Sand slips through your fingers. Clay clods defy your shovel. And somewhere in-between is the sweet spot with bits and pieces of soil just the right size for plant roots. Whatever the size, these chunks are called soil aggregates.
To learn about soil aggregates, you will need a scoop of dry soil from your garden. Put the soil in a bowl. Are there a lot of different sized pieces or are they mostly the same? If you look closely at the photo below, you can see a clear line between the old clay layer and all the decomposing mulch and compost that I have been putting on top. Over time, those organic materials work their way down, into the clay, reducing compaction and improving drainage. These improvements will occur because of soil aggregates.
Take another look at your soil. Stir it around a bit. Pick some of it up and roll it around in your hand. Rub it with your fingers. Does it feel gritty? Or powdery? Do the clumps mostly hold together? Do they crumble completely or do they feel like rocks? Soil aggregates, also known as ‘peds’, are the clumps that tend to stay together when you work the soil.
Why do soil aggregates matter?
Healthy soil has a variety of aggregate sizes, with plenty of large spaces (macropores) between the aggregates and tiny spaces (micropores) inside the aggregates. These spaces are used by roots and gases to move through the soil. These spaces are also what allow water to soak in, increasing your soil’s water holding capacity. And plant nutrients stick to these clumps.
In some cases, aggregates are not as important. Sand, for example, has no aggregates, but there are so many spaces between grains of sand that plant roots, water, and gases have no trouble moving around. [Hanging on to water and nutrients is something else altogether!] Soils with low bulk density are another case where aggregates don’t matter as much. For the rest of us, the soil aggregates in our gardens have a huge impact on plant health, especially tender seedlings. If your soil’s aggregates are unstable, seedlings can suffocate.
Aggregates are described according to their stability. If your soil crumbles into dust, you probably have a lot of clay or silt and that can mean your soil has low aggregate stability. Low aggregate stability increases problems with erosion, gas exchanges, root development, and permeability. More immediately, as rain, irrigation, or sprinkler water strikes the soil surface, flimsy aggregates can be broken. Those tiny broken bits clog the spaces in the soil, making life difficult for plant roots, worms, and soil microorganisms. It also causes crusting which can kill seedlings before they get a chance to grow.
How do soil aggregates form?
Healthy soil aggregates are held together by clay, organic matter, and glomalin. Glomalin is a protective fungal excretion that helps the fungi feed your plants and binds soil aggregates together. Bacteria have similar excretions which are not as effective.
Organic materials in the soil usually mean decomposition is taking place. Decomposition means fungi, worms, bacteria, and microorganisms are present. Those life forms excrete coatings and other materials that help soil aggregates form and stabilize. Finally, as clay particles become moist, they act as a cement, holding molecules and particles together into aggregates.
Test your soil for aggregates
Returning to your soil sample, select a few particularly large clods and gently set them aside to dry completely. Once they are really dry, dip them into a glass of water. If they break up quickly it means your soil has low aggregate stability. If the clods retain their shape for 30 minutes or more, your soil’s stability is high. Because my soil contains so much clay, it pretty much dissolves immediately. As more organic material is incorporated, my soil will breath better, hold its shape better, and provide plenty of pores for roots, water, and microorganisms.
How can you improve the aggregates in your soil?
Start by taking a look at your soil test. If your soil contains a lot of calcium or iron, it probably already has good aggregates. If your soil holds too much salt, aggregates are harder to come by. The biggest indicator of good soil aggregates is the amount of organic matter found in the soil. By mulching and top dressing your soil with manure and aged compost, you are encouraging all the life forms that help soil build healthy aggregates. This is why no-dig gardening has become so important. We learned that excessive digging, plowing, and rototilling disrupt the soil dwelling populations that create and maintain good soil aggregates.
If your soil aggregates are unsatisfactory, use these tips to encourage better soil structure in your garden and landscape:
How did your dip test turn out? Let us know in the Comments!
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