Excessive N and brix levels

alan haighMay 10, 2012

Because I already hijacked another thread on this discussion I decided, at another person's suggestion, to start another thread. I realize this won't be of much interest to most of you.

A participant in this forum has often stated here that excessive nitrogen leads to more watery, lower brix (level of soluble solids)fruit, which was always my belief as well, until someone questioned me on the basis of my opinion.

At that time I found a couple pretty extensive studies that indicated that excess N did not affect brix levels in fruit but I was unable to locate those studies this time around. However I did find several that shows the issue is more complicated than brix levels.

While it may be correct that excessive N can damage fruit flavor, it is apparently not because of lower brix levels. Here's a quote from an English study I found in the ISHS library.

Fruit ground colour was greener and red colour poorer in high nitrogen than in low nitrogen fruit. The concentration of titratable acid sometimes was highest in fruit from low nitrogen plots. Moreover, decomposition of the acids seemed to progress more slowly in low nitrogen than in high nitrogen fruit. Soluble solids were not affected by nitrogen supply. There is a close negative relationship between colour and flavour. Thus the higher rates of nitrogen were reflected in the poorer flavour of the fruits.

Here's some other studies that generally indicate that brix levels are not affected by rate of N.






When I researched this last year, the studies I looked at only evaluated brix levels in relation to N and not the overall quality of the fruit. I mistakenly assumed that the two were synonomous.

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Thanks for the links. We do have to be careful about applying this knowledge to pear trees as discussed in the other thread, since none of the studies involve pears or even related trees like apples, but I think we can all agree, that overfertilizing by definition is not good for plants, and also not good for the environment and we should do what we can to only use what's required.

    Bookmark   May 10, 2012 at 12:31PM
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alan haigh

The study I quoted from is about 3 varieties of apples but response varied from variety to variety so you may need to be even more careful. The ISHS library is members only so I didn't include a link. I renewed my membership just to try to get info. $90. Worth it if you are a plant nerd.

I appreciate your thoughtfulness, Capoman.

    Bookmark   May 10, 2012 at 12:39PM
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fruitnut Z7 4500ft SW TX


Thanks for the references. By and large it seems like N had little effect on brix. But these studies are difficult to interpret without seeing the trees, the leaf color, light interception, crop load, etc, etc.

I'll admit my bias stems mainly from 22 years studying sugarbeet response to N while working for Texas A&M. In sugarbeet excess N lowers brix and makes what sugar is present more difficult to extract, ie more ends up in molasses.

I'm pretty certain that more N than needed to achieve well colored leaves on fruit trees doesn't help brix. And if it reduces flavor, apart from brix, there's not much to argue about.

    Bookmark   May 10, 2012 at 4:07PM
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alan haigh

No, Fruitnut, nothing to argue about, but lots of interesting stuff to discuss. I agree (research shows it) that too much N can be very bad for fruit quality, but I'm still interested in how and why. I had expected brix to go down when N rates go up.

    Bookmark   May 10, 2012 at 4:19PM
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Scott F Smith(6B/7A MD)

Thanks Hman, very interesting. Its hard for me to get too many comparisons with high N commercial fruits since the grocery store ones are usually harvested too early, but in the few reliable comparisons I have been able to make I have noticed something similar: plenty of sugar but less flavor.


    Bookmark   May 10, 2012 at 9:27PM
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olpea(zone 6 KS)


I'm still inclined to believe excessive N can reduce brix in apples. There's too much out there for me to think otherwise.

Following are three studies dealing with apple that indicate excessive N does reduce brix.

The first I posted on another thread:

"A factorial design was used with N and K annual fertilizer rates (0, 50, 100, and 200 kg haâÂÂ1 of N and K2O) replicated in three orchards....Nitrogen fertilization negatively affected fruit color, flesh firmness, and TSS (sugar) content."

The second is a study of N levels on Fuji:

"Lower rates of N fertilizer were related to lower concentrations of leaf and fruit N, "redder"; fruit skin color, greater fruit firmness and soluble solids concentration, higher fructose levels, and a lower incidence of bitter pit and scald than the higher rates of 100 or 150 lbs N AâÂÂ1"

The third is research that actually deals with different types of mulches/understory for apples. However they attribute the lower brix of apples with clover understory probably due to higher N levels:

"Fruit grown with a red clover understorey had lower soluble solids (11.1� Brix) compared to the other three treatments (11.8 - 12.0� Brix), greener background colour and were less firm, probably due to higher nitrogen levels."

