4 ways to lower and maintain soil pH

dirtydan(8-9 Lancaster, CA)May 5, 2010

Why is our PH is so high despite the level of sulfur?

I am not a chemist however I have now been dealing with issues (native soil pH 8.4) for 2 seasons in the Mojave Desert. I have spent a lot of time trying to figure out the chemistry involved and it is quite complex since micro-organisms, irrigation water, and soil composition all factor into the final equation.

As you may be aware, elemental sulfur only modifies the pH of soil after it is oxidized by soil bacteria and converted to sulfuric acid. Secondly, the sulfur eating bacteria will only flourish if the right environmental conditions are met; Moisture, heat, and oxygen. If no oxygen is present, elemental sulfur may be digested by anaerobic bacteria producing toxic hydrogen sulfide gas. Its my understanding that elemental sulfur is unavailable to the plants as a nutrient and must be in a different form in order to be utilized by the plant. Conversely there may be forms of sulfur that cannot be utilized by soil bacteria.

If so, what can we do besides adding more sulfur to even out the PH a bit?

I don't believe that there is any likely hood that you will damage plants from excess sulfur fertilization. Its virtually unheard of.

I have used four different products to lower my soil pH.

Elemental Sulfur

The most commonly available elemental sulfur preparations with 10% bentonite clay, break down very slowly in the soil, may take years to be effective, and if the right conditions are not present, could be useless or detrimental. If properly utilized, elemental sulfur can be a powerful tool in maintaining a lowered pH over a long period of time but provides no immediate benefit.

Ammonium Sulfate

Fertilizing with ammonium sulfate will lower pH over time since sulfuric acid is formed during the break down of this chemical. This will not meet your immediate concerns.

Aluminum Sulfate

Another product that is commonly used is aluminum sulfate. Flower growers use it to lower soil pH around hydrangeas thereby turning them blue. You can think of this product as acid in powder form. When it contacts water it forms sulfuric acid (which you want), and aluminum (which you don't want). Excessive use of this product may cause aluminum toxicity.

Sulfuric Acid

The fourth, most effective, immediate way to lower soil pH is to add liquid sulfuric acid . The good part is that you will instantly change the pH of your soil.. The bad part is that this stuff is highly corrosive so you must take safety precautions very seriously. Contact will cause severe burns to your body. Read precautions on the bottle carefully.

I apply it prior to planting. In high concentrations it will sterilize the growing medium and kill vegetation that it comes in contact with. First, I dilute the product before application then later I thoroughly drench my planting beds with tap water to disperse the acid throughout my beds. After application you may consider tilling the soil and adding compost to recolonize the soil food web. I have planted within 24 hours of application with good results.

Concentrated sulfuric acid is found in the janitorial section of many national home improvement stores such as Lowe's and marketed as sulfuric acid "Drain Opener". It will be in a plastic bottle with a sealed clear plastic bag fully enclosing the bottle.

So there you have it, four very different products for acidifying your soil, each with its own unique pros and cons. And remember, its best to modify the soil pH in increments rechecking your results repeatedly, rather than over-correcting.

I welcome any corrections or discussion since I am always endeavoring to increase my understanding of soil chemistry and the soil food web.

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I was under the impression that sulfur inhibits the growth of fungi. Since at least 90% of plants form mycorrhizal relationships, it seems that the use of sulfur would be a very bad thing.

I'm a little curious to know how soil can naturally have low pH. I know that rainwater can play a part. I also know that organic acids, the end-product of the decay of organic matter, can play a large part. Other than those two sources of acidity, how else does soil achieve the neutral or slightly acid state? What does Calcium and Magnesium bind to in order to reach a neutral pH.

I suppose the composition of the soil itself has a lot to do with its natural-state pH. Clay soils, because they are the end result of chemical weathering (as opposed to sand and slit, which come from mechanical weathering), is naturally disposed to being alkaline.

    Bookmark   May 5, 2010 at 7:44PM
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I am using elemental sulfur, 90% sulfur/10% clay, to lower soil pH, in our blueberry beds. It works well, taking about 6 months for an application to become effective. However, our soil conditions are different from the Mojave, I suspect. Our soil is fairly high in clay, with a pH around 7.6, but it also contains considerable organic content, decomposed leaves and grasses. I am adding compost to our soil when planting blueberry shrubs, and I am also mulching shrubs with shredded tree leaves, to help retain soil moisture. So there is a big dose of vegetable fiber being added to the soil, in addition to sulfur. I fertilize in the spring, each shrub gets a tablespoon of Miracle-Gro-acidic formula, dissolved in 4 gallons of water, in April and again in May. I know the pH is falling, from testing. Last year our blueberry shrubs had soil pH around 6.5, and this year, the pH range is 3.8 to 4.5, as tested. So it is clear that we have the correct bacteria for metabolizing elemental sulfur. I don't know to what extent mycorrhizal fungi are present. The shrubs are healthy and productive, so I suspect that we have fungi as well, but I haven't figured out how to test for that yet. With regard to native soil pH, we have lots of limestone here, as well as clay soil. To my knowledge, this is left over from the time when most of North America was covered by seawater. In northern Wisconsin, there are some soils with lower pH, caused by the accumulation of evergreen leaves on the forest floor, over thousands of years. I would think that volcanic ash deposits help lower soil pH, but the presence of decomposed vegetation seems to be a factor as well. The Pacific Northwest has both volcanic ash, and decomposed evergreen tree leaves.

