Should I use aluminum sulfate (sulphate) to acidify my soil?

Question

It is often recommended online or by nurseries to apply aluminum sulfate to plants like blueberries, rhododendrons, azaleas and to hydrangeas to make the flowers blue. Is this advice I should follow?

Answer 1

A few months ago I called my local nursery to ask if they carried ammonium sulfate. He said "Oh, you want aluminum sulfate to acidify the soil for blueberries." I cringed in horror that this advice is being dispensed so regularly.

• Is aluminum a nutrient or do plants use aluminum in any way?

Aluminum is not known to be a nutrient for plant growth in any quantity and is actually more well-known for being a toxin.

Aluminum (Al) is the most abundant metal in the earth's crust, comprising about 7% of its mass. Since many plant species are sensitive to micromolar concentrations of Al, the potential for soils to be Al toxic is considerable. Fortunately, most of the Al is bound by ligands or occurs in other nonphytotoxic forms such as aluminosilicates and precipitates. However, solubilization of this Al is enhanced by low pH and Al toxicity is a major factor limiting plant production on acid soils.

http://www.plantphysiol.org/content/107/2/315.full.pdf

Why would you want to add more of this to your soil? Aluminum doesn't acidify the soil, but aluminum becomes more readily available to plants as the soil becomes more acid. Ph is a measure of H+ ions in the soil. It has nothing to do with aluminum.

• What does Ph mean?

In technical terms, pH is the negative logarithm of the activity of the (solvated) hydronium ion, more often expressed as the measure of the hydronium ion concentration.

The exact meaning of the "p" in "pH" is disputed, but according to the Carlsberg Foundation pH stands for "power of hydrogen".

http://en.wikipedia.org/wiki/PH

• What do acid-loving plants actually need?

Actually, what blueberries like is Magnesium. They grow well in acid soils because calcium is low, which lowers the Ca:Mg ratio and provides the plant with more access to Mg with less competition from Ca.

pH is a measure of free Hydrogen ions in water. It measures Hydrogen ion concentration, H+ and OH-, and that’s all it does. One can change the soil pH with any acid or alkali. You can raise the pH with sodium hydroxide, which is lye, drain cleaner, or lower it with hydrochloric acid, for instance, but they aren’t going to give you much growth stimulus. They will probably kill the plant. A slightly acid pH of about 6 or 6.5 is ideal, because it gives just the right amount of electrical conductivity in the soil, but plants aren’t nearly as finicky about pH as they are about having the right balance of soil minerals.

Rhododendrons, for instance, are supposed to require an acid soil. What they really prefer is a high Magnesium soil. Experimenters in Scotland raised the pH of soil from 5.0 to nearly 8.0 with Magnesium Carbonate, and the rhodies grew better and better as the soil pH went up because the Magnesium level was going up. pH had little to do with it.

So, this is a good thing to know if you are trying to grow rhododendrons in New Mexico, for instance, where the soil is frequently alkaline to start with, although there you would want to use an acid form of Magnesium like Magnesium sulfate, Epsom salts. But your garden, your farm crops and your fruits and berries wouldn’t necessarily like it (except the blueberries). High levels of Magnesium in relation to Calcium are common in Organic gardening and farming, though, because people are told to lime their soils with dolomite lime, which is high in Magnesium.

http://www.soilminerals.com/AgricolaII.htm

So, its not ph that matters as much as its the right nutrient combination for the plant. It just so happens that the right nutrient combination is most often found in a naturally acid soil. That observation doesn't necessarily mean that the soil must be acid and in no way infers anything about aluminum.

• How do sulfates acidify the soil?

Aluminum sulfate acidifies the soil because of the sulfate, not the aluminum. Actually, its possible aluminum, being Al+++, could take the place of 3 hydrogen ions (H+), though more likely is it would take the place of a Ca++ and 1 H+. Anyway, its the sulfate which causes acidity.

SO4 + 2 H2O = 2 H2SO4 + O2

Sulfate + water = sulfuric acid + oxygen

Ammonium sulfate is NH4 + SO4, so not only will the sulfate make sulfuric acid, but the NH4 (ammonium) will break down to NO3 (nitrate) and release extra H+ into the soil, which makes the soil more acid.

Same thing with epsom salts. Mg + SO4. Sulfate when added to water takes the hydrogen out of water and makes an acid.

• Why is aluminum sulfate especially harmful to blueberries?

Blueberries have sensitive roots which lack root hairs found on most plants. The application of nitrates is known to "burn" blueberry roots and could kill the plant.

