First, let's examine the role microorganisms play in aggregate formation. There are three main ways microbes produce stable aggregates. First, filaments of microbial tissue can form networks in groups of mineral particles, causing the particles to stick together. This sticky nature causes mineral particles to stick together. These may be such organic molecules as enzymes.
Plants can influence aggregate formation in a number of ways.
This organic material is fed upon by numerous microorganisms, amongst which are included algae, bacteria, and fungi. As an earthworm moves through the soil, digesting organic constituents and minerals within, it leaves a cast behind it. These casts consist of soil that has, in essence, been eaten by the worm. Parent material and time, the remaining factors of soil formation, effect aggregate formation but in a less profound way than factors such as biota or climate. Time determines the amount of weathering a soil will undergo.