Category: Cool Research

First, let me give a blanket apology for all of us GPs – this is the first time ever all four of us have NOT posted in the same week.  I’m on the road this week with my high schooler checking out colleges, and I think the other three are out drinking beer and tipping cows somewhere.  So our visiting GP veggie specialist extraordinaire has graciously stepped in to answer a reader’s question about the apparent decline in vegetable nutrition.  Here’s Charlie:

Your United States Department of Agriculture tracks information about all kinds of things, like dry bean production and farm wage data.  They also measure nutrient content of foods (not pesticide residues–that’s for the FDA).  Some curious researchers have wondered if the nutritional content of vegetables has changed since the mid-20th century.  The data exist, so why not look through them?

Authors of a well-cited publication from 2004 have done just that.  Specifically, Davis, Epp, and Riordan did (J. Amer. College Nutr., 23:669-682).  What they found, for example, is if you ate cauliflower in 1950, you probably ate more protein, phosphorous, iron, and thiamin than if you ate the same amount cauliflower in 1999.  They measured the ratio of the nutritional concentration in 1999 compared to the concentration in 1950 [smartly, they adjusted 1950 moisture content to match that of 1999]. If ratio was 1, there was no difference in the concentration.  If the ratio was 0.5, then 1999 cauliflower had half the nutrition of 1950 cauliflower.  They had to use some statistical trickery (they didn’t know error or the number of samples from 1950), but some people might just call that ‘educated assumptions’.  When these ‘educated assumptions’ must be made, I’m a big fan of being conservative with them–in this case, that means that if there is a tiny difference, the researchers wouldn’t catch it.  Being conservative with statistics makes the differences that show up more robust.  Even with the most conservative assessment, the authors show that 26% of the time when nutrients are studied in vegetables, the concentration was lower in 1999 than in 1950.  However, 11% of the time, the concentration was higher in 1999.

The primary author of that paper published a summary of evidence in 1999 (HortScience 44:15-19).  The average numbers for a bunch of studies show similar declines, but statistically, there seems to be a significant decline in specific nutrients in about ¼ to ⅓ of vegetables studied over time. 

‘Jade Cross’ brussel sprouts

Why would this be happening?  Well one reason might be dilution.  The review article gave an example of raspberries: growing raspberries with more phosphorous fertilizer gave more yield (on a dry weight basis), and higher phosphorous concentration in the fruit.  But the plants still took up the same amount of calcium (or only slightly more), irrespective of how many pounds of raspberries were produced.  More pounds of raspberries with the same pounds of calcium removed from the soil means less calcium per pound of raspberries.  That makes sense. The plants can make much of their own dry matter (photosynthesis!), but they can’t make calcium.  I have some questions about using the dilution argument for the 2004 paper: if dry matter didn’t change, but concentration of macronutrients went down, the concentration of something else had to go up–but what?  Is the decline in specific things large relative to the concentration of that thing but small relative to the total dry matter? 

‘Graffitti’ cauliflower

The dilution effect may be the cause sometimes, but what causes the dilution effect?  Atmospheric CO2, or changes in production practices like irrigation, pest control, and fertility might be important, but I like the ‘breeding’ explanation.  Breeders don’t care how much calcium the plant has.  They care if it yields well (dry matter), is resistant to pests and diseases, is pretty or unusual, tastes good, etc.  If a trait is not selected for in a breeding program, it might go away over time.  So maybe the answer is to breed veggies that accumulate (or make, if it’s a vitamin) more nutrients, or to grow more of the existing varieties that might, by chance, already have relatively high nutrient concentrations (they do exist).  There may be a market for selling broccoli that has certifiably more calcium in it, and for change to happen in the marketplace, it has to be profitable.  For right now, you have no idea if the broccoli you buy is a low-calcium or a high-calcium variety because consumers don’t demand to know.

The un-interesting headline reads “some vegetables may be declining in average nutrient concentrations over time”.  The interesting (and false)
620
headline would be “vegetables aren’t good for you anymore”.  From the cauliflower example above:  in 1999, a serving of cauliflower would have about 2.5% of your recommended daily iron, 6.3% of your phosphorous, 3.5% of your protein, and 4.8% of your thiamine.  In 1950, it would have been 6.1% of your iron, 10.3% of your phosphorous, 4.3% of your protein, and 9.2% of your thiamine.  Your vegetables aren’t devoid of nutrition, they’re good for you.  Easter candy probably has none of those things.  If you’re worried, have a multivitamin, or better yet, eat MORE vegetables.  But vegetables grown in 1950 are rather old by now, I’d avoid them if I were you.  Meanwhile, know that a) science is aware of the issue, b) it’s not universal.

 

With Ray’s recent photos of the peach, crabapple, and hydrangea planted too deeply, a discussion of tomato planting depth arose in the comments. I’ve seen the prolific adventitious roots start to form near the base of tomato plants, and I plant tomatoes to the cotyledon or deeper, but tomato planting depth not an area I have extensive research experience in. So I did a little literature search.

