In container culture, the pH of the soil solution is much more important than the pH of the medium. What happens to container media pH is too complicated to draw generalities, but more often than not, media pH tends to rise as they age due to an accumulation of bicarbonates, but we can impact that affect by the pH of our irrigation water and by the fertilizers we use. Fertilizers deriving their N from urea and ammonium salts tend to acidify, while fertilizers deriving their N from nitrate sources tend to move the medium pH toward basic.

Dolomitic lime's solubility varies with soil/soil solution pH, temperature, moisture content, and very importantly, the size of the limestone particles. Particles that won't fit through insect screen should be considered useless as a liming agent because of their reduced surface area:bulk density makes then essentially insoluble for container culture. The lime you often buy that is in round pellets of varying size is actually prilled. A slurry of pulverized lime and a binding agent is shot from tall 'prilling' towers. It forms small spheres on the way down and hardens. This is done to make the pulverized lime easier to broadcast. When the lime gets wet, the prills quickly break down into pulverized form, so the 'prills' are much more soluble than unpulverized limestone of the same size would be.

The lime fraction of the limestone doesn't leach from the medium very quickly at all. I have (slow-growing) plants that I've kept in the same medium for 5 years or more that showed no signs of Ca deficiency with no lime applications subsequent to the original incorporation into the medium. Part of that is due to the tendency for bicarbonates to accumulate in the soil, which also supply a source of Ca. The Mg fraction of dolomite is much more soluble, up to 125x more soluble than the Ca fraction. I usually try to include a source of Mg (Epsom salts) in my fertilizer solutions for plants in the same medium for growth cycles subsequent to the first annual cycle.

Al

Thanks, Al. When I read this, I tested the soil solution of a plant I'd had potted for about 8 months. The pH was the same as when I potted it. But, I wanted to hear from someone else that this was false - not just me testing one pot!

It's very interesting that the form of N determines, in part, whether the media acidifies or gets more basic. In the case of many growers, they are using peat based mixes; I've read the peat acidifies as it breaks down. If you were using a peat based mix with a nitrate source fertilizer, would you nullify the pH move in either direction?

Almost certainly not. Peat forms various acids as it breaks down, but most are flushed from the pot if you water correctly. The only time it would be likely that you wouldn't see an upward creep in the pH of your container media is if you are consistently irrigating with water in the 5.8-6.0 pH range or lower and are using fertilizers that get more than half their N from urea or ammonium salts.

Regularly flushing the soil slows the upward creep, but it's important to temper everything I just said with the fact that media pH is less important than soil solution pH. If you're fertilizing on a regular basis, and the nutrients are in the fertilizer solution, your (containerized) plants are going to have access to those nutrients.

It's important we understand that fertilizer elements don't behave quite the same in the low bulk density container media containing large organic fractions the same way they do in high bulk density mineral soils.

Al

OK, so the upward creep is inevitable if using a nitrate based fert, even when using peat, as long as you're flushing the media when you water. It's when you don't flush that things build up.

The mention of urea brought up another question. Why do some people say not to use urea based ferts? I just saw someone on another forum saying that the urea can't be broken down and used by container plants because they don't have the necessary bacteria present. It's the first time I've seen a reason; usually I just see the admonition not to use it.

While I grow many outdoor veggies and a few things houseplants, the majority of my indoor plants are gesneriads. Chirita, African Violet, streptocarpus. If you frequent any AV forum, you will hear over and over not to use urea based ferts. The post I mentioned above suggested it was because there were missing bacteria to break it down in containers.

In this document, http://www.avsa.org/AVMFiles/AVSA%20Culture.pdf, it says not to use urea based ferts because they can burn the roots.

This document doesn't tell you why, just to use one low in urea: http://www.baystateafricanviolet.org/avinfolinks.html

In a description of one of the ferts sold on this page, it says urea hinders the ability of the plant to uptake the fert: http://www.bluebirdgreenhouse.com/html/supply01.htm

It's truly everywhere in the AV world. Now, burning the roots? That's different than preventing uptake, though I suppose if you're burned you can't uptake.

I haven't a clue as to if this is true or not or what the real reason is, if there is one. I do know that it is permeated throughout the AV community and everyone spends their time trying to find non-urea based ferts.

If you over-fertilize using urea, you can 'burn the roots' because of the conversion of too much ammonia (or from a high level of TDS/EC), but that's a dosage issue.

Plants can safely store abundant N in nitrate form, but they can't store much in ammonium form. When you apply urea, it is either broken down in the soil into ammonium and CO2, as I outlined above, or is taken up intact and converted to ammonium inside the plant. Only when luxury (super abundant) levels of N are available will plants store N. At THIS time, the possibility of ammonium toxicity is increased, but again, that is due to a heavy hand on the fertilizer measuring spoon.

