Think of a well-aggregated soil like a city. The aggregates are the buildings. The network of pores and voids between them is the road system. The microbes, fungi, and roots living in that soil are the citizens going about their work. Carbon and water are the groceries that the workers need to survive. A soil without good aggregation is a city with no roads, crumbling infrastructure, nowhere for the workforce to live, and nothing for them to eat. It simply cannot function.

How Soil Aggregates Form

There are only two things that create soil aggregates, and both of them are biological. You cannot till your way to good soil structure. You cannot buy good structure. You can only grow it. 

The first ingredient is root exudates. Plants send a significant percentage of the carbon they capture through photosynthesis down to their roots, and a good share of that gets pumped out into the soil as exudates. Root exudates are liquid carbon compounds loaded with sugars, acids, enzymes, and dozens of other substances. These exudates feed the soil biology and also help glue the smallest aggregates together. The plant is running the whole operation. It captures sunlight, converts it to liquid carbon, and trades that carbon to the microbial workforce that builds and maintains soil structure.

The second ingredient is biotic glues. These are sticky compounds produced by soil organisms. This category includes glomalin, a sticky protein produced by arbuscular mycorrhizal fungi (AMF). It also includes what you could call “dead microbe goo” – carbon-based compounds left behind as organisms die and consume each other. These glues along with fungal hyphae are what stitch tiny microaggregates together into the larger macroaggregates that give healthy soil its chocolate cake look.

This is why living roots are non-negotiable. On average, tiny microaggregates only hold together for around four weeks before the glues break down and have to be replenished. Biotic glues are carbon-based, and carbon is microbe food. Microbes will literally “eat their house” if they run out of other sources or readily available carbon. The main source of readily available carbon is root exudates. 

In other words, aggregation is not a permanent condition. It's a living process that has to be fed continuously. Mycorrhizal fungi cannot survive without root exudates. Pull the living root out of the equation for months at a time, as we do on most cropland, and the whole system starts to fall apart. 

This is also why our first rule when managing soil should be: do no harm to aggregates. Tillage shreds them and disrupts AMF. An excess of readily available nutrients in the root zone inhibits root exudation. Overuse of pesticides kills off the organisms that build aggregates. Bare soil starves the whole system.

Why Soil Aggregation Is Important for Soil Health 

Aggregates are the engine that drives soil function, and it's important to understand why. Within and between those clumps you get water films for biology to live in and micro-sites of high and low pH and Eh (reduction-oxidation reactions). That diversity of conditions is what makes rapid nutrient cycling possible. A well-aggregated soil is a bustling, functioning ecosystem. A compacted, poorly aggregated soil is a hostile work environment for the microbes that do the work of cycling nutrients and feeding plants.

Well aggregated soil also enhances nitrogen production from the soil. There are single celled organisms in the soil that can fix nitrogen without associating with a legume root. But here’s where it gets interesting. Many of these nitrogen-fixing organisms are “anaerobic” meaning they do their work without oxygen, but we want our soil to be “aerobic” – containing oxygen for other microbes and plant roots. 

So how do we get aerobic and anaerobic conditions at the same time? Aggregation. Soil aggregates have tiny anaerobic pockets where nitrogen fixation occurs right next to aerobic pockets that allow plant roots to access oxygen. You get the best of both worlds and have everything plants need to survive and thrive inside these tiny engines of life we call aggregates.

Run down the list of things that limit crop productivity and you'll notice the same answer keeps coming up. Poor gas exchange? Build aggregates. Water running off instead of soaking in? Build aggregates. Slow residue decomposition and sluggish nutrient cycling? Build aggregates to boost biological activity. Compaction? That's a symptom of poor aggregation, not an inherent property of your soil. 

Even photosynthesis is tied to soil aggregation.  Better aggregation means better water availability, and water is often what limits the plant's ability to capture sunlight in the first place. Better aggregation also means that the soil can “breathe” i.e. it can release CO2 from the soil that was produced underground by plant roots and aerobic microbes. This CO2 is the other half of the photosynthesis equation. Increase biological activity with well aggregated soil and you increase CO2 production that drives photosynthesis. This is how soil is intended to work. Plants are mirror images of themselves and are recapturing the CO2 that they helped to produce. This miracle is what drives almost all life on Earth.