From: “The Fruit Gardener”, vol. 40 no. 5.
Soil Nutrients and Fruit Quality, Dr. Tom Ruehr.pdf (1.4 MB)
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Should be required reading! Excellent article that gives the reader an idea of how important each nutrient is, and how complicated it can be to do it right. Although it’s of particular interest to the commercial grower it is still full of information readily understood by rank amateurs like me.
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That article was so good I added it the the Reference Section (which is an under utilized but fantastic source of information.)
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The first paragraph on N lost me when he lumps urea with other organic N sources saying it takes weeks to months for it to become available in CA soils. Kind of shattered credibility. More like a day or two- if the species is capable of absorbing urea whole, even sooner. From the endorsements here, I guess I should read on, but this should have been edited out- if he’s this wrong about what I know, how can I trust him with what I don’t know? . Especially when he doesn’t include references to enforce his assertions, such as that late season N reduces fruit quality. I need to see the research because I’ve seen contradictory research to this with apple trees.
I would tread carefully here, the author is not widely known and this paper is not solidly academic. I could find none of his published work in peer reviewed journals.
Maybe you can help here, Richard.
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The article seems like mostly speculation and not science based. I didn’t see much useful take away information. For instance I don’t think many commercial orchards are applying soluble P via drip. And the stuff about deep and shallow N and K and affects on fruit quality seems pretty far out and can’t be very widely applicable. Soils, rainfall, and irrigation patterns vary too much.
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Hmmmm …
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Ruehr is a huge fan of fertigation.
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I figure California soils are so dry most of the time that this probably true about urea. 
Most of the soil science is reasonable. But not all of it. It has a particularly western bent and half of it is irrelevant to many soils east of Lincoln, NE (Pedocal - Wikipedia). Cation leaching in soils (particularly with high organic matter and/or clay present) is an over many years process or due to acidification through addition of much urea and ammonium nitrogen. But I suppose if you consider an essentially all sand soil, then it might be accurate.
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To me his credentials are impressive (PHD and professor of Earth & Soil Science Cal Poly). He has an email address and invites questions if anyone wants to challenge him or get clarification but sadly he has died so that isn’t going to happen. It would be nice to find a published review of his article.
I don’t think he intended it to be academic with a title “Plain talk on the basics.” He is a published author. Google Thomas A. Ruehr for more info.
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Earth and Soil science is not a credential associated with fruit production- I believe you could get that degree without a single course related to commercial fruit production. If he was involved in fruit research at U.C. Davis that would add credibility- if he worked as a consultant in the industry, that would add credibility. I can’t even know if he taught soil science related to agronomy- let alone fruit production.
Irrigation is always involved with N fertilization to the soil (or even to foliage) when it isn’t dependent on rain- without water no form of N works- urea is urea west or east. Fertigation is common practice in CA. His mistake on this was egregious, IMO.
@alan, do you know Harvey Correia? I believe he took at least one of Dr. Ruehr’s classes at Cal Poly.
Hard to believe he could be completely wrong although science is always evolving. Maybe his education and experience was inadequate for the article he wrote. Or maybe newer research shows that his ideas from 2009 and prior where wrong. Like to see some contradictory information.
Here is a statement regard his experience:
“He conducted research in agriculture, food systems, and bioremediation, was awarded many grants and authored many publications and presentations. He also reviewed many articles and books for his profession, and worked in curriculum development.
Professor Ruehr conducted educational and training sessions for fertilizer companies, and frequently served as a consultant.”
I know @alan has plenty of practical experience so I will reread Ruehr’s article knowing that not every experience fruit grower agrees with what he is saying.
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No Richard, why do you think I’d know him?
Dan, none of what you mention includes research with fruit trees or is specific to the article we are discussing. Assertions such as made in his article need a foundation based on research. Maybe they are, but I just haven’t been able to find a link from his biography that tells me he would have any way of knowing of what he writes. It isn’t a matter of agreeing or disagreeing, it is a matter of wondering where his conclusions come from. I don’t know if the foundation of his assertions is valid or not at this point. I’d feel the same way even if I had no experience with fruit trees.
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Prior fruit growing bulletin boards, NAFEX, CRFG, …
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Haven’t lived in CA for over 40 years.
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Here’s a few of them. It’s a Google Scholar search, so articles by others also appear.
https://scholar.google.com/scholar?q=Ruehr+Fruit+Cal+Poly&btnG=&hl=en&as_sdt=0%2C5
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Harvey is a good guy. I’ll ask him about Dr. Ruehr at our next fig gathering.
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Interesting Richard, he clearly was a part of the mainstream research community and not some outlier kook as I feared. The statement about urea in the paper you originally linked was clearly a mistake and lumping urea in the statement couldn’t have been intended, although it’s hard to understand how it stood after proof-reading it. I’m no academic and it stood out to me like a flair, maybe because urea is what eastern fruit growers almost always use. Nitrate gets washed away pretty easily and is more expensive. With free standing trees in the humid regions I’m not sure there is much difference in what the roots absorb, regardless of original form.
