Compaction is a pretty big deal when you’re trying to grow plants.  How far can roots go down before they hit “hard pan” and must go sideways?  How far can water infiltrate until it, too, must go sideways, leaching soluble nutrients and potentially carrying precious topsoil away? 

Compaction can be caused by the weight of rainfall on bare soil (in that case you’ll likely find it 4 inches or so down), heavy equipment (expect it 18 inches down) and tillage (depth of damage will depend upon the equipment).  Even walking in your garden can cause compaction.  Tillage and chemical agriculture directly assault soil biology – the very thing that builds soil structure and mediates compaction.  Imagine a layer of sand, silt, clay, rocks and pebbles packed so tightly that no air can enter.  As structure disintegrates and compaction increases, more nutrients disappear as nitrogen, sulfur and phosphorus are lost as gasses.  

Without air, organisms that are beneficial to plants (soil structure-building aerobic microorganisms) either die or go dormant and anaerobic organisms flourish.  Roots turn not only because the layer is impenetrable, but because anaerobic organisms make plant-toxic compounds like organic acids, alcohol, formaldehyde and toxic phenols.  You wouldn’t put your roots down there either!  And sideways roots do not support happy plants.

One way to find out where compaction is in your soil is to use rebar shaped into a giant “T”.  Use the top of the T as handles to push the bottom into the soil.  Record the depth at which you can’t push any further.  As you restore soil biology, you will be able to push deeper over time.  This is what we saw in the corn experiment out at Hope Farm this past season.  Measuring with a penetrometer, compaction receded an average of 2.8 inches across the 8 biologically managed plots, compared to an average of 1.7 inches of increased compaction in the 8 plots that received both chemical and biological management.  

So if there is any bare soil left in your garden, quick!  Go cover it with a layer of mulch.  And consider exploring cover crops for next year so that you will have living plants in place by the time the rains come again. The biology in your soil will thank you for it.