Several forces bind soil

HAVE you ever wondered why clay soils have such a good ability to absorb water? And yet, these same soils can become muddied and are prone to standing water in wheel tracks from vehicles traversing a pasture. Some might call this an effect of soil compaction, and it might be, depending on the circumstances. However, we also need to consider another process that occurs in soil, and that is the process of soil aggregation.

Soil aggregation is the clustering of the smallest soil particles — clay and silt-sized particles that would often be too small to see as individual particles — into larger soil crumbs that resemble small stones or pebbles. These clusters of soil particles are bound together by a variety of organic and inorganic gluing mechanisms.

Historically, soil science has focused on inorganic glues that bind soil particles by electrostatic forces, chemical bridges, and chelating agents. However, soil biological mechanisms can be equally important in cementing soil particles together into stable crumbs that create a diversity of soil pore sizes between these crumbs but also keep their strong cationic attraction forces.

Root of attraction

Plant roots often find their way through soil by exploring the path of least resistance, following water and nutrients in the spaces between soil crumbs (or aggregates). These growing roots deposit water-soluble and other by-products of growth onto and between soil crumbs. Earthworms and other soil animals might also push soil particles to the side to consume soil and organic resources deposited in soil.

All of this tugging and pushing, along with the deposition of fecal pellets and various reworked soil and carbon substrates, leads to a dynamically structured soil. Soil fungi and bacteria are consuming these root-derived inputs continuously, but they are also leaving behind some of the glues that bind soil particles together. Further, as soil dries and rewets over and over, it becomes more structured over time.

The activity of soil microorganisms and roots are key elements of soil aggregation. Since these factors are most active near the soil surface, soil aggregation is also strongest near the soil surface. One way to measure the impact of soil management on aggregation is by determining the soil stability index. This index measures the average size of aggregates following a short period of immersion and oscillation in water relative to the average size of aggregates following light crushing of dried soil.

A grassland advantage

Vigorously growing forage plants produce an abundance of roots to extract water and nutrients. They are also secreting carbon substrates that feed soil microorganisms to glue soil particles into aggregates. In a recent survey of paired land uses across 25 sites in North Carolina, soil stability index was 94% under grassland fields. This meant that nearly all dry aggregates were stable in water and did not fall apart. The index was the same as soil under undisturbed woodlands. Croplands had lower stability index values, falling to 75% when managed with no-till and only 62% when managed with conventional disk tillage.

Land management effects were stronger in fine-textured soils (more clay and silt), but the positive impact of conservation management with grass roots occurred even in coarse-textured soils (more sand). Therefore, forage-based agriculture can improve soil aggregation in a variety of soils.

Stable soil aggregation means that soil is functioning to allow rapid water infiltration when it rains, thereby lowering water and nutrient runoff and storing more water in soil. It also means that communities of microorganisms living within these aggregates are protected from their neighbors seeking to consume them, which ultimately leads to more stable nutrient cycling. Excessive trampling, tillage, and vehicular traffic during wet conditions are forces that can degrade soil aggregates, so make pasture management decisions accordingly to preserve and enhance soil aggregation. •

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Alan Franzluebbers

The author is a soil scientist with the USDA Agricultural Research Service in Raleigh, N.C.