Aggregation is an excellent indicator of soil health, especially when a shift in management practices happens on-farm. Water and air movement in the soil along with trafficability improve when aggregates develop. Organic matter, biological activity (whether it be earthworms or microbes) and root development are all linked to aggregation and generally improve as aggregation increases. The best part about aggregation is that it's easy to evaluate in the field with a shovel.
Let's go over some basics first - aggregates are bundles of individual sand, silt and clay soil particles that are held together by roots, fungal hyphae, microbial glues and clay in soil. The more you have of each of these (roots, hyphae etc.), the higher the aggregate stability.
A range of aggregate sizes, both large and small, are needed to get soils functioning. Larger aggregates, generally held together by roots and hyphae, are important for creating large pore space for water to drain through the soil, protecting recently added organic matter for short-term release and keeping the soil from blowing away in the wind. Smaller aggregates, held together by clay and microbial glues, create small pores that help store water for crop use and protect decomposed organic matter rich in nitrogen for long-term release. You can see benefits of having both large and small aggregates to ramp up soil function.
How do you evaluate aggregates in the field? Taking a shovel to the field is an easy way to do this. Set up a comparison, for example - a field where tillage is used and one where no-till is part of the management plan. You can use a fence row too as a comparison. Here's my thought process:
Start digging and ask yourself, "how easily does the shovel slide into the soil? Can I push the shovel into the soil (2 or 3 inches), let go and not have it tip over? Do I hit a compacted or resistant layer below 2 or 3 inches? If the shovel stays standing and you don't hit a resistant layer, then the soil probably has good aggregation. If the shovel falls over and/or you do hit a resistant layer, the surface soil is not aggregated and there is compaction likely at the depth of current or previous tillage equipment. The non-aggregated surface soil is susceptible to wind erosion and the compacted layer will restrict water movement into soil. In some cases, though, the shovel may not slide easily into a no-till field. Don't be alarmed. Here's why: roots are holding the soil together, so it's not compacted, it's probably that you are cutting through roots and a highly stable soil. Check the soil on the shovel before jumping to conclusions.
Next, look at the soil on the shovel. Do you see large and small aggregates? You should see something that resembles cottage cheese with all different sizes of aggregates. If it's windy out and the soil is somewhat dry, an aggregated soil will stay on the palm of your hand and not blow away like a non-aggregated soil. When you put pressure on the aggregates between your fingertips, do they hold together or break apart? This will provide some indication of stability. Check out how many roots there are in the soil and if they are wrapping around the soil particles.
Start with getting familiar with aggregation differences in your comparisons. You can also watch the Soil Health Minute titled: Evaluating Soil Health (web: ndsu.edu/soilhealth, then go to the Video tab). In the next Soil Health Minute, I will follow up with management approaches to build soil aggregation.