pH basically stands for the “Potential of Hydrogen”. Acidic solutions or soil (those with higher concentrations of H+ ions) are measured to have lower pH values than basic or alkaline solutions which have lower concentrations of H ions.
When you increase the H ions in soil the pH decreases. Acids contain more active H ion than Alkalines do.
To bring this back to the lawn most grasses will uptake nutrients most efficiently when soil pH is around 6.3 to 6.6 or so. This is why we care about what our soil pH is and why some people take great measures to adjust their soil pH from year to year.
In my lawn I’m starting out with a pH in the neighborhood of 6.8 to 6.9. I’d like my pH to get down to 6.5 but I also know that lowering pH more than 0.1 to 0.2 permanently takes a lot of time.
Here are the main ways of lower pH; some of these methods are longer lasting than others and some take longer to see the effects of your applications.
In the late winter and early spring I like to core aerate my lawn. By doing this I open up physical channels in the soil reaching 2.5 to 3 inches into the soil. When these cores are open I will be applying a top dressing of peat most onto the soil surface and will brush it all into the cores.
Peat moss is a naturally acidic growing medium. I have seen references to it having a natural pH level of 4.5 but in my own home testing I’ve never gotten a pH reading on peat moss below 6. I’m not using a full lab however so my number is probably high.
You should also know that peat moss is highly carbon based product – in a compost pile it is used as a brown or a carbon source as it has a 30-1 carbon to nitrogen ratio. You should also keep in mind that finished compost is typically pH neutral or very close to it meaning whatever you put into a compost pile will come out as finished compost near pH 7.0 – even if you add peat moss to the pile.
If you put peat moss on the lawn or in the cores you will be adding an acidic organic product that adds lots of trace minerals and carbon components to the lawn. The peat moss will improve your soils water holding capacity and will provide some channels of slightly more acidic soil to your lawn.
However as soil microbial life breaks down the peat moss on or in your lawn you can surmise that the biological breakdown of the peat moss in the lawn will neutralize the pH of the peat moss just like it does in a compost pile and over time the peat moss will become neutral and stop acidifying the soil.
The caveat here however is that the breakdown of peat moss will not be perfectly neutral because as microbes break down peat moss and other organic materials some organic acids are formed along the way. The resulting pH will probably be higher than fresh peat moss but likely not acidic enough to have a meaningful effect on the pH of your top soil over the long term.
Of course you will get the benefits of adding organic carbon based material to your lawn for many years to come so I think this is worthwhile even if the pH change is slow and not exactly permanent.
Of all the standard homeowner techniques, this is probably the quickest way to lower pH in the lawn and the least dependent on temperature and microbial activity, however it’s also probably also the most dangerous because over applying can result in aluminum toxicity on your plants, grass or otherwise.
Applying aluminum sulfate can also deposit an excess of undesirable salts into your lawn which can be detrimental in the long run if you are regularly applying them.
As you apply these the sulfate is what actually lowers the pH and the aluminum simply builds up in the soil resulting in toxicity issues down the road. As the sulfate reacts to water sulfuric acid and oxygen forms resulting an acidifying effect on the soil.
Applying these products may work fast because it’s a chemical reaction rather than a biological reaction but you also have to apply a greater physical quantity of them which can be harder for large yards as well as more expensive… and how do you get rid of the extra aluminum later on? IMO this isn’t worth it.
Even if you do go this route the effects of long term pH changes to your top soil are still not going to significantly last over the long term. Some lingering effects may last for years but only minimally which is why reapplication throughout the years is common.
Because my pH is already slightly acidic 6.8 I will not be applying either of these two options to my lawn. I’d rather go a safer route.
This product works similar to Aluminum sulfate. The chemical reaction to create sulfuric acid is the same however the Iron builds up in the lawn and gets too high if applied too often or in application rates that are too high.
Elemental Sulfur or Soil Sulfur
Sulfur is usually the most common amendment added to soils to create a lasting effect on soil pH to the down side. Sulfur can be applied to lawns in smaller portions and at a lower cost than other alternatives and it can move the pH needle further downward for longer periods of time than by adding organic matter or applying acidifying fertilizers to the soil regularly.
Sulfur however lowers pH partially through a biological process instead