The Goldilocks principle applies, not too much or too little, neither too high nor too low, not too hard and not too soft. Good compost, by definition, has a wide diversity of organisms that compete with, inhibit and consume disease causing and pest organisms. Compost and Aggregate Structure Aggregates develop through a rather complex biological process. As a result of soil organism actions, soil or compost develop structure which allows oxygen and water to easily move through the material. Bacteria make the smallest microaggregates, as they glue clay colloids, soil organic matter, silt and sands together.
Fungi grow as long threads, which bind the microaggregates together into macroaggregates.
The physical placement of macroaggregates is engineered by microarthropods, nematodes, insect larvae, and earthworms. All these things working together result in good soil pore structure. This means there is plenty of space for water to infiltrate into the soil, for oxygen to diffuse to the root system, and to allow carbon dioxide to move out of the soil. Without good soil structure, soil becomes anaerobic. Water puddles on the soil surface, and erosion can become a problem.