Aggregate stability refers to the ability of soil aggregates to resist disruption when exposed to outside forces like water. It is an indicator of soil quality, with greater stability suggesting better soil quality. Stable aggregates provide pore space for air, water, and biological activity in soil. Factors that influence aggregate stability include soil texture, clay type, organic matter, iron and calcium levels, and microbial populations. Management practices like tillage can degrade stability while conservation practices like cover crops and reduced tillage can improve it.

2. Soil Quality
Indicators By Allah
Dad Khan

7. Aggregate stability
• What are soil aggregates? Soil aggregates are
groups of soil particles that bind to each other
more strongly than to adjacent particles. The
space between the aggregates provide pore
space for retention and exchange of air and
water.

8. What is aggregate stability?
• Aggregate stability refers to the ability of soil
aggregates to resist disruption when outside
forces (usually associated with water) are
applied. Aggregate stability is not the same as
dry aggregate stability, which is used for wind
erosion prediction. The latter term is a size
evaluation.

9. Why is aggregate stability important?
• Changes in aggregate stability may serve as early indicators
of recovery or degradation of soils. Aggregate stability is an
indicator of organic matter content, biological activity, and
nutrient cycling in soil. Generally, the particles in small
aggregates (< 0.25 mm) are bound by older and more
stable forms of organic matter. Microbial decomposition of
fresh organic matter releases products (that are less stable)
that bind small aggregates into large aggregates (> 2-5
mm). These large aggregates are more sensitive to
management effects on organic matter, serving as a better
indicator of changes in soil quality. Greater amounts of
stable aggregates suggest better soil quality. When the
proportion of large to small aggregates increases, soil
quality generally increases.

10. Stable Aggregation
• Stable aggregates can also provide a large range in pore
space, including small pores within and large pores
between aggregates. Pore space is essential for air and
water entry into soil, and for air, water, nutrient, and biota
movement within soil. Large pores associated with large,
stable aggregates favor high infiltration rates and
appropriate aeration for plant growth. Pore space also
provides zones of weakness for root growth and
penetration. Surface crusts and filled pores occur in weakly
aggregated soils. Surface crusts prevent infiltration and
promote erosion; filled pores lower water-holding and air-
exchange capacity and increase bulk density, diminishing
the conditions for root growth.

11. How is aggregate stability measured?
• Numerous methods measure aggregate stability.
The standard method of the NRCS Soil Survey
Laboratory can be used in a field office or in a
simple laboratory. This procedure involves
repeated agitation of the aggregates in distilled
water.
• Soils that have a high percentage of silt often
show lower aggregate stability if measured air-
dry than the field behavior would suggest,
because water entry destroys the aggregate
structure.

12. What influences aggregate stability?
• The stability of aggregates is affected by soil
texture, the predominant type of clay,
extractable iron, and extractable cations, the
amount and type of organic matter present,
and the type and size of the microbial
population. Some clays expand like an
accordion as they absorb water. Expansion
and contraction of clay particles can shift and
crack the soil mass and create or break apart
aggregates.

13. Ca Ions
• Calcium ions associated with clay generally
promote aggregation, whereas sodium ions
promote dispersion. Soils with over about five
percent iron oxides, expressed as elemental
iron, tend to have greater aggregate stability.

14. Soils
• Soils that have a high content of organic
matter have greater aggregate stability.
Additions of organic matter increase
aggregate stability, primarily after
decomposition begins and microorganisms
have produced chemical breakdown products
or mycelia have formed

15. Microorganisms
• Soil microorganisms produce many different
kinds of organic compounds, some of which
help to hold the aggregates together. The type
and species of microorganisms are important.
Fungal mycelial growth binds soil particles
together more effectively than smaller
organisms, such as bacteria. Aggregate
stability declines rapidly in soil planted to a
clean-tilled crop. It increases while the soil is
in sod and crops, such as alfalfa.

16. Practices that lead to poor aggregate stability
include:
• Tillage methods and soil disturbance activities that
breakdown plant organic matter, prevent
accumulation of soil organic matter, and disrupt
existing aggregates,
• Cropping, grazing, or other production systems that
leave soil bare and expose it to the physical impact of
raindrops or wind-blown soil particles,
• Removing sources of organic matter and surface
roughness by burning, harvesting or otherwise
removing crop residues,
• Using pesticides harmful to beneficial soil
microorganisms.

17. Conservation practices resulting in aggregate stability
favorable to soil function include:
• Conservation Crop Rotation
• Cover Crop
• Pest Management
• Prescribed Grazing
• Residue and Tillage Management
• Salinity and Sodic Soil Management
• Surface Roughening

18. Conservation practices