    Bookmark   May 5, 2010 at 10:55PM
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DrHorticulture_(Z3 Central Saskatchewan)

Good points by both the OP and posters. In response to idaho gardener - besides the factors you stated, high rainfall can also cause low pH by washing out basic cations like Ca,K and Mg as their hydroxides.

Just wanted to add a couple things:
Ammonium sulfate has a one-two punch. Apart from hydrolyzing to sulfuric acid, the ammonia is oxidized to nitric acid by bacteria. Other ammonium salts, and ammonia itself, work in similar ways.
I think cost is the most important reason why liquid acids are rarely used, except in hydroponics.

    Bookmark   May 6, 2010 at 1:36AM
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Soil pH is a measure of how many, or how few, free Hydrogen Ions are floating around in the soil. A soil will be acidic if there are a lot of those free H Ions while an alkaline soil will have very free H Ions.
Perhaps this from Clemson Ubiversity can help.

Here is a link that might be useful: About Alkaline soils

    Bookmark   May 6, 2010 at 6:52AM
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I'm a little curious to know how soil can naturally have low pH. I know that rainwater can play a part. I also know that organic acids, the end-product of the decay of organic matter, can play a large part. Other than those two sources of acidity, how else does soil achieve the neutral or slightly acid state?

The parent material of the soil is the primary source of soil pH. Soils are made up of ground up rocks, created over the eons by erosion, wind and rain. And the chemistry of the rocks natural to various areas will give rise to a similar soil pH - basic or calcareous rocks (eg. limestone, basalt) will result in alkaline soil; acidic rocks (eg. granite, rhyolite) will generate an acidic soil.

Rainfall plays another major role. Rain and snowfall in unpolluted areas tends to have a moderately acidic pH of around 5.5. Pollution can lower that significantly ("acid rain"). In high rainfall areas (more than 30" per year), the passing of rainfall through the parent material leaches out basic cations like calcium, magnesium and phosphorus which are replaced by acidic cations like aluminum and hydrogen. For this reason, high rainfall areas tend to have acidic soils - low rainfall or arid areas tend to have more alkaline soils.

Clay can be either alkaline, neutral or acidic. Chemical weathering - as described above - can create acidic conditions just as easily as not.

Decomposing organic matter has the least amount of impact on a soil's pH - most likely because it has such a transitory existence. Conifers or needled evergreens do not make a soil acidic - the trees grow there in the first place because they prefer the acidic conditions that naturally exist. The decomposition of the needles - or any other OM - over time will release a small amount of carbonic acid but that only adds to the existing conditions. It doesn't create them. By far the two biggest players will be the underlying parent material and the amount of rainfall that area receives. That is why it is difficult to make any significant and permanent changes to a soil's natural or stasis pH - you cannot easily alter the parent material nor can you change the amount of rainfall.

    Bookmark   May 6, 2010 at 11:07AM
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dchall_san_antonio(8 San Antonio)

dirtydan, have you had a soil test done to determine why your pH is so high? Where are you in the Mojave? What are you trying to grow that is having trouble?

As gardengal says, it is the parent material that sets up the pH. My soil is limestone rubble. The pH runs about 8. I supplement that with silicon sand as a topsoil. My lawn seems to generate small amounts of acidity but that is easily washed away in heavy rains. When that happens the iron binds with the calcium and my lawn turns bright yellow-green.

Pollution can contribute nitric acid as well as sulfuric acid depending on where you are. Nitric acid comes from high traffic areas where there are a lot of cars burning nitrogen along with oxygen in the air. Sulfuric acid comes from places where there are a lot of coal fired heat.

Also I don't see those as four different materials. They are all sulfur. Some may do other things but the active ingredient is sulfur. And I have not heard anything good about sulfur or sulfates and soil fungi.

    Bookmark   May 6, 2010 at 11:49AM
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dirtydan(8-9 Lancaster, CA)

Thanks to everyone for the interesting responses.

dirtydan, have you had a soil test done to determine why your pH is so high?

Unfortunately there is no one in this area that will do a soil test for less than $70 per sample. I recently found a link to University of Mass Amherst that offers soil tests for $13 so I am planning to send them some samples. http://www.umass.edu/plsoils/soiltest/soilbrochure2009.pdf

I use home test kits for nutrient and pH. I also have a Hanna pH meter that comes in handy for quick tests.

I don't know my native soil composition. Its pretty bad though. It could be described as dense gray clay with some occasional small pockets of white material. When dry, its is normally cracked and coated with a layer of frosty looking crystals that taste like salt. The salty crystals grow up the side of masonry, concrete and stucco walls. When wet, it turns to a slippery slime that will coat the bottom of your boots with a couple pounds of thick clay. It has no vegetative matter whatsoever.

