Wednesday, September 14, 2011


Gardens start with dirt
          Not to overstate the obvious, all gardens start with dirt. If you’re a recent transplant to North Carolina’s coast, you may have noticed that you’ve traded your familiar “real dirt” for sand or, worst-case scenario, antediluvian swamp muck over concrete-like hardpan (a layer of soil so compacted and/or so mineral-laden that it has become impervious, a horticultural term meaning “impossible to dig through.”) Not to worry: you just need some perspective adjustment, a little education, a lot of patience and a good attitude.

No matter what you may have read or been told, native coast soils are NOT acidic. In our experience, pHs generally range between 6.0 and 9.0 (fill dirt can be an exception, depending upon where it came from).

What’s pH and why does it matter? you ask.

The measure of a substance’s acidity or alkalinity is called pH—little “p,” big “H”—standing for “percent of Hydrogen.” The pH scale runs from zero to 14, but actually starts in the middle, at number seven, or neutral. Seven to zero is the acid end of the scale, seven to 14 alkaline, or basic. The progression between numbers is geometric: four is ten times more acid than five, nine is ten times more basic than eight. Vinegar tests out to about pH 3; chalk’s about a pH 12.

            It matters because soil pH values influence how plants absorb nutrients, which translates to how well a given plant will perform in a given place. By way of illustration, let’s talk iron absorption. An essential micronutrient, iron is most readily available to plants rooted in soils with pHs between 4.0 and 6.5. If pH is higher than 6.0, iron absorption is curtailed. You can pour all the Ironite® around you want—the plant can’t take it in. Most soils contain sufficient iron, but that’s a moot point if the pH is too high. Iron deficiency frequently manifests as chlorosis, characterized by yellow leaves with green veins. 

            Many other nutrients are absorbed better at lower pHs as well; optimum absorption of most nutrients occurs at pHs from 5.0 to 7.0. (See table.)

(Click on the table to enlarge it)

            Unamended native coastal soils fall in the neutral to somewhat basic range of the pH spectrum, 6.0 to 9.0, regardless of what we want to believe. Pine trees do not indicate acidic soil, so get that notion out of your head right now. The marvelously adaptable genus Pinus will grow where the dirt tests a vinegary pH 3.0 and with equal aplomb in calcified soils of pH 12. Moreover, forget what you’ve heard about the acidifying properties of pine needles. They do not contribute significantly to lowering pH. Repeat after me: I do not have acid soil. I do not have acid soil. I do not have acid soil.

Don’t believe me? Get a soil test. Get a soil test even if you do believe me. Call your local Cooperative Extension Office to find out how. They may also be able to translate for you when you get the results. (Maybe not, in North Carolina. Tim and I had to take a seminar before we were able to decipher all those symbols and figure out what they told us about our yard.) 

What do you need to know in order to optimize plant performance in the soil you do have? Just because it’s more common, let’s start with sand.

My yard: sand to China
           On the plus side, sand is a dream to dig in. Remember those happy childhood hours spent in the sandbox or playing on the beach? Well, chances are your present yard, except for the mat of roots just below the surface, has a similar consistency. The other really good thing about sand is that it drains beautifully. The day after 1999’s Hurricanes Dennis and Floyd finished dumping about 45 inches of rain on us in a little over a week, our yard—at about 26 feet above sea level—had no standing water. When Tim and I dug our back garden, we finished the job in one day: a 28-by-32-foot area, double dug, in six hours. Never found one stone, one worm, one ant, to the depth of two feet. Just sand, sand and more sand. Coming from the foothills of the Adirondacks, we thought we’d died and gone to heaven. While it is possible to induce root rots in such an environment, you have to work really hard at it. Much easier to do is water insufficiently, especially during the one to five years it takes for a new planting to establish.

Swamp muck is another, sadder story. Don’t let its rich-looking black color fool you. Stick your nose near it: that pungent smell means it’s anaerobic, what farmers call “sour.” Millions of years of periodic flooding have resulted in a densely compacted layer of organic matter. The first 12 to 24 inches or so may be tillable, given a sharp shovel and good muscle tone: below that, the compaction increases to form a stratum of hardpan, an impervious layer of super-compacted soil. The worst thing about hardpan (not counting trying to dig through it) is that water flows across it laterally instead of sinking straight down, dramatically affecting drainage. If your hardpan layer is less than 12 inches from the surface, your best bet will be to: a) blast; or b) raise your beds. The good news is you won’t need to water so much.
Despair not: both soil types make fine gardens with some amending. Both lack high (or even moderate) levels of microbial activity, but you can give the little critters a boost by adding generous amounts of composted organic materials. Why do I want to do that? you ask. Because good soil is, by definition, alive, full of organisms including the ones we can see, like worms, centipedes, ants and slugs, as well as the ones we can’t: bacteria, fungi, nematodes and protozoa.

