The lone voice of horticultural reason

Soil Sacrifice

1-2-2009 — You’ve read it a hundred times, in plant books, catalogs, websites and on plant tags: such-and-such perennial, “prefers well-drained soil containing sufficient organic matter.”

As if that’s what you’re dealing with the first time you thrust a shovel into the ground. You may discover a sandy mix with only faint traces of organic content. Or dense clods of clay that after a hard rain could be lumped on a potter’s wheel and thrown into a lovely set of soup bowls. Maybe your shovel proves inadequate for the task, but dynamite would do, as you learn that the “soil” you intend to garden is a gravelly, concrete-like hardpan containing more fossils than minerals.

When it comes to growing trees, shrubs, perennials and annuals, success begins with soil. So let’s look briefly at basic soil science, then build on it to learn ways to turn devilish dirt into supreme soil.

Soil Basics

Soil serves three main functions in plant growth. First, it serves as the anchor for roots so that plants grow upright. Second, soil holds water and air around the roots. Third, soil contains the essential nutrients (either naturally or via fertilizers) that plants need to enable healthy growth.

Some definitions are in order:
LOAM: A rich soil composed of clay, sand, and organic matter.
CLAY: Soil composed of mineral particles of microscopic size.
MINERALS: Inorganic substances occurring naturally in the earth that are neither vegetable nor animal. Coal is a mineral. Sand is made of minerals.
SAND: Loose particles of disintegrated rock between 1/16 mm to 2 mm in diameter.
ORGANIC MATTER: Decomposed plant material; also, the live microorganisms (bacteria, fungi, insects) found in soils.
MINERAL SOIL: Soils with more than 80% mineral material. The remainder is organic material.
SOIL STRUCTURE: The shape the soil takes based on how individual soil granules clump or bind together.
BLACK DIRT: Mineral soil.

That’s just enough to make you dangerous. These terms are important to know, as you will find them used in books and articles (such as the rest of this one) about soil, and in the report you receive if submitting your soil for a university soil test (highly recommended).

Soil Fixes

Start by digging a hole eighteen to twenty inches deep in an area you wish to plant. What have you, based on the information above? If you are lucky enough to have around ten inches of easy-to-dig, black topsoil, then the start of a layer of loose clay or sandy loam, never sell your house, because what you have is, “well drained soil containing sufficient organic matter.”  But quite often that’s not the case. Here’s how to fix your soil, depending on what you find:

Clay Soils – Though clay can occur naturally as topsoil, a better bet is that the homebuilder scraped off the loamy topsoil and sold it, then left you the clay layer to deal with.
THE PROBLEM: Clay soils lack organic matter and have a soil structure so dense that water has a tough time penetrating, though when it does it remains in excess, leaving no pockets to hold air. Plants that die in clay don’t drown so much as suffocate.
THE SOLUTION: Add copious amount of organic matter such as compost or peat moss. Spread a six-inch layer atop the clay and spade it in, or till it down as deeply as possible with a power tiller. Each year in late fall, top-dress the bed with an inch or so of additional organic material. Never add sand to clay soils. Adding gypsum (a mineral) will also help loosen clay. When moving perennials, fill any holes with compost. Your clay soil will begin to loosen, the level of microorganisms essential for soil and plant health will skyrocket (they feed on organic matter), and earthworms, also your partner in creating healthy, “live” soil, will return.

Sandy Soils — These occur naturally, the luck of the glacial draw.
THE PROBLEM: Sandy soils lack organic matter and have a soil structure so loose that water and nutrients flow quickly through, not giving roots much time to absorb them.
THE SOLUTION: With the exception of adding gypsum, the solution is the same as for clay soils. You need to till in organic matter so that the soil begins to retain moisture, the microbe activity increases, and nutrients have more material to bond with. Or order in some mineral soil and till in half that, and half organic material. The addition of mineral soil helps add stability.

Hardpan Soils — For the sake of brevity, hardpan is a combination of clay, sand, gravel, and perhaps remnants of the concrete, plaster, stucco, and roofing nails left over from your home’s construction. The solution? You guessed it: copious amounts of organic matter.

Here’s an additional solution for all the above: Raise the beds. Add the layer of organic material, till it in, then ring the area with a twelve- to sixteen-inch-high border material such as concrete wall block, if you can put up with the vapid look, or wallstone or fieldstones (the latter two will look best). Fill the bed with blended garden soil available from nurseries and landscape supply yards, thus creating about a twenty-inch layer of good gardening soil. Yes, you had to write a check, but it sure is easy.

Peaty Soils — In rare cases, you may have naturally occurring “peaty loam,” soil that stays wet due to too HIGH an organic content. The solution here is to add coarse sand, to aid drainage.

Final note: Power tillers greatly disrupt soil structure. Only till soil once, to introduce the organic matter, then leave power tillers out of the equation. Good soil structure establishes slowly, greatly aided by the flourishing microorganisms. Don’t go back each year, or every three years, and goof it all up by hitting it again with a power tiller. Those little high-strung, high RPM mini tillers that spin their little razor tines a zillion times a minute are the kiss of death. People buy those and then think they are doing their garden a favor by running them through their flowerbeds and vegetable patches every spring, when all they are doing is ripping the poor soil into silt.

Annually top-dressing your beds with a one-inch layer of organic material is the best way to keep healthy soil healthy. If you use compost that you make, or order a load in, and spread it over all the places you grow perennials and annuals, you’ll probably find you don’t need to fertilize. Ever.

Don Engebretson
The Renegade Gardener