Assessment of Soil Aggregation
Introduction:
Aggregates change the physical, chemical, and biological characteristics of the soil. They improve structural integrity, facilitate drainage and aeration, reduce erosion, and protect microorganisms, life processes, and organic materials from degradation.
The difference between topsoil and subsoil is space, life, and the remains of life. When we dig into topsoil with living plants (the rhizosphere), we can see that the soil is darker. It clings together in large and small clumps, with plenty of pores and spaces. Where there are roots, there is life, and life builds structure.
The formation of soil aggregates.
(Credit: global soil biodiversity atlas-LBNA27236ENN https://data.europa.eu/doi/10.2788/2613)
Soil microorganisms work to decompose all soil carbon inputs in soil from dead crop residues and roots, root exudates from live roots, and other organic sources from compost or manure. In such a process, they produce compounds that begin to bind soil mineral particles together into microaggregates. With increasing organic matter inputs these microaggregates begin to bind to one another, forming larger and larger aggregates and resistant to decomposition from biological activity, due to the changes in the chemical structure of soil organic matter that occurs in microaggregates. However, the carbon in these micro aggregates is generally more protected from decomposition than the carbon in macroaggregates. Thus, the formation and retention of soil aggregates pertains to the basics of soil health improvement.
Aggregates form when soil organisms reproduce, hunt, graze, and produce waste. The largest ones are called macroaggregates. These range from 0.25 (250 μm) to 2 millimeters in diameter, and they are made up of microaggregates, which are tiny clusters of particles cemented by chemical and physical bonds, and are less than 250 μm in diameter.
However, the specific effects of organic matter on soil aggregation have been more closely linked to various fractions of organic matter, especially polysaccharides and humic and fulvic acids, particularly the latter. These components cause physicochemical linkages with soil clay particles to form stable soil aggregates, a process enhanced where soils are high in iron oxides.
Repeated inverting and pulverizing of soils expose soil organic matter to aeration and thus mineralization. This process leads to reduced potential biological and biochemical activity, leading to soil aggregate destruction.
As roots decompose, soil microorganisms bind soil mineral particles together.
Credit (https://ucanr.edu/).
An amazing work from USDA scientist Devin Rippner's research group using open-source software to generate models to segment X-ray µCT images.
The work shows ~2 mm soil aggregate created from a neural network analysis of X-ray CT data. In gray are the mineral grains, in blue is the pore space, and in brown is particulate organic matter.
Stable soil aggregates form from an unhindered and productive interaction between physical, biological, and mineral components, which is why they’re one of the most important aspects of soil health functions.
Devin’s “hyperfine” analysis and work will inform future research approaches that show meaningful differences in soil management strategies and overall soil health.
Rippner et al, 2022: A workflow for segmenting soil and plant X-ray computed tomography images with deep learning in Google’s Colaboratory (DOI: 10.3389/fpls.2022.893140)
Measurements of Soil Aggregate Stability:
Following dry sieving, three replicates of 50 g dry aggregates are proportionally sampled to the total weight distributed on different sieves. Samples moistened slowly by micro-cracking on filter paper are placed on small dishes.
After 30 min, wet sieving was carried out for 2 min. Sieves of 2.0, 1.0, 0.5, and 0.20 mm mesh were used. Sieves were oscillated (amplitude 10 cm) 100 times and removed from the tank, and dry weights of the water-stable aggregates retained on a 0.5 mm sieve were determined to provide macro-aggregation measurements. The test was replicated three times for each soil sample.
An additional specific index of stability is provided by subjecting the 2.0–1.0 mm dry aggregate-size fraction to wet-sieving following the Soil Survey Laboratory Methods Manual (1996). Samples of 10 g of the 2.0– 1.0 mm dry-sieved fraction were wetted overnight, then placed on a 0.5 mm sieve, and subjected to disruption by wet sieving. The sieve oscillated 20 times for 40 s, after which the water-stable aggregates on a 0.5 mm sieve were removed from the tank, dried, and weighed. The sand content of the > 0.5 mm water-stable aggregates was determined after dispersion with hexametaphosphate solution, and aggregate stability was calculated as follows:
Please, watch the video to follow the process of collecting water-stable aggregates assessed by the dispersion method and using end‐over‐end handshaking of a 1000 ml standard cylinder.
The collected soil was oven-dried for 24 hours at 230c.
Water- stable aggregates training course.
Please, watch below, a complete procedure for measurement of Water Stable Aggregates Stability.
Wet_sieving_Main.pdf