One of the most amazing things about soil — or dirt, as we so often call it — is its interaction with water.  And there is no better time to watch this interaction than during a storm like the one we had this weekend in Northern California.
There are thousands of different types of soil, each with its particular composition and origin.  Each one has the ability to soak up and hold a certain amount of water.  Soils with lots of sand in them can soak up lots of water, but they can’t usually hold onto it.  The water runs through the soil quickly.  Soils with lots of silt or clay can hold water much better.  Rocks and gravel don’t absorb any water at all.
For farming purposes, soils that are a mix of sand, silt and clay (loams) are best suited for growing most crops.  They can absorb lots of water, and slowly release it back to plants.  Some of the soils that we farm can hold their own volume in water — you can pour five gallons of water into a bucket with five gallons of soil in it, if you give it enough time to soak in.
The storms we had back in December dropped rain onto most dry soil.
Intense rain can overwhelm the soil’s ability to absorb it, causing the water to run off quickly into creeks and streams and cause flooding.  While areas like Sonoma County experienced this type of heavy rain in December, we did not.  The rain fell over a relatively long period, soaked in, and was absorbed entirely.
The storm that ended this past Sunday was different.  The ground was still mostly saturated from the earlier storms.  So by Sunday morning, the soils on our farm and all around us could no longer soak up the water as fast as it was falling from the sky.  It began to fill up fields and flow down hills into dry creeks and arroyos that have been dry for over two years.  Even the next day, after the rain had stopped, low spots were full of water.
Water running over soil can be extremely destructive.  It picks up soil particles and nutrients as it flows, washing them into ditches, creeks and eventually rivers.  On slopes, it can dig deep gullies and even cause landslides.
Plants cushion the impact of heavy rain on soil, and their roots hold it in place.  At Terra Firma, we grow so-called “cover crops” in the winter on as much of our land as possible to help protect our soil.  In a storm like this last one, the difference between those fields and the ones without cover crops is dramatic.  On bare soil, large amounts of muddy water flow across and out of the field.  Where the cover crops are growing, less water runs off the field and what does it clear.  When the rain-hammered soil dries, it is crusty and compacted.  The soil under the cover crops is soft and fluffy.
I hope we get a few more chances to witness soil and rain working together this winter.  A lot depends on it.