Prairie Soil Aggregates - Soil Research

Argonne National Laboratory
Julie Jastrow, Project Manager

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Intro - Core Collection - Aggregates - Roots - Graphs and Charts

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Soil aggregates are the basic structural unit of soil. These structures include the soil particles (sand,silt and clay) and humus found in the soil. Aggregates stabilize the soil and maintaining the productivity while preventing erosion and deterioration.

Well developed aggregated soil from virgin prairie remnant (West Chicago Prairie)

One way of evaluating soil structure is to separate the soil aggregates into different sizes compare the amounts of each size. This is achieved by wet sieving soils through a nest of sieves of appropriate sizes.

First a soil core is carefully broken apart along natural breakage points to pass through a large mesh sieve (8-10 mm openings.)

Then the soil is dried, weighed and placed on top of the nest of sieves (largest size on top.)

The sieve nest with soil is submerged in water and allowed to soak for 10 minutes. After soaking, the sieve nest is moved up and down in the water about 1 inch at the rate of 30 times per minute for 10 minutes. Water is not allowed to flow over the top of the uppermost sieve. After the 10 minutes has elapsed, the nest is removed from the water reservoir and allowed to drain. Mesh sizes and the number of sieves in the nest can vary depending on the purpose of the study but aggregates caught in this study's sieves are macroaggregates (>0.2 mm.) We used 4.75 mm, 2 mm, 1 mm, 0.5 mm, and 0.2 mm.

To collect microaggregates (<0.2 mm) the water in the reservoir (plus soil that made it through the bottom 0.2 mm sieve) is drained out the bottom of the reservoir through a 0.053 mm (53 µ m) sieve. Particles passing through this sieve are silt or clay sized particles which are not considered aggreagates.

Any soil adhering to the sides or bottom of the reservoir is flushed out and passed through the 53 µm sieve with a stream of fresh water.

The soil collected on each sieve is dried. The soil is removed from the sieve and weighed to give the percentage of the total soil in each size.

The total quantity of soil in each of the 6 size classes measured for a 12 year old restored prairie plot at Fermilab is shown. Illustrated are the stable aggregates in size classes of 2 - 4.75 mm (top center), 1-2 mm (top right), and 0.5 -1 mm (bottom left).

Comparisons are made of the amounts of soil in microaggregate (0.053 - 0.212 mm) and larger macroaggregate (2 - 4.75 mm) size classes for a long-term cultivated corn field and 12 year old restored prairie (previously long-term cultivated) at Fermilab. In this experiment the prairie soil has more large aggregates and less smaller aggregates.