One should be mindful of academic leaps of logic, where an academic starts with a researched truth and then formulates a creative hypothesis and represents that hypothesis , even in their own minds, as established fact. The way to see the basis of academic assertions is by way of checking the research they are based on. I’m still not sure how much, Dr. Ruehr’s statements are research based or are logical leaps. I don’t see any research that establishes his ideas about form of N and fruit quality (with tree fruit). Furthermore, what I read is about fertigation, which most of us do not use in our fruit production. Not saying his assertions are or aren’t research based, only that I can’t establish it.
In research about the human body, and diet, for example, the public is well aware of these kinds of leaps of logic, such as the danger of cholesterol in eggs and meats automatically leading to congested arteries according to experts and then similar experts stepping back when subsequent research proved the relationship much more complicated. Research on the human body is much, much more extensive than on root uptake of fruit trees.
Nevertheless, thanks for bringing his ideas to the forum. Not sure how useful they are to us but they certainly are interesting to me.
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For a novice like me Ruehr’s article on soil nutrients is useful but no written document should be assumed to be 100% correct. Overall it conveys the importance and complexity of soil nutrients that effect fruit trees. With @alan’s critique, other responses and further research by me I’ve increased my knowledge of the how, why and when of nitrogen fertilizer.
The first paragraph from Ruehr’s article certainly is confusing and misleading. Perhaps he is referring to very specific conditions (and unimportant to most readers) which he should have explained. Doing some research I did come across this article from University of California Cooperative Extension with the following statement;
“Soil temperature is the main factor influencing the rate of
nitrification in our agricultural soils. The warmer the soil, the faster ammonium nitrogen will be
converted to nitrate nitrogen. Winters in California generally do not get cold enough to stop the
nitrification process entirely. However the process is slowed down during cooler months and
typically occurs in the range of several weeks to months. Under hot conditions, the transformation can occur within a few days to weeks.”
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I suppose like Alan, I’ve always read nitrification of urea is very rapid. My guess is under some conditions Urea is slower to convert to nitrification. Alan’s point is well taken by me that it’s assumed CA growers irrigate, which would speed nitrification in the desert.
I really have no experience w/ growing practices out there.
My thought is that I’ve sat in lectures of respected Profs who said some fringe things (which I of course took notes on for the test). Mind you, I think these things were true in a narrow context, but lacked any wide application. I sort of see that with this article.
I think it has some value, but some of the specifics probably lack wide application (what Fruitnut said). For example, I don’t think high N automatically results in increased fungal diseases. Certainly there is narrow research which supports this, but I suspect there is little research testing this against real world growing (where some fungicides are applied under typical commercial - or even home grower - conditions).
Dr. Ruehr also cautions using urea without being immediately watered in. Again this is a broad brush. There is much research on the denitrification of urea in the Midwest. While everyone knows this is a risk, the risk is mostly over played. Nitrogen losses from Urea are generally pretty minimal on our soils when it’s applied in the spring. But any PhD worth his salt should probably mention it, because it’s such a documented phenomena (ignoring how severe it is, which is not much here). The problem is that when people take it as a universal, they are afraid to apply it except under the most perfect conditions, which isn’t really borne out by the research.
Likewise, Dr. Ruehr proposes zinc nails in the tree as a means to reduce zinc deficiency. While there is research to support this, there are also colleagues who call into question the wide applicability.
All in all, I think the paper is worth reading, but wouldn’t take it as the final truth in every situation.
In terms of bringing up P in the soil, I have read that P is very hard to bring up, and seen multiple recs to use it in drip irrigation, if low in P. By overloading the irrigation zone in P, you can get it down to the roots.
Some forms of P are better than others. My understanding is that DAP is the most mobile form of P, but still not super mobile. Rock phosphate (the natural form of P) is basically immobile under almost all conditions.
All in all, if I may sound condescending from my local experience, I’d give Dr. Ruehr a good grade (lol) on his paper to the audience it’s targeted. A lot of good basic info, along with the processes involved.
He sounded like quite a Prof. when he was alive:
http://polyratings.com/eval.php?profid=557
“Ruehr is crazy. He talks about rocks one minute and dropping ACID at a rock festival the next. If you answer half of the questions on his test right, you’re doing fine. Everyone thinks they’re failing the class until he curves the grades at the end. He also must have a deep hatred for the chalkboard. He will suddenly, and for reasons only Ruehr knows, scream and hurtle erasers into the blackboard. It’s very scary for those of us who are trying to sleep. I still have bruises on my knees from hitting the top of the desk from when he decided to do his insane tarzan scream.”
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