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Ministry of Agriculture, Food and Rural Affairs

SOIL STRUCTURE

Soil structure refers to how the soil particles (sand, silt and clay) are arranged into clumps or aggregates. Soil aggregates are bound together by clay, organic matter and root exudates. Soil structure has a significant effect on crop establishment, growth and productivity.

Soil structure influences:

  • the movement of water into and through soil;
  • the degree of aeration;
  • the ability of crop roots to grow through the soil profile; and
  • the ability of the soil to resist soil erosion.

The structure of a soil is influenced by soil texture; climate; biological activity; and farm management practices, including tillage, crop rotation and machinery/equipment use patterns. Soils are often described as structureless, spheriodal, blocky or platy:

  • Structureless soils have no observable aggregation and no definite arrangement of the soil particles. Sands and sandy-loam soils are often structureless with a single grain arrangement of the soil particles. Clay soils may also be described as structureless when the particles form a massive structure with no small aggregates within. This is more commonly seen in finer textured soils like clays, particularly when they have been worked wet or exposed to heavy loads under wet conditions.
  • Spheriodal soils have a granular structure. The aggregates are 1- 10 mm (1/25- 2/5 in.) in diameter with rounded corners. This structure is usually seen in the A horizon or the upper layer of fine or medium-textured soils. Granular structure in the seed or transplant zone is important for good contact and early establishment.
  • Blocky soil aggregates are often larger in size (5- 50 mm or 1/5- 2 in. in diameter) and form irregular-shaped or cube-like blocks. This structure promotes good drainage, aeration and root penetration. Usually these structures are found in the B horizon or just below the colour change in many soils. Blocky structures greater than 50 mm (2 in.) in diameter are often an indication of compaction or soil management problems.

  • Platy soils form thin layers or horizontal planes. This type of structure can be found in both surface and subsurface soil horizons. It is commonly seen in undisturbed or no-till soils. Platy structures in sandy soils often indicate compaction problems.

Management Notes
Soil structure can be improved through a variety of management techniques including: crop rotations involving cereals and forages; the use of cover crops; the addition of organic matter sources like manure and compost; and reduced tillage. Be aware of the number of traffic passes, axle loading, harvest travel patterns and the depth of tillage.

Crust Rooting and soil structure Granular soil structureClick to enlarge.