Why soil tests matter: lessons from my vegetable garden

Regular blog readers will remember that we moved to my childhood home a few years ago. With an acre or so of landscape I finally have enough room to put in a vegetable garden. My husband built a wonderful raised bed system, complete with critter fencing, and we’ve been enjoying the fresh greens and the first few tomatoes of the season.

Jim puts on the finishing touches to our first raised bed garden.

We filled these raised beds with native soil. During a porch addition I asked the contractor to stockpile the topsoil near the raised beds. The house was built almost 100 years ago and at that time there were no “designed topsoils” (thank goodness) – soil was simply moved around during construction. Some of this soil had been covered by pavers and the rest had been covered with turf. [You can read more about designed topsoils in this publication under “choosing soil for raised beds.”] There had been no addition of nutrients for at least 7 years so I was confident that this was about as natural a soil as I could expect.

Our native soil, ready for adding to our raised beds.

I’ve always advised gardeners to have a soil test done whenever they embark on a new garden or landscape project, so before I added anything to my raised beds I took samples and sent them to the soil testing lab at University of Massachusetts at Amherst (my go-to lab as there are no longer any university labs in Washington State for the public to use).

What I already knew about our soil was that it’s a glacial till (in other words it’s full of rocks left behind by a receding glacier). The area is full of native Garry oak (Quercus garryana), some of which are centuries old. The soil is excessively drained, meaning it’s probably a sandy loam. And that’s about all I knew until my results came back.

Some of our massive, centuries old Garry oaks.

Because nothing has been added to this soil for several years, and because I had removed all of the turf grass before filling the beds, I assumed that the organic matter (OM) would be quite low. Most soils that support tree growth have around 3-7% OM. Hah! Ours was over 12%! All I can figure is that centuries of leaf litter has created a rich organic soil.

I never expected this level of OM.

So here’s lesson number one: Don’t add OM just because you think you need it. Too much OM creates overly rich conditions that can reduce the natural protective chemicals in vegetation. This means pests and diseases are more likely to be problems.

I think these may be the lowest P levels I’ve seen in home garden soils.

I was pleased to see our P level was low. First soil test I’ve ever seen in my area where P was below the desirable range! Does that mean I’m adding P? No – because there is no evidence of a P deficiency anywhere in the landscape. And my garden plants are growing just fine without it.

No sign of any nutrient deficiencies here (though the mesclun mix got out of control).
Or here either.

 

 

 

 

 

 

 

 

 

 

Lesson number two: Just because a nutrient is reportedly deficient, look for evidence of that deficiency before you add it. It’s a lot easier to add something than it is to remove it.

Likewise, our other nutrient values are just fine, and I was pleased to see that lead levels were low. Given that this is an older house that had lead paint at one time, and given the fact that the soil being tested was adjacent to the house, I was prepared for lead problems.

Surprisingly low lead given the original location of this soil next to an older house.

However – we do have high aluminum in the soil. Exactly why…I don’t know. Perhaps the soil is naturally high in aluminum? There’s no evidence that aluminum sulfate or another amendment was ever used. In any case, that was an unexpected result that does give us some concern for root crops. I’ll be doing some research to see what vegetables accumulate aluminum.

The aluminum levels may bear some watching if I’m growing root crops.

Finally, note our pH – 4.9! This is completely normal for our area, which is naturally acidic. In addition, the tannic acid accumulation from centuries of oak leaves has undoubtedly pushed the pH even lower. Are we going to adjust it? Again, no. There is no evidence of any plant problems, and even our lawn is green. Why would we adjust the pH if there is no visual evidence to support that?

No, this is not a typo.

Which leads to lesson number three: Don’t adjust your soil pH just because you think you should. If your plants are growing well, the pH is fine. Plants and their associated root microbes are pretty well adapted to obtaining the necessary nutrients. If you have problems, don’t assume it’s a pH issue. Correlation does not equal causation! You’ll need to eliminate all other possibilities before attempting to change your soil chemistry. And remember it is impossible to permanently change soil pH over the short term. Permanent pH changes require decades, if not centuries of annual inputs (like our oak leaves).

The cat agrees – no pH issue with this lawn.