It seems that the practice of planting tomatoes is more than just friendly garden folklore. There’s some evidence that it works. Or, at least it works sometimes. Early in the season. And maybe not everywhere. Oh these darn scientists, with their wishy-washy caveats.

There just isn’t a whole lot of peer-reviewed research on the subject. One of the recent papers (from 1996) cites a “Crockett’s Victory Garden” (circa 1977) as a source of information for Northern gardeners about the benefit of deeply-planted tomatoes. Jim Crockett was no horticultural slouch, but just because successful gardeners do something doesn’t mean it’s sound practice (but like I said, I do it too…).

Southern tomato growers, on the other hand, have some peer-reviewed evidence to go on. It seems that in some years and some locations in Florida, tomatoes planted to the first true leaf were able to be harvested earlier (Vavrina 1996) than if they were planted to the top of the root ball. The overall trend was for a bit more and a bit larger fruit per plant. The paper is pretty sparse on detail (statistics and design), but that’s what they conclude. Similarly, a study of fall-planted tomatoes in Louisiana (Hanna, 1997) showed a benefit of planting to the first true leaf instead of to the top of the root ball in two years at one location. In that study, treatments that lowered the root zone temperature tended to increase yield. If you can get an extra 3 pounds of medium or larger tomatoes from a 50 square foot plot in some years, what’s the harm? Deep planting certainly helps prevent lodging in tender young plants, so there’s a clear benefit there if the supports aren’t adequate.

Most of the work suggests that, like mulch, deep planting helps to moderate root temperature, and fluctuating or high root temperatures are stressful to the plant. I can get behind that I suppose, but it’s hot in Florida and Louisiana in the summer. What about stuff planted in Minnesota, where I live? We use plastic up here to get warmer soil temperatures at planting, not cooler temperatures!  Well, I didn’t find any work on that.  But interestingly, some of the yield benefit seen in planting peppers to the cotyledon or true leaf in Florida (equivalent to 5 extra pounds per 50 square feet, in 3 of 4 years, with commercial spacing and fertilizer rates; Vavrina, 1994) don’t show up in Massachusetts (Mangan, 2000). But deep planting did help prevent lodging, which allowed for faster crop maturity in both places.

So the verdict on deep planting tomatoes? It doesn’t hurt, it helps sometimes, and it helps prevent lodging, so why not? One caveat: don’t plant grafted tomatoes deep. The scion will make roots, negating some of the benefit of the rootstock.

(And an addition from Linda:  here’s a diagram from TAMU demonstrating planting depth for tomatoes; this link will take you to the article itself.)

This summer, I’ll be giving a seminar on “Arboriculture Myths” at the ISA conference in Portland, OR. I’ve been quizzing arborist-types for a few months now to find out what myths they would most like to see debunked during my talk. Intermixed with the suggestions of dubious products and questionable practices there was this question: “How often do the results from research with limited scope get over-extrapolated?”

I like the question a lot, because this is the fine line that we Garden Professors walk in bringing you the newest scientific information we can find.  As a rule, I tend to hold back on recommending anything that has only been tested in a lab situation.  I like to see field test results, where environmental variation will quickly swamp anything with marginal effects.  In other words, if something can make it through an experimental, replicated field test, I can get excited about it.

Which brings me to a recent article in Arboriculture and Urban Forestry (2012, Vol. 38, Issue 1, pp. 18-23. And no, I can’t post it on the web). Briefly, the article describes an experiment where water evaporation was measured in pots filled with various substrates, which were either left uncovered or mulched with about 3” of pine bark.  The results showed little difference between the mulched and unmulched containers.

As the authors point out in the discussion, it’s an artificial system that includes no trees, nor any way for water to move through the soil except from the top down.  And I really don’t have a problem with the methodology, or the data generated, or even most of the discussion. What bothers me is a single sentence at the end of the abstract:

“Given the minor reduction in evaporation, and reported disadvantages of mulch application close to the trunk, landscape managers might consider changing mulch application practices for newly planted trees.”

Wow. How did we get from a series of containers with no plants in them to this recommendation?

Every gardener knows the value of mulching – a perception that’s substantiated by hundreds of publications. Since I’ve written about mulches on the blog a number of times I’m not going to belabor the point. But I will refer readers to a short Ecological Restoration article I published a few years ago that most definitively linked mulch application to plant survival in restoration sites; Bert also published an article on the benefits of mulching and lent me a few photos to illustrate. And Jeff has even more data on the topic, including some that may radically change the perception that mulch against tree trunks is a bad thing.

Mulch increases soil moisture

 

Which plot would you rather have in your garden?

Those of you who read scientific journals probably read the abstract first – I know I do. If it interests me, I’ll read the entire article. But sometimes the abstract is the only thing you can find online. And for this reason, the peer-review process in many of the journals asks whether the contents of the abstract are justified by the results. Honestly, I don’t think this article meets that standard.