Soil Science & Preparation

Soil Science and Preparation: An Introduction to Building Soil in the Piedmont

by Don Boekelheide

When we dig in our gardens we become artists of the soil, as well as the keepers and protectors of this precious and threatened resource.  We take our place as apprentices to nature, the Great Artist, helping to mix the colors of life.  

In Renaissance times, Leonardo trained apprentices by example before he allowed them even to pick up a paintbrush.  Before we start digging, nature can train us as well, if we slow down and observe her carefully, the same way we might look at a great painting. 

First lesson: Healthy soils are alive.  No gardening insight is more important.  

It’s not simply that soils contain organic matter; they are vibrant ecosystems in their own right.  The number of living microorganisms in one teacup of rich soil is greater than the Earth's entire human population.

Soils also teach patience and nurturing.  Natural soils build slowly over time.  In a woodland forest, it takes 500 to 1,000 years to create an inch of topsoil.  Conversely, unfortunately, soils can also be lost, but at a much faster rate.  Due mostly to unwise agricultural practices, North Carolina's Piedmont lost an estimated average of 1.5 meters of topsoil from 1700 to the 1950s.  In South Carolina, the amount is thought to be 2.5 meters—more than eight feet! 

Finally, each soil is unique.  Not only do they differ greatly from place to place, soils transform as you dig deeper, developing distinct layers that soil scientists call “horizons,” with the richest “top soil” on top, and less fertile layers below. 

When preparing your soil, start by keeping such basics in mind.   For instance, encouraging healthy life in your soil is always a top priority.  The obvious way to do this is by following nature’s example, by using mulches, compost and soil building crops (i.e., cover crops) to add organic matter and feed your microbial allies.   

Protect your soil from erosion through wise planting choices and garden management.  And apply a common sense notion too often neglected: when preparing soil, keep the topsoil on top.  It makes no sense at all to mix your best soil with your subsoil. 

Pretty much anyone who’s cracked a gardening book knows all this in theory, but how do you apply these appealingly green notions in a practical way?  The fact that all soils are different is a good place to begin.  It follows logically that all the soils in your landscape do not need to be prepared and maintained in the same way.  If Leonardo had taken a similarly obsessive monochromatic approach to painting, the Mona Lisa would have looked like a Mondrian or a late Mark Rothko.   

Before digging a single spadeful, ask yourself where your soil actually needs “preparation.”

Start by making a working soil map of your landscape, divided into two zones.  One is for your vegetable garden and annual beds, your “active” zone.  The other is everything else, a “stewardship” zone where the wisest choice is to let Nature do its job with the gardener in a supporting role. 

Then circle specific places where you want to establish new plants or plantings this year.  Even inside natural areas, you'll sometimes need to prepare soil, when for example you want to transplant shrubs and trees, or create a new perennial bed or lawn. 

Fortunately, most of the time, the stewardship zone is much larger than the active zone.  It’s at least 90% of my own home landscape, though having a community gardening bed for our family vegetables means I do the bulk of my digging there. 

In the stewardship zone, soil preparation couldn't be easier.  Simply work with nature.  Never leave the soil bare, and maintain a 3- to 4-inch layer of natural organic mulch (leaves are ideal and free).  Choose plants that fit local gardening conditions and your specific characteristics of site, shade and slope.  Occasionally, if need be, add a dusting of high quality compost, and once in a while, but minimally, aerate with a good digging fork, without turning the soil or chopping roots.  That's pretty much it.  Let nature do what she does best. That also leaves you free to focus on the much smaller areas where active soil preparation makes sense. 

Beyond traditions of gardening that go back to ancient times, annual soil preparation in your active areas is ecologically sensible in the vegetable garden and annual beds.  When we eat veggies and cut flowers we export soil nutrients, so we need to replace them or see our soils lose fertility.  Also, many of these plants are annuals, or biennials managed as annuals, close cousins to plants ecologically adapted to quick colonization of disturbed soils. 

In addition, it makes sense to prepare soil to accommodate new woody plants, such as shade trees, shrubs or fruit trees.  As Bob Kourik, one of my favorite writers on the nuts and bolts of gardening, points out, by doing a good job on initial soil preparation, you lay the groundwork for managing naturally over the long term, without a lot of additional tillage or expensive inputs. 

In the series of three articles that accompany this introduction, we’ll take a closer look at active zone soil preparation: the nuts and bolts of digging a vegetable bed, doing transplant preparation and dealing with buying soil.  (Go to www.mastergardenersmecklenburg.org, Garden Articles, and find the articles in this Soil Science & Preparation series.) 

In the meantime, sketch out your soil zones, and take time to enjoy whatever art form fills your well.  (Garden catalogs count.  The Baker Creek seed catalog is coffee table book quality.)  Not only will experiencing art enrich your life and spill over into your gardening, the attentiveness inspired by great and beloved art is exactly what you need in the garden.  No tool can serve you better as you prepare the soil.  © Don Boekelheide 2010 all rights reserved ​