Soil Structure
Defined as "arrangement of soil particles into aggregates"
Soil structure can be altered by the addition of organic material such as compost or humus because organic material helps to form aggregates. Aggregates are composed of many soil particles held in a single mass or cluster such as a clod, crumb, block, or prism.

Structures distinguishes soil from weathered rock
Structure is degraded in cultivated soils
Structure impacts pore spaces between aggregates

Particle Arrangement
Some aggregates are stronger than others
Clay and humus are most important binding agents
Aggregates or peds Soil particles held together by stabilizing agents Described according to their shape, size, and stability = how easy they can be broken apart Soil has structure when most soil particles are aggregated Structure is an important morphological characteristic Controls the size and number of pores Kind of structure is determined by the kind of aggregates in the soil

Structure-less soil – does not form aggregates
Non-aggregated soil is not bound together such as sand
Massive All particles are stuck together No individual aggregates Single grain Each particle acts individually Sand dunes and beaches Very likely to erode

Structure Strength
Particles stuck together = structure Water, shrinking, swelling, freezing, thawing, roots and animals cause particles to be pushed together and pulled apart Weak Aggregates break apart with little handling Moderate Soil removed from the horizon is aggregate Strong Aggregates do not readily fall apart when handled Structure is described for each horizon Structure differences help define horizons Once particles are stuck together, they form structure Water and organic matter help particles stick together

Measured as the resistance of aggregates to destruction by water Stabilizing agents found in soil (commonly found in B horizon) = helps to bind particles into aggregates Silicate clay Organic materials Oxides of iron (Fe) and aluminum (Al)

Measures soil cohesion (like +like) and adhesion (water + soil) and resistance to deformation or rupture Dry Sandy loam: individual grains do not stick together Moist Loose to extremely firm depending on the soil Wet Sticky
• Highly decomposed organic matter
• “organic glue” forming aggregates
• Highly charged
• Large surface area per unit mass

Clay influences soil as much as water. Why?
Increases the hardness of dry soil Increases the firmness of moist soil Increases the stickiness and plasticity of wet soil