Palm | Kelapa Sawit

The stability of aggregates is affected by soil properties that change relatively little and by properties that change in response to changes in vegetation and management. As a result, measurements of the aggregate stability of a given soil should be compared only with measurements for the same or similar soils with similar textures.

Soil properties.—Soil properties that change relatively little include texture and type of clay. Expansion and contraction of clay particles as they become moist and then dry can shift and crack the soil mass and create or break apart aggregates. Calcium in the soil generally promotes aggregation, whereas sodium promotes dispersion. The quantity of calcium and sodium is specific to each type of soil.

Vegetation.—Management affects the plant community. Changes in the composition, distribution, and productivity of plant species affect aggregation-related soil properties, including aggregate stability, the amount and type of organic matter in the soil, and the composition and size of the soil biotic community. The amount of plant cover and the size of bare patches also are important. The centers of large bare spaces receive few inputs of organic matter and are susceptible to degradation.

Grazing.—Disturbance of the soil surface by grazing animals has both beneficial and detrimental effects on aggregate stability. It breaks the soil apart, exposing the organic matter “glues” to degradation and loss by erosion; however, it also can incorporate litter and standing dead vegetation into the soil, increasing the content of organic matter in the soil. Heavy grazing that significantly reduces plant production disrupts the formation of aggregates by reducing the inputs of organic matter. Grazing is more likely to increase aggregate stability in areas where an unusually large amount of standing dead material is on the soil surface and the risk of erosion is not increased by removal of plant material and disturbance of the soil surface.

You can improve the productivity of rangeland through good range management which normally increases aggregate stability. A few practices that you could use include:

• Maintain the optimum amount of live vegetation and litter
in order to maintain the content of organic matter and soil
structure and control erosion.
• Decrease the number and size of bare areas.
• Minimize soil surface disturbances, especially in arid
areas.

Try putting these practices into action on land that you have access too.

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Stable aggregates are critical to erosion resistance, water availability, and root growth. Soils with stable aggregates at the surface are more resistant to water erosion than other soils, both because soil particles are less likely to be detached and because the rate of water infiltration tends to be higher on well aggregated soils. Unstable aggregates disperse during rainstorms, then form a hard physical crust when the soil dries. Physical crusts restrict seedling emergence because they have few pores for air and water entry into the soil. The crusts result in more runoff, more erosion, and less available water. Aggregated soils hold more water than other soils and provide pores for root growth. Large, stable aggregates can resist degradation and removal by wind better than small, weak ones.

Aggregate stability is a good indicator of the content of organic matter, biological activity, and nutrient cycling in the soil. The amount of organic matter increases after the decomposition of litter and dead roots begins. Stable aggregates result from this process because soil biota produce material that binds particles together. “New” organic matter stabilizes the larger aggregates, while the smaller aggregates are more likely to be bound by “old” organic matter. New organic matter holds and can release more nutrients. Changes in aggregate stability may serve as early indicators of recovery or degradation of soils and, more generally, of ecosystems. Perennial plants can often persist long after the soil and plant community have become too degraded to support.


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Soil aggregates are groups of soil particles that are bound to each other more strongly than to adjacent particles. Organic matter “glues” produced when soil biota break down dead roots and litter hold the particles together. Threadlike strands of fungi also bind particles into aggregates. Microscopic aggregates are the building blocks of larger aggregates. The larger aggregates and the arrangement of them, along with chemical attraction between particles, determine soil structure. The structure of the surface layer commonly is granular or blocky, but a degraded surface layer can be crusted, platy, or structureless. Pores important for the movement of air, water, and plant nutrients occur within and between aggregates. Pores also provide thoroughfares for soil organisms.

Aggregate stability refers to the ability of aggregates to resist degradation. Additions of organic matter to the soil enhance the stability of aggregates. Raindrops, flowing water, and windblown sand grains can break apart soil aggregates, exposing organic matter to decomposition and loss. Physical disturbances, such as vehicle traffic and trampling, can break down soil structure. Soils can resist degradation differently when wet or dry. For example, dense, cloddy soils can be very stable when dry but unstable when wet.

Listen to this short video that explains soil aggregates.


Soil & Compost:
What are soil 'aggregates'?

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