I'm not sure why anyone would think there are not nitrifying bacteria in the soil that would convert reasonable doses of ammoniacal N to nitrate N, but they're there. Again, I would point to the thousands of plants and the millions of planters that are using urea-based fertilizers with stellar results.

Admittedly, anything that suppresses the work of these bacteria (saturated or compacted [low oxygen] soil, low media pH, low [lower than 55*] soil temps are the main influences) increases the possibility of ammonium toxicity, but again, these are adverse cultural conditions that if avoided would not contribute to the possibility of ammonium toxicity.

I have no dog in the fight over what should or shouldn't be used for someone else's plants, and I'd never claim to be particularly proficient at growing AVs, but the pictures I recently posted showed perfectly healthy plants that had been reared in the gritty mix using FP 9-3-6 (40% urea-N), and I have owned equally healthy plants in the past that were fertilized with MG 24-8-16 or 12-4-8 (100% urea-N), Micromax having been added to the soil when it was made. I'm not sure what I might be doing differently, that allows me to use urea-based fertilizers where others struggle with them, but obviously I have to come down firmly on the side of 'it can be done w/o issues'.

As is usual, the mileage of others may vary.

Al

Based on what you've said, my guess is that the delicate AV roots cannot take overfertilizing with a urea based fert but aren't hurt by overfertilizing with a non-urea based fert. Instead of educating people about using fertilizer properly, AVSA and other sources simply say to avoid urea.

I am using FP on AVs and other gessies in a perlite/peat mix and they are growing very well. I use it for hippeastrum in a gritty mix. I have not had any root problems using the perlite/peat mix with the AVs (2:1 with a bit of chopped sphagnum and lime to bring the solution to a low 6 pH).

There are two exceptions. When I recently forgot to add lime, all the plants declined. The roots were dead, brown and had stopped growing in all cases (about 50 young plants). I assumed it was the very low solution pH (it tested in the high 4's) which caused the decline. I cut off the root balls and repotted in a mix with lime and the roots are doing fine. It makes sense that it was the low media pH given what you said above, not the fert itself.

Thank you, once again.

Al, Dyna-gro prides itself on being urea-free. I am surprised that you say that it's 9-3-6 nitrogen source is 40% urea.

L.O. -here is a different explanation (see link) of what happens with mixes as they age --one that I'm more familar with:

Salient points - organic mixes such as peat and pine bark acidify with age unless tendency is offset with alkalinity in water, dolomite incorporations, base fertilizers, etc.

Acceptable pH of mixes in containers is broader than soil as nutrients are being supplied on a regular basis. Though I did read that tomato yield in greenhouses is 25% less when the pH reaches 7 so it does have an impact.

Here is a link that might be useful: pH of mixes over time

Farkee, this is the explanation I am also most familiar with. I have only tested one plant, and the mix was only 8 months old or so, but it was identical to when I potted it.

What this article does not say is how long this can take. They said something like - sometimes quickly - but what does that mean? Weeks? A few months? 1 year instead of 5 years?

Yes, I understand. I've explained dozens of times that the upward creep in pH is related to an accumulation of base compounds from our tap water that offsets the acids resultant of humification, as well as how to offset it if you see indications of (particularly) micro-nutrient deficiencies.

OK - Dyna-Gro doesn't use urea, and I SHOULD have said that 9-3-6 gets 40% of its N from ammoniacal N (probably ammonium nitrate and/or phosphate) to be technically correct. What does urea break down into when it's hydrolized or is combined with urease in the soil? CO2 and ammonia.

Al

I went to the Dyna-Gro website to see what I could find out about the urea content. The first thing I found was this: "The ammoniacal and nitrate forms are used directly by plants for stem and leaf growth. The urea form of nitrogen must be broken down by soil borne microorganisms or urease before it can be utilized by the plant. Urea can cause leaf tip and root burn. Deficiency (Def.): reduced yields, yellowing of leaves, stunted growth. Excess nitrogen can delay fruiting and flowering."

That could be why the person I mentioned above said that urea shouldn't be used. He/she probably only knows the first part of the above statement, not that urease is readily available in containers.

I finally found, after many pages, a comparison chart to other commercial fertilizers, where is says that there is no urea on the 7-9-5 Liquid Grow. Below that, there is a bullet point, "DYNA-GRO does not add urea to its formulas, since it may cause burning of the roots and is not immediately available to the plants."

Again, statements like this could be why people believe that urea shouldn't be used in containers. If you don't know all the pieces of the science, a statement such as that can lead you to believe something that isn't the whole story.