Dishing on dirt

A summary of what all gardeners should know about soil


Back in the day, some farmers used to test their soil by tasting it. If it was sour, it was too acidic; to sweeten it, they added ground limestone or wood ashes.

These days, the Thurston Conservation District will test your soil for $25. They’ll tell you whether it’s in the right range on the Ph scale that measures acidity and its opposite, basicity. They’ll also report on its levels of nitrogen, phosphorous, potassium and other essential soil ingredients.

Or you can just guess, which is what I’ve been doing since forever. I justify not testing for two reasons: first, I read up a long time ago on all things soil, so I think I know what it needs. Second, my garden generally does well.

Here’s the executive summary of what all gardeners should know about soil, which might save you that $25:

The Big Three elements of good soil are nitrogen, phosphorus and potassium. If you ever read a fertilizer label, you may have noticed three numbers on it, and those three numbers are the Big Three: nitrogen, phosphorus and potassium. Those three should be engraved in your memory, in that order.

Here’s a useful over-simplification of what they do: Nitrogen = green leafy growth; phosphorus = flowers and fruit; potassium = strong roots. (The most egregious oversimplification is about potassium; it actually does many useful things.

So if you want green leafy growth – in your lawn, perhaps, or a bed of lettuce, you will want soil with plenty of nitrogen. If you want abundant tomatoes, you’ll need plenty of phosphorus.

That’s why a lawn fertilizer label might say 5-1-1 – lots of nitrogen and not a lot else. A fertilizer for tomatoes might be 5-10-10 – much heavier on the phosphorus and potassium.

There also a host of minor characters known as trace minerals in a healthy soil. For $48, the Conservation District will test your soil for a bunch of them.

Different plants prefer different nutrients

There are variations among plants about both the nutrients they need and their preferred Ph. Rhododendrons, azaleas, potatoes, strawberries and blueberries like a more acidic soil; most garden vegetables like it closer to neutral. The Ph scale is 0-14; 7 is neutral, less than 7 is more acidic and more than seven is base. (And please forgive me if you learned this in 8th grade. Not everyone did.)

Vegetables need to grow fast and therefore need richer soil than perennial flowers or shrubs. In fact, making the soil too rich around slower-growing shrubs can freak them out. I once killed a favorite shrub with too much manure.

The good news is that compost and manure, dug into vegetable garden soil each year, will generally do what needs to be done. Over time, it builds an ever-healthier soil that will provide the nutrients plants need and the soil texture that holds moisture but doesn’t stay soggy and drown plant roots. And it moderates our generally acidic soils to the Ph to levels most vegetables like.

I supplement compost and manure with bone meal to beef up the phosphorus for both flowers and vegetables like tomatoes. In early spring, when the soil is quite wet and more likely to be acidic, I used to add lime before I planted lettuce, but this year I forgot, and the lettuce did fine without it. That was humbling, proving once again that I often know less than I think I do.

In the garden, science and experience are partners, but they do quarrel sometimes. When that happens, more of both are probably needed. Now that I think about that, maybe I really should get my soil tested just out of curiosity.

Jill Severn writes from her home in Olympia, where she grows vegetables, flowers and a small flock of chickens. She loves conversation among gardeners. Start one by emailing her at jill@theJOLTnews.com


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thanks, Jill! I have found the Conservation District soil testing to be not only interesting, but very helpful! They even told me very specific amounts of each additive for each raised bed, AND where to find the additives in bulk so I didn't have to buy more than I needed.

Monday, June 21