• What Affects Soil Structure?

What Affects Soil Structure?

What Affects Soil Structure?

Susceptibility to Compaction

Soil susceptibility to compaction is related to texture coarseness, packing density, and organic matter content, together with the prevailing climate and land use (Jones, 2003). Coarser soils, soils with low packing density, and soils with higher organic matter content are usually more susceptible.

Consolidation

At a reduced level, compaction can be beneficial where it increases seed-to-soil contact, capillary movement of water and root-to-soil contact; generally, this “positive” compaction is referred to as “consolidation.”

Tillage Systems

Tillage has many influences on soil quality. Less aggressive tillage methods generally result in improved soil structure and, when combined with Controlled Traffic Farming (CTF), show significant savings in energy required to establish the crop. Conventional tillage techniques have a place in the control of resistant weeds, alongside crop rotations and ahead of certain crops, but need care in their application if soil health is to be improved.

Field Traffic

Vehicle traffic such as tractors, combines, and other implements penetrating deeply into the soil profile may cause semi-permanent compaction, which will reduce yields for many years (Raper, 2006).

Natural Factors

Soil structure can be affected by many natural factors such as freeze/thaw cycles and wetting/drying cycles (depending on soil type); however, most research points to only gradual improvements to soil structure caused by natural processes.

What is an Ideal Soil Structure?

In general terms, the ideal ratio of solids to water to air in a well-structured soil profile is approximately 50:23:23, with organic matter (OM) at 4% or greater. OM is made up of carbon, nitrogen, phosphorous and potassium, together with trace elements. It influences plant growth by affecting soil pH, assisting structure, easing cultivations, and supplying nutrients.

In addition to these basic factors, many other elements contribute to good soil structure and health. These include the vast array of micro- and larger organisms that co-exist to form the soil ecosystem. Additionally, maintaining the correct soil mineral balance helps optimize the health and productivity of crops, plus assists in creating a stable soil structure. Soils with a high cation exchange capacity (CEC) are capable of providing high levels of nutrients to the crop (Scamell, 2000), and the correct balance of these elements (particularly Ca and Mn) assists soil stability.

Tillage has the ability to destroy or enhance these balances, and the type of tillage system can greatly impact soil quality. 

Tillage systems should never be applied as a “blanket” approach, but carefully managed from season to season. Timing and choice of tillage techniques depend on the field circumstances at the time; however, certain guidelines can be adopted, according to the general soil type and condition.

Heavy Soils: Poor Structure

(1) Surface is rough and cloddy with poor seed-to-soil contact.

(2) Surface is too fine and prone to moisture loss and erosion.

(3) A clear distinction exists between zones. Such layers create barriers to moisture movement.

(4) Layers below the surface are dense and blocky, and cracks and fissures are generally horizontal.

(5) Horizontal layers are frequently present, forming barriers to roots, water, air and nutrients. Trash inverted above such layers is often anaerobic, creating additional barriers to root growth.

When digging these structures: It is often difficult to break apart the soil mass by hand, or by dropping it two or three times onto a firm surface from 36" to 40" height, when it is in a friable state.

Heavy Soils: Good Structure

(6) Surface horizons are a combination of stable aggregate sizes and organic matter, consolidated for good seed-to-soil contact, moisture retention, and weatherproofing.

(7) There is a gradual transition to larger, structured aggregates at depth. The structure has varying sizes of open pores to hold water, air, and nutrients, allowing roots to access these through the profile. No distinct layers exist. Cracks and fissures are generally vertical, allowing water, air, and roots to easily pass through the profile to depth. Such fissures are generally supplemented by worm channels.

When digging these structures: It is easy to break open the larger aggregates by hand. Additionally, the soil (again, in a friable state) will easily break open when dropped onto a firm surface two or three times from 36" to 40" height.

Light Soils: Poor Structure

(1) Surface can be capped and structureless, forming a barrier to emerging plants, which causes erosion and run-off (crusting).  This also prevents the upper layers from drying after rain, which delays seeding. If proper drying doesn’t occur, additional passes may be needed to loosen the surface and speed up the drying process. Such passes can risk compaction by tractor wheels with the soil in such a state.

(2) A clear distinction exists between zones. Such layers create barriers to moisture movement.

(3) Horizontal layers below the surface can be dense and blocky, resulting in poor root growth and poor water and nutrient access. Cracks and fissures are often horizontal, as opposed to vertical.

(4) Density-change layers exist, forming barriers to roots, water, air and nutrients.

When digging these structures: It is often difficult to break apart the soil mass by hand, or by dropping it two or three times onto a firm surface from 36" to 40" height, when it is in a friable state.

Light Soils: Good Structure

(5) Surface horizons are a combination of stable aggregate sizes and organic matter, consolidated for good seed-to-soil contact. In soils prone to crusting, organic matter provides stability, and methods of consolidating the soil are available to minimize surface crusting and its restrictions to seedling emergence.

(6) There is a gradual transition to larger, open granular aggregates at depth. The structure has open pores of varying size to hold water, air, and nutrients, allowing roots to access these through the profile. No distinct layers exist, and roots and worm channels are widespread to depth. Cracks and fissures are generally vertical.

When digging these structures: It is easy to break open the larger aggregates by hand. Additionally, the soil (again, in a friable state) will easily break open when dropped onto a firm surface two or three times from 36" to 40" height.

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Tillage represents a significant part of the cost of crop establishment and growth and, when correctly applied, delivers increased yields. Assess the health of your soil to determine how tillage can improve your soil structure.