Will I test my soil again? Probably not. I have the baseline report and since I don’t plan to add anything I don’t expect it to change much. If I had a nutrient toxicity I would retest until the level of that nutrient had decreased to normal levels. But with everything growing well, from lawn to vegetables to shrubs and trees, there really is nothing to be concerned about.

Viburnum plicatum (I think) is one of many established shrubs on the property.

Lesson number four: Unless you have something in your soil to worry about, don’t.

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Linda Chalker-Scott

Dr. Linda Chalker-Scott has a Ph.D. in Horticulture from Oregon State University and is an ISA certified arborist and an ASCA consulting arborist. She is WSU’s Extension Urban Horticulturist and an Associate Professor in the Department of Horticulture, and holds two affiliate associate professor positions at University of Washington. She conducts research in applied plant and soil sciences, publishing the results in scientific articles and university Extension fact sheets. Linda also is the award-winning author of five books: the horticultural myth-busting The Informed Gardener (2008) and The Informed Gardener Blooms Again (2010) from the University of Washington Press and Sustainable Landscapes and Gardens: Good Science – Practical Application (2009) from GFG Publishing, Inc., and How Plants Work: The Science Behind the Amazing Things Plants Do from Timber Press (2015). Her latest effort is an update of Art Kruckeberg’s Gardening with Native Plants of the Pacific Northwest from UW Press (2019). In 2018 Linda was featured in a video series – The Science of Gardening – produced by The Great Courses. She also is one of the Garden Professors – a group of academic colleagues who educate and entertain through their blog and Facebook pages. Linda’s contribution to gardeners was recognized in 2017 by the Association for Garden Communicators as the first recipient of their Cynthia Westcott Scientific Writing Award. "The Garden Professors" Facebook page - www.facebook.com/TheGardenProfessors "The Garden Professors" Facebook group - www.facebook.com/groups/GardenProfessors Books: http://www.sustainablelandscapesandgardens.com

18 thoughts on “Why soil tests matter: lessons from my vegetable garden”

  1. Glad your gardens are growing well, Linda. One comment regarding the “mystery” of the high aluminum values. It will be directly linked to the low soil pH, as aluminum solubility from soil minerals increases rapidly below a pH of 5 to 5.5. If you are concerned about aluminum toxicity, a moderate addition of lime may be in order.

    1. All of the soils in the western (wet) part of Washington are acidic. This is the first time I’ve seen a test in this region (and I’ve seen humdreds) where aluminum is so high. It is not typical. We also have relatively low clay content, so not a lot for Al to bind to.

  2. Linda, your vegetable garden looks great! You mentioned not adding any P whatsoever. So you don’t fertilise at all, not even before a new season after last year’s plants have sucked up what’s in the soil?

    1. Thanks Lukas! This is the first year I’ve had the garden. The crop you see is what I got with the unmodified soil, and none of the plants has any signs of any deficiency. Over the winter I’ll keep the soil topdressed with wood chips to keep weeds down and to provide a slow feed of nutrients. I don’t plan to add anything unless I can see a deficiency in the next season.

    1. Hi Al, I’m assuming you mean me? The timbers are pressure-treated wood – the newer preservatives are copper based. When I re-do soil tests at some point I’ll pay close attention to the copper level in the soil (which right now is in the optimal range. If it becomes a problem I will re-think my use of these timbers. But right now evidence points to no real problem to plant or people health.

  3. Dr. Linda: “I’ll be doing some research to see what vegetables accumulate aluminum”

    from reading many posts on Garden Professors blog, i thought the concept of “dynamic accumulators” was debunked.

    1. We’re not talking about “dynamic accumulators” which is not science. We’re talking about the ability of plants to take up metals – many of which are necessary enzymatic cofactors and some of which are toxic metals. Some plants avoid uptake; others take up and accumulate in their roots or leaves. It’s different for every plant. There is a lot of research on heavy metal uptake by vegetables and that’s the literature I look at.

      1. i’m now trying to understand the difference between “mineral uptake” in all plants and the claim that certain plants are “dynamic accumulators”. is it the claim made that minerals are “stored” in the plant?

        1. “Dynamic accumulators” isn’t a scientific term. It’s a belief that certain plants can have deeper roots/accumulate more minerals than other species. It’s not true. It has to do with soil aeration, hydration, nutrient levels, and microbes. Period.

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