Have you ever seen a finisher screed a concrete slab?  First, smooth boards and then, large trowels are run back and forth over liquid concrete.  The motion separates the tiny bits of fine sand and cement from denser bits of gravel.  The “fines” rise to the surface where they are trowelled into a thin smooth skin.  The same thing happens when humusless soil is rained on or irrigated with sprinklers emitting a coarse, heavy spray.  The droplets beat on the soil, mechanically separating the lighter “fines” (in this case silt and clay) from larger, denser particles.  The sand particles sink, the fines rise and dry into a hard, impenetrable crust.

Organic matter decomposing in soil opens and loosens soil and makes the earth far more welcoming to plant growth.  Its benefits are both direct and indirect.  Decomposing organic matter mechanically acts like springy sponges that reduce compaction.  However, rotting is rapid and soon this material and its effect is virtually gone.  You can easily create this type of temporary result by tilling a thick dusting of peat moss into some poor soil.

A more significant and longer-lasting soil improvement is created by microorganisms and earthworms, whose activities makes particles of sand, silt, and clay cling strongly together and form large, irregularly-shaped grains called “aggregates” or “crumbs” that resist breaking apart.  A well-developed crumb structure gives soil a set of qualities farmers and gardeners delightfully refer to as “good tilth.”  The difference between good and poor tilth is like night and day to someone working the land.  For example, if you rotary till unaggregated soil into a fluffy seedbed, the first time it is irrigated, rained on, or stepped on it slumps back down into an airless mass and probably develops a hard crust as well.  However, a soil with good tilth will permit multiple irrigations and a fair amount of foot traffic without compacting or crusting.

Crumbs develop as a result of two similar, interrelated processes.  Earthworms and other soil animals make stable humus crumbs as soil, clay and decomposing organic matter pass through their digestive systems.  The casts or scats that emerge are crumbs. Free-living soil microorganisms also form crumbs.  As they eat organic matter they secrete slimes and gums that firmly cement fine soil particles together into long lasting aggregates.

I sadly observe what happens when farmers allow soil organic matter to run down every time I drive in the country.  Soil color that should be dark changes to light because mineral particles themselves are usually light colored or reddish; the rich black or chestnut tone soil can get is organic matter.  Puddles form when it rains hard on perfectly flat humusless fields and may stand for hours or days, driving out all soil air, drowning earthworms, and suffocating crop roots.  On sloping fields the water runs off rather than percolating in.  Evidence of this can be seen in muddy streams and in more severe cases, by little rills or mini-gullies across the field caused by fast moving water sweeping up soil particles from the crusted surface as it leaves the field.