have been considered as having "good" or "restorative" effects on soil structure whilst others have been considered "bad". It is almost impossible to separate the direct effects of the crop itself and the effects due to its field management and harvest. Indeed, the individual management of a crop often plays a dominant role in its overall effect on soil condition. The general influence that various crops have on soil condition is outlined below. The major points are summarised in Table 1.
Table 1: Summary of relative effects of growing various crops on soil organic matter content and soil structure
| Relative benefits | ||||
To soil |
To soil |
Other |
Other |
|
| Crops | organic matter |
structure |
benefits |
Possible adverse effects |
| Grass | Very good | Very good | Soil surface protected | Treading damage by grazing livestock |
| White clover seed crop | Poor | Poor | N input via N2 fixation | Treading damage if grazed |
| Autumn cereals | Moderate | Good | Stubble protection | Structural decline caused by annual cultivation |
| Spring cereals | Moderate | Moderate | Stubble protection | Structural decline caused by annual cultivation |
| Maize and sweetcorn | Poor | Poor | - | Structural decline caused by annual cultivation |
| Oilseed rape | Moderate | Moderate | Stubble protection | Structural decline caused by annual cultivation |
| Peas/beans | Poor | Poor | N input from Residues | Structural decline caused by annual cultivation, risk of damage if harvested when land is wet |
| Forage crops | Good | Moderate to poor | - | Structural decline caused by annual cultivation, risk of soil damage if grazed when land is wet |
| Vegetables | Poor | Poor | - | Severe structural decline caused by repeated rotary cultivation, risk of damage if wet when harvested |
| Fallow | Decline | None or negative | Reduction of weeds, pests and diseases | No drying and cracking of the subsoil |
Pasture Plants
Grass
Grass can be grown in rotation with arable crops either as a seed crop or in a grazed pasture in association with clover. It has a well-deserved reputation for maintaining and improving soil organic matter content and soil structure. Soil organic matter accumulates under pasture primarily because of the exceptionally dense, ramified grass root system. Plant roots are continually growing and dying so that their growth adds considerable organic material to the soil. The stability of soil aggregates and the porosity of the soil is also increased under pasture. The increase in stability of soil aggregates under grass occurs because:
- the build up in humus under pasture results in soil aggregates being more strongly bound together,
- a large microbial population develops around the dense root system and this population produces gums that help bind the soil particles together,
- the growing roots and associated fungal filaments help "enmesh" and bind soil particles together, and
- earthworm populations increase rapidly under pasture. Earthworms ingest large amounts of soil below the soil surface. The casts have a high microbial activity and become firmly bound together.
Porosity of the soil is increased under grass because:
- growth and death of the extensive root system causes formation of "root channels",
- the large water use (transpiration loss) from a growing pasture sod results in a high intensity of drying and rewetting cycles and thus crack formation, and
- the extensive borrowing activity of the large earthworm population creates a network of large pores.
It should be noted that to some extent the improvement in soil structure that occurs under pasture can be offset if grazing is practised when the soil is very wet (at or above field capacity). Under such conditions, compaction caused by treading can occur (see page 22).
Clover
White clover, and sometimes other clovers, are present in grazed pastures. In addition, white clover can be grown as a seed crop within arable crop rotations. The major benefit of clover is to the N fertility of the site. In clovers, as well as other legumes, N is accumulated from the atmosphere via biological N2 fixation in their root nodules. Clover, however, has a rather sparse root system and has little positive effect on soil structure. Inputs of organic matter to soils are generally much greater from grasses than clover.
Contact for Enquiries
MAF Information Services
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PO Box 2526
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