I have been prepping my raised beds with compost, sand, clay, fertilizer and various sulfur products to lower pH.

Where are you in the Mojave?

I am located in the city of Lancaster, CA. This is the high desert north of Los Angeles at about 2,400 ft elevation. We only get about 5 inches of rain per year, most of it is in February. In fact we will probably not see any significant rain here until next fall or winter. The arid conditions are one of the reasons for the high soil pH and lack of organic mater, also the fact that we are almost entirely dependent on alkaline municipal water for irrigation.

What are you trying to grow that is having trouble?

When I first moved here I put tomatoes, squash, cucumber, melon, and several others in the unamended native soil. They were all killed or stunted from the salt or nutrient deficiencies. Iron chlorosis and micronutrient deficiencies were evident in the plants that didn't die immediately. The only exception were some tomatoes that I managed to save by side dressing with aluminum sulfate and a highly acidified nutrient solution, they remained quite stunted however and suffered from BER.

Since I have built raised beds and worked the soil, I have a productive garden and only seem to have problems with cucurbits in beds that have been freshly sulfured, and typical aphid and leaf miner infestations.

There are two main reasons as to why sulfur is the preferred material for acidifying soil.

1. Economics
Sulfur is one of the most plentiful and inexpensive elements on earth, said to comprise 2% of the mass of the earth. Sulfur is a byproduct of oil and gas refining and there is a massive oversupply of the material, especially in Canada where its is extracted from oil sands during refining.

  1. High concentrations are not toxic to vegetables.
    Cucurbits being the exception. Avoid sulfur contact with the leaf surface of cucurbits.

Other possible acidifying agents (some impractical):
ammonium nitrate
liquid nitrogen
diammonium phosphate
acetic acid
hydrochloric acid HCl (also called muriatic acid)
phosphoric acid

Elemental sulfur has been used for centuries as a fungicide and miticide. I don't know if, or to what extent, it affects beneficial soil fungi. I suppose that a reduction in soil fungi is OK as long as our vegetables are not negatively impacted, and there is no damage to the environment. Once the sulfur it oxidized by soil bacteria I would speculate that soil fungi will return to pre-application population size.

    Bookmark   May 6, 2010 at 8:08PM
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Part of your soils pH problem is due to the lack of rainfall. Rather then trying to change your soils pH you probably will be better off building raised beds. and making the soil in those raised beds what you need.

    Bookmark   May 7, 2010 at 7:33AM
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Hey, dirtydan
You can get soil tests cheaper than that in your area. It just depends on how much information you want. You should try

Top Choices:
Perry Labs in Watsonville.
Soil and Plant Laboratory in Anaheim.
Good choice:
A&L Wetern in Modesto.
Last resort
Wallace Laboratories in El Segundo.

By looking at what you get on the UMass website I'd bet dollars to donuts that they're running a Meilich 3 extraction, which yields very unreliable results when run on alkaline soils. The above labs all use other extraction techniques that work a lot better for your alkaline soil.

I've done a bunch of work out in Lancaster and what you said here:
"It could be described as dense gray clay with some occasional small pockets of white material. When dry, its is normally cracked and coated with a layer of frosty looking crystals that taste like salt. The salty crystals grow up the side of masonry, concrete and stucco walls. When wet, it turns to a slippery slime that will coat the bottom of your boots with a couple pounds of thick clay."
sounds about right for Lancaster. ;)
I would bet you'll find high salinity and a sodium hazard (either ESP or SAR, depending on which lab you choose).
Sodic soil conditions can also bump your pH way up. That's on top of your already alkaline conditions. But, from what I know of the area and your description, I'm gonna say the pH will likely be the last thing you'll need to worry about.

    Bookmark   May 7, 2010 at 8:25AM
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....and by "cheaper than that"
I mean cheaper than $70.00.
$13.00 is dirt cheap and also points towards the quick and easy Meilich 3 one extraction test.

    Bookmark   May 7, 2010 at 8:30AM
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dirtydan(8-9 Lancaster, CA)


Kimmsr, I couldn't agree more, in fact I am in the process of building my eleventh raised bed this week and I am experimenting with different soil compositions in each bed.


Gargwarb, thanks for the links to CA soil testing companies.

I agree 100%; The native soil here is a salt hazard. Fortunately I am using raised beds that can be leached by flooding for a few hours. Its an ongoing battle but I think I have the salt problem under control for now. Here are a few pictures from last year before I built raised beds, and pics from today. Care for a little salt with those vegetables?

I actually saved this little squash by digging it up and placing in a raised bed. It turned out to be a huge producer.

Here are pics taken today, a year later

    Bookmark   May 8, 2010 at 4:23PM
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BahamaDan ZTropic

Looks great! Have any more recent pictures Dan?

    Bookmark   February 9, 2015 at 2:14PM
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