Physics

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SEEDBED SOIL STRUCTURE DECLINE

BackgroundSurface soil structure decline generally results in one of two things—hardsetting or crusting. A surface crust is typically less than 10 mm thick and when dry can normally be lifted off the loose soil below (Mullins et al., 1990). Crusting forces the seedling to exert more energy to break through to the surface thus weakening it. A surface crust can also form a barrier reducing water infiltration.

Soil structure breakdown caused by rapid wetting can lead to hardsetting. The unstable soil structure once wet, collapses and then shrinks as it dries. This leads to a ‘massive’ soil layer with little or no cracks and greatly reduced pore space (Needham et al., 1998). This hardset ‘massive’ structure is associated with poor infiltration, low water holding capacity and a high soil strength (figure 1). In many instances, this causes patchy establishment and poor crop and pasture growth.

Development and indicatorsNaturally hardsetting soils are unable to develop water-stable aggregates (Mullins et al., 1990). This means that during wetting, soil aggregates start to swell and become soft. This occurs prior to “slumping” (also referred to as “slaking”) when the aggregates collapse and disintegrate.

Hardsetting can also occur on soil with a high exchangeable sodium percentage (ESP) through “dispersion” of soil aggregates. This results in clay and silt becoming suspended in soil solution and causing a breakdown of aggregates (Mullins et al. 1990). Other factors that influence dispersion of soil aggregates include soil electrical conductivity (EC), calcium/magnesium ratios and organic matter content (Needham et al., 1998).

Soil types more prone to soil structure decline are sandy loams to clay loams (between 10 to 35% clay) (figure 2) —particularly those low in organic matter (<2%) (Needham et al., 1998).

Key points Hardsetting or crusting soils are usually indicators of poor soil structure.

A “massive” soil has significantly reduced pore space resulting in poor infiltration and low water holding capacity.

Bulk density is a good indicator of soil structure.

Increasing organic matter and decreasing traffic and stock can improve soil structure.

Gypsum can help in alleviating problems with hardsetting or crusting.

Figure 1: Soil structure decline in a hardsetting soil and associated agronomic restrictions. Adapted from Mullins et al. (1987).

Cultivated seedbed

Rainfall

Softening & partial disintegration of soil aggregates

Slumping

Drying (via evaporation) of soil surface

Suction causes movement of smaller particles (clay & silt) into empty pore

spaces

Surface hardening/crusting

Drying (via roots) of soil profile

Hardening extends downwards

Hardset profile

1. Reduced soil aeration

2. Reduced infiltration

3. Poor seedling emergence

4. Increased resistance to root growth

5. Timing and ease of future cultivation restricted

Agronomic restrictions

This soilquality.org.au fact-sheet has been funded by the Healthy Soils for Sustainable Farms programme, an initiative of the Australian Government’s Natural Heritage Trust in partnership with the GRDC, and the WA NRM regions of Avon Catchment Council and South Coast NRM, through National Action Plan for Salinity and Water Quality and National Landcare Programme investments of the WA and Australian Governments.

The Chief Executive Officer of the Department of Agriculture and Food, The State of Western Australia and The University of Western Australia accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.

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Figure 2: An example of a soil type prone to hardsetting—grey clay from the WA Great Southern region. Note the small topsoil depth (less than 10 cm).

It is important to make the distinction between crusting, hardsetting and compacted soils. Soil compaction results from repeated or long term movement of agricultural machinery and stock compacting (or “squashing”) the soil profile when it is moist, often remaining hard when wet. In many cases, this compaction layer occurs at depth. Crusting refers to a surface crust, and hardsetting generally affects the whole A1 horizon (Mullins et al., 1987), but in both cases the soil softens when moist. This gave rise to the term “Sunday soils”—the soil being too wet to seed Saturday and too dry to seed Monday!

Management to improve seedbed soil structureTo decrease the level of crusting or hardsetting in soils, it is necessary to stabilise soil structure. For example, amelioration of a hardsetting grey clay was found to be most effective using management practices that increased soil organic matter and reduced trafficking, thereby improving soil structure (Hamilton et al., 2005). Removing or reducing stock when the soil is saturated also helps avoid

compaction, smearing and “pugging” of the soil surface.

Another option for stabilising soil structure in soils prone to hardsetting or crusting is through the addition of gypsum on dispersive soils. This effectively displaces sodium, and causes clay particles to bind together helping to create stable soil aggregates. A resulting reduction in the ESP and increase in the calcium/magnesium ratio may be observed. Addition of lime also adds calcium to the soil, but is generally only used for soils with a low pH.

Measuring seedbed soil structure Bulk density (see Bulk Density fact sheets) is often used as an indicator of soil structure. It is the measure of the oven dry weight of soil in a known volume normally reported as g/cm3. A soil corer is hammered into the soil and removed without disturbing the soil inside (figure 3).

Bulk density is a good indicator of soil porosity—the higher the bulk density the lower the total soil pore space (Needham et al., 1998). This will also dictate the compressibility of soil but is not always related to soil strength (Needham et al., 1998). Coarse textured soils may already have a high bulk density (>1.6 g/cm3) so compaction forces will only increase it slightly but increase soil strength significantly.

Figure 3: An example of bulk density sampling where the topsoil has been sampled and the core is ready to be hammered into the 10 to 20 cm layer.

Further reading and referencesNeedham P, Moore G and Scholz G (1998) Soil structure decline. In ‘Soilguide. A handbook for understanding and managing

agricultural soils’. (ed G Moore) Department of Agriculture, Western Australia Bulletin 4343, pp 64-79.Mullins CE, Young AG, Bengough AG and Ley GJ (1987).Hard-setting soils. Soil Use and Management. Vol 3(2) pp 79-83.Mullins CE, MacLeod DA, Northcote KH, Tisdall JM and Young IM (1990) Hardsetting soils: behaviour, occurrence and

management. Advances in Soil Science 11: 38-108. Hamilton G, Fisher P, Braimbridge M, Bignell J, Sheppard J and Bowey R (2005) Managing grey clays to maximise production

and stability. Department of Agriculture, Western Australia Bulletin 4666.

Authors: Jessica Sheppard (Avon Catchment Council), Fran Hoyle (Department of Agriculture and Food, Western Australia)