See this link and take note on page 9 of the pic of blueberry roots with NH4 and NO3.

Blueberries, and their relatives’ cranberries, lingonberries, and bilberries have somewhat unique N requirements. They are not able to use nitrate forms of N (NO3-N) effectively. These plants have evolved in soil conditions that do not naturally contain a significant amount of NO3-N and they depend more on ammonium-N (NH4-N). Blueberries take up both forms of N, but they have limited nitrate reductase activity. Nitrate reductase is an enzyme that is needed to convert nitrate to amino acids and proteins. The limited nitrate reductase system in blueberries means that they cannot efficiently utilize nitrate forms of N. Some reports also state that excessive nitrate fertilization can lead to leaf burn.

(I'm pretty sure "leaf burn" is a misprint, it should probably be "root burn")

Also consider from the 1st page:

Blueberries have fine, fibrous roots that do not develop root-hairs.

Going back to the first link I posted:

The most easily recognized symptom of Al toxicity is the inhibition of root growth, and this has become a widely accepted measure of Al stress in plants. In simple nutrient solutions micromolar concentrations of Al can begin to inhibit root growth within 60 min.

So if Al harms roots and blueberries have fine, sensitive roots, why would you want to add aluminum to your soil for the benefit of blueberries?

Furthermore, aluminum sulfate dissolves in water and becomes immediately available to plants (as a toxin). As opposed to the aluminum in soil that is locked-up.

Lastly, from page 45 of spectrum analytics site which contained the root pics:

Aluminum deserves special attention with blueberries because of the very acid soil pH which the crop requires. At these acid pH’s, there is often a considerable amount of soluble Al in the soil solution. This can cause several negative results. First, soluble Al has a strong affinity for soluble P. This is the same form of P that the blueberries require. The result of the excess soluble Al can be the requirement for a higher soil P test than suggested by us and other authorities. Another potential problem is the cation competition caused by excessive soluble Al. The likely result of this cation competition is reduced uptake of one or more of the cation micronutrients (Cu, Mn, Fe, and Zn). About the only way to identify either of these potential problems is with leaf analysis.

And then from page 117 here:

Do not use aluminum sulfate to lower the soil pH because aluminum is toxic to blueberries and is already present in many soils in the region in quantities that can negatively impact blueberry plants once the pH is lowered.

From page 2 here:

Do NOT use aluminum sulfate, as this material is toxic to blueberries.

Hopefully one day nurseries will stop recommending aluminum sulfate for anything.

Answer 2

If you just want to use it to keep hydrangeas blue, then fine, but sequestered iron is more useful for rhododenrons, usually. Also works on the hydrangeas, in my experience. Sold in the UK as Sequestrene Iron Tonic, but other companies produce it with added ingredients such as magnesium or seaweed extracts. A useful remedy for chlorotic lime hating plants, and useful for blueberries too, because they, like all the other lime haters, have trouble taking up iron in alkaline conditions.

Answer 3

Okay, a few things here. Firstly, Aluminum Sulfate in too high volume is not good. Nor is anything else. This depends where you live and how rapidly you'd like to adjust the soil. When mixed with water it will act with an acidifying factor to your soil. It's MUCH more immediate than most other things you might add. It's cheap and readily available and when you add the correct portion it WILL NOT harm your plants. That's because people do silly things and go outside the recommendations. I would ONLY use it as a soil amendment initially. No need to add this stuff year over year. That's why you'd add elemental sulfur. My soil starts at about 7.25 which is too extreme so I can add this and mix in with my initial planing and that's it. I've never seen harm come from following the recommended levels. Do a pH test first! Determine how badly you need it. And yeah, there are other ways to accomplish this task. The acid is what releases the nutrients for the plants. With alkali soil those nutrients are locked up so no, it does no good to just keep on adding those nutrients without getting pH right from the very beginning. Before you follow some of the recommendations abouve do something easy, ask a farmer, it's their livelihood to grow green things. They's NEVER use Aluminum Sulfate but they will say pH matter MOST. So, pH first, then get the nutients right. If you get the pH corrected then there's usually enough nutrients in the soil to get things started.

Answer 4

Why not calcium sulfate/ gypsum ? Very cheap and available , eg. dry wall. The sulfate does the acidifying , aluminum or calcium don't do much of anything. A problem I had with putting broken dry wall on the garden , over time paper comes off its' surface and needs to be picked up. Then ,if you can go straight sulfate - sulfuric acid , if you can get it. I put sulfuric on my blue berries and they grew very well. Sulfuric acid requires great care in handling ; on second thought don't use it unless you are a chemist or engineer.