Soil structure refers to the arrangement of sand, silt, and clay particles into larger units called aggregates. A well-structured soil forms stable crumbs, which creates a mix of large and small pores necessary for plant survival. The spaces between these aggregates determine how easily water infiltrates the soil, how much air is available to roots, and how deeply roots can penetrate to access nutrients and moisture. Improving soil structure is a foundational practice for ensuring healthy plant growth and increasing the soil’s resilience against erosion and compaction.
Building Structure with Organic Matter
Organic matter (OM) is the primary driver of long-term soil structure improvement, acting as the foundation for creating stable aggregates. Fresh organic material feeds soil microorganisms, which then produce the binding agents needed to glue soil particles together. These microbes excrete sticky substances, such as polysaccharides and a glycoprotein known as glomalin, that function like biological cement.
Glomalin is a tough, hydrophobic substance secreted by the hyphae of arbuscular mycorrhizal fungi, and it acts as a protective coating for aggregates. This glycoprotein binds soil minerals and organic matter into water-stable clumps. Polysaccharides also contribute to this process, providing the initial sticky glue that holds smaller particles together.
To increase the biological activity that generates these glues, practitioners should continuously introduce organic material into the soil profile. Applying compost, which is already partially decomposed, provides a readily available food source for microbes. Utilizing mulches and planting cover crops (green manures) introduces fresh plant residues both above and below ground.
The decomposition of these inputs eventually leads to the formation of humus, the stable fraction of organic matter that can persist for decades or even centuries. Humus, a dark and chemically stable substance, contributes to structure by stabilizing existing aggregates. It also provides a large capacity for nutrient retention, offering enduring structural benefits.
Physical Management to Protect Existing Aggregates
While adding organic matter builds structure, careful physical management is necessary to prevent the destruction of newly formed aggregates. Tillage, the mechanical turning of soil, is a major threat because it physically breaks apart stable clumps created by microbial activity. This disruption exposes organic matter within the aggregates to oxygen, speeding up decomposition and leading to the rapid loss of carbon and structural stability.
Adopting reduced tillage or no-till practices is the most effective way to protect existing soil aggregates and allow microbial networks to flourish undisturbed. Repeated disturbance makes soil susceptible to compaction, often forming a dense hardpan below the tilled zone that restricts root growth and water movement. Minimizing the use of heavy machinery and reducing foot traffic also prevents compaction, which crushes the pore spaces necessary for air and water flow.
The timing of physical work is also a factor in maintaining soil structure. Working soil when it is too wet rapidly destroys aggregates because water lubricates the particles, allowing them to smear together and dry into dense clods. Protecting the soil surface with plant residue or mulch shields aggregates from heavy raindrops, which can shatter them and lead to surface crusting.
Targeted Mineral Amendments for Soil Flocculation
Specific mineral amendments can be used to improve the structure of certain problematic soils, complementing biological and physical management. Gypsum (calcium sulfate) is primarily used to treat sodic soils, which have an excess of sodium ions. Sodium causes clay particles to disperse, clogging soil pores and leading to poor drainage and surface crusting.
The calcium ion in gypsum is divalent, possessing a stronger positive charge than the monovalent sodium ion. When applied, calcium displaces the sodium ions attached to the clay particles through an ion exchange process. This replacement encourages the clay particles to flocculate, or clump together, increasing the soil’s porosity and improving water infiltration. The displaced sodium is then flushed below the root zone by water, effectively reclaiming the soil.
Adjusting soil pH also indirectly supports better structure by optimizing the environment for microbial life and nutrient cycling. For example, applying lime (calcium carbonate) to overly acidic soils raises the pH, improving nutrient availability and enhancing soil organism activity. Healthy microbial populations are necessary for the continuous production of the organic glues that stabilize aggregates. Building a resilient soil structure requires the continuous input of organic materials alongside careful management practices and, when necessary, targeted mineral applications.