Ewww, you say, but since I need them, where do I get them? I told you already, composted organic materials. You can produce your own, simultaneously reducing overcrowding in our landfills, by composting your food waste and yard debris. Keeping small herds of bovines, ovines or equines and flocks of chickens, turkeys or bats would be nice, too—less packaging—but many communities frown upon urban farming.

Not all commercial composts
are created equal:
read the label
Another option is buying commercially available compost mixtures. Most blends feature some kind of animal manure. It’s not as bad as you might think, smell–wise. (Except for Milorganite, which I don’t recommend because it’s industrial sludge and heavy on the heavy metals.)  Plus bags of already-aged stuff are easier to manage, ready to use, consume less space and cost far less than feeding a herd or flock of anything.

Besides attracting and feeding our bacteria buddies, adding composted organic matter to the soil raises your chances of gardening success in other ways. It makes sand more moisture-retentive by giving water something to soak into. It loosens up the close-packed particles of swamp muck, improving its ability to drain. Most herbaceous plants (the ones that die back to the ground every winter) appreciate eight to 12 inches of good soil to root in: woodies (plants like trees and shrubs) prefer 12 to 18 inches. Compost also provides micronutrients in forms plants can access without additional chemical reactions taking place. Used at planting and as an annual topdressing, compost encourages the arrival and multiplication of soil bacteria, fungi, protozoans, nematodes, micro- and macro-arthropods, and earthworms. Bottom line: compost is good.

Permit me to expand the scope of my little rant vis-à-vis encouraging microbiota in soil.

1.                  When you want to create new planting beds from lawn or natural areas and don’t feel up to digging them out by hand, consider this method: place a layer of newspaper six to ten sheets thick over the unwanted vegetation. Cover the newspaper with organic mulch three to four inches thick. Go do something else for three or four seasons. (Have I mentioned that gardening is not the best of hobbies for personalities requiring instant gratification?) Light deprivation discourages the growth you want discouraged while the newspaper and mulch biodegrade, enriching the soil.

2.                  Be careful about amending with topsoil. “Topsoil” is a vague catch-all sort of word meaning something like “the stuff scraped off the top of something.” One lady we know wanted to transform her flat back yard into an undulating landscape, so she bought ten double-truckloads of what the seller assured her was "topsoil” and had it sculpted into a lovely rolling berm. When everything she planted the first season only survived long enough to get really ugly before dying, she had the “topsoil” tested. Well, it may indeed have been scraped off the top of something, but mostly it was home to nematodes of the most unbeneficial kind. The lady subsequently spent four years, pots of money and untold hours outside sweating and swearing under her breath to remediate the damage. Less horrendous but still not good, another friend paid good money for a load of “topsoil” that arrived chock-full of asphalt chunks. When it comes to “topsoil,” caveat emptor. 

Peat moss:
detrimental to soil bacteria
and non-renewable
3.                  Avoid amending with peat (a.k.a. sphagnum) moss. Yes, it adds some acidity to the soil. Yes, it holds water. Like a sponge. Once you manage to get it wet. But why would you want to put a wet sponge into your swamp-muck? Also, its very high acid level is detrimental to bacteria. In fact, it kills them. Besides, if high-acid soils are so hot, why do Northerners spread lime all over everything twice a year? One other, global consideration: because peat bogs take skillions of years to form (perhaps I exaggerate a teensy bit), peat moss—like oil and coal—counts as one of your basic non-renewable resources. Yes, the Canadian economy will falter, but it’ll be all right if more Americans start buying their prescription medications north of the border.

A Southern Living garden spread
            To recapitulate, chances are excellent that sand and/or swamp muck is the land-hand you’ve been dealt if you've relocated to southeastern North Carolina. Chances are also excellent that, with a little knowledge, planning, a judicious amount of compost and time put into soil building, you can have the garden those Southern Living spreads promised.

This post is an excerpt from my excellent but unpublished The Best Gardening Book Ever. Be sure to tell all your publisher friends what they’re missing.

Thanks for dropping by.