Beyond dead dirt: healthy soil is alive

This post is modified from an article originally published in The Delmarva Farmer (2/13/2018)

Most people would probably be surprised to know that bacterial cells outnumber human cells in our bodies by 10-to-1 and that just one teaspoon of healthy soil contains more than 1 billion bacteria and fungi (microbes for short). Yuck, right?  Well, not exactly.

SoilMicrobes have gotten a bad rap because the small fraction of bacteria and fungi that cause disease get all the attention.  In fact, most microbes are friendly, and neither humans nor plants can live without them.

Although the chemical and physical properties of soil have dominated discussion (and soil testing) in the past, the focus is now changing as soil is recognized as a living ecosystem.  With this change, it is becoming clear that sustained agricultural productivity requires farming practices that protect the soil and increase the diversity of life underground.  Home gardeners can also benefit from gardening strategies that protect and promote the living things in their garden soil.

Soil organisms are behind 90% of the services that healthy soil provides.  Microbes decompose organic matter and cycle those nutrients back to plants.  Healthy soil allows water to infiltrate after a rain while holding on to some of it for plants to use when it is hot and dry. Water that soaks into healthy soil is filtered, and soil microbes clean it further by degrading chemicals and toxins.  The microbes in healthy soil protect plants from disease and insects, provide plants with nitrogen, phosphorus, and water, and increase plant growth in a whole list of other ways.

Soil may seem like just “dirt” to the casual observer, but it is actually a highly structured environment built by the soil organisms themselves.  This habitat is composed of groups of water-stable aggregates, and these are what give healthy soil that good crumbly texture.  A soil aggregate is formed of tiny soil particles that are loosely stuck together by roots, fungal filaments, and sugary-gluey materials that leak from roots and soil fungi. Within a high-quality aggregate, there is plenty of space for the air, water and organic matter that plants and their microbes require.  In contrast, unhealthy soil is often blocky and compacted, low in organic matter and with little microbial life.

The sad truth is that most US agricultural soils aren’t very healthy. In the Midwest, tilling and leaving fields bare in the winter has allowed more than half of the region’s topsoil to be lost to wind and water erosion.  Tilling is very hard on soil.  It breaks up soil aggregates and exposes organic matter to air, allowing eager microbes to digest it, which releases carbon as CO2.   With routine tillage, life in agricultural soils has dwindled.  Now, much of the nation’s agricultural soil is indeed little more than dead dirt.

Dirt

Here in Maryland, though, we can be really proud because our farmers are national leaders in soil health.  Over 70% of the acres planted to corn, soybeans, and grains are farmed using no-till practices.  This means that farmers skip the traditional spring and/or fall tilling.  Instead, they plant directly into the residue from a previous crop without disturbing the soil.  The crop residue protects the soil from erosion, and the lack of regular tilling allows soil organisms to go happily about their business within the soil aggregates that they built.

Maryland grain farmers also routinely plant cover crops, which provide a huge boost to soil health.  Winter cover crops protect the soil from erosion and soak up excess nutrients from the previous crop that would otherwise flow right to Chesapeake Bay.  But cover crops also feed the soil microbes, which depend on a constant supply of living roots.  Leaving ground bare over the winter essentially cuts off the food supply of the microbes that are so helpful to plants.  Other strategies like increasing the number of different crops in rotations or planting a mixture of cover crop species increase the diversity of soil organisms, further boosting soil health. When farmers “plant green” into living or recently terminated cover crops, the cover crop becomes mulch that fights weeds, holds water and adds additional organic material.

Cover crops - winter wheat and hairy vetch

Cover crops – winter wheat and hairy vetch

So how about your garden?  Have you been able to use no-till or cover crops?  If you’ve ever created new beds with the “lasagna” method instead of with your rototiller, then you’ve practiced no-till!  But what about your garden beds?  Do you rototill each spring?  Even turning over the soil can be somewhat damaging, though it is not as harsh as using a tiller and light digging can be useful to incorporate compost and manure into extremely poor soil.  However, tilling and digging are not necessary for the long term, and halting physical disturbance entirely can be a real boon to soil health.  Once you get beyond soil that has little organic matter or is very compacted, amendments placed on top filter down fairly readily.

No-till is relatively easy in corn and soybeans where weeds can be controlled by herbicides without damaging the crops.  But that doesn’t work in home gardens.

One promising strategy for no-till weed control is to use overwintering cover crops as mulch for spring crops.  Gardeners can easily dispatch the overwintering cover with a mulching lawnmower set to the lowest setting.  Then you can make a small hole for transplants or a slit for seeds and just plant into the undisturbed soil.  Adding more mulch around the transplants or seed row will help prevent regrowth of the cover crop and prevent germination of weed seeds.  Mulched leaves are great for this, so is straw.

“Living mulch” is another no-till weed-control strategy.   Here’s how it works.  First plant a cover crop like red clover early enough in the fall to get established.  Then mow it in spring to reduce competition with your crop but not low enough to kill it.  After mowing, plant your seeds or (even better) transplants immediately without tilling as described above.  The difference between this and pure no-till is that the cover crop will start growing again around your vegetables.  A legume like red clover will not only crowd out weeds, it will fix nitrogen and reduce your fertilizer needs.  Win-win!

 

No-till and cover crops work because they prevent disturbance of the all-important soil organisms and provide them with food during the winter while leaving their habitat intact.  Adoption of these strategies is just beginning in the gardening world.  However, I suspect we will be hearing a lot more about them as people become more aware of the value of nurturing soil organisms.

Why not try planting a cover crop in your empty beds this fall?  Then in December when you are warm in front of your fire, you can think of your army of soil microbes happily carrying on in your winter garden amid a tangle of living cover crop roots.

Sara Via, Professor & Climate Extension Specialist, UMD College Park

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