Following is a quote from on orchard management out of the Midwest Tree Fruit Pest Management Handbook. It's not research, but I include it to point out that I've read many similar statements in professional literature:

"Excess nitrogen increases fruit size and cork spot, but it delays maturity and seriously reduces color, sugar content, flavor, shelf life, and storage life."

I'm sure you recognize of the four different studies you list, none deal specifically with apples. That doesn't mean I dismiss the research. In fact I do think there is some crossover of the research you linked, just as I think the grape research listed in the other thread has crossover.

Additionally, I'm sure, given enough time, you could produce the previous apple studies you read, and more, just as I could probably find more apple studies that support the converse.

I think all these studies have crossover in that there seems to be a "sweet spot" for the amount of N applied. Too little N results in lower brix. Likewise, too much N results in lower brix.

There seems to be a balance needed for optimum brix. Just as the grape research suggests, I agree that balance is different for different soils (as some soils naturally have more N than others), different irrigation and cultural practices, as well as different climates. Due to all these variables, it's no wonder different studies produce different conclusions.

    Bookmark   May 11, 2012 at 1:04AM
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alan haigh

Olpea, the studies I've seen were carefully constructed with varying rates of N applied and a comparison of brix levels of harvested fruit. I've seen 3 such studies (all in either England or Australia) with apples, including the one I took the excerpt from.

Delaying maturity would affect sugar in correlation to climate, probably, and brix levels would also perhaps be affected by variety if later ripening occurs, with earlier apples not affected. In areas with long growing seasons, such as Australia, and for that matter, England, the affect would be different.

As Fruitnut says, it's really not important whether excess N reduces brix if it clearly reduces quality. I think this is one of those things that research produces contradictory results for and maybe it will never be resolved because it is not practically important.

    Bookmark   May 11, 2012 at 5:54AM
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Scott F Smith(6B/7A MD)

I just read the abstracts of the first 20 relevant articles that pop up when you Google for apple "soluble solids" nitrogen (only about a third of the links are the relevant studies). There is no clear conclusion that can be drawn in my mind, different studies give differing results. One study of trees grown in sand showed the more N the better, but that is probably because there was so little nutrition in the growing medium. In these sand growing studies I would not interpret their results to mean anything much -- who cares about sand-grown apples. A few studies showed a peak at a low but non-zero N level, that to me is the "right" answer. One reason why trees with more N will produce less SS in some contexts is the total fruit mass will be larger, so even though there is more N there is more apple flesh to "feed". This kind of subtle interdependency is what makes the results so inconsistent.

Anyway the bottom line is pretty clear to me, fertilize a small amount, and be aware that too much is probably worse than too little in most settings.


    Bookmark   May 11, 2012 at 8:55AM
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alan haigh

Scott, if later ripening affects brix levels and high N causes this, how do you separate cause and affect?

N applied in early spring is not the same as N applied later, as well. And what about the water and N correlation?

I guess you can just draw the conclusion you started with as there are too many variables to really get a handle on this but I believe that a urea application, either foliar in the early spring (around tight cluster) or ground ap in late summer can improve fruit quality in most soils if trees are not on a mulch program that started a few years earlier.

I think there may be a problem with mulch increasing the supply of N when you want it to be winding down with bearing trees. Or maybe it just holds too much moisture on wet years in the humid regions. Like to see some research on this.

    Bookmark   May 11, 2012 at 9:55AM
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fruitnut Z7 4500ft SW TX

I agree, one needs to know crop load and light interception among many other things to interpret any of these studies. I also think there is a difference between going for maximum economic commercial yield and a hobby situation where eating quality is first priority. For the later I want only enough N to color up the leaves, an open canopy, and a light crop load.

Leaf color to me is the best indicator of nitrogen status. It's easy to see and very definitive. So I don't apply N until I can see leaves that are light green and then only very little.

    Bookmark   May 11, 2012 at 10:08AM
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alan haigh

Scott, I wish you had listed links to relevant studies. I used your search words and spent 20 minutes finding very little of use. I don't trust studies of late ripening apples in cool regions because it doesn't separate the affect of later ripening from other aspects of N influence.

    Bookmark   May 11, 2012 at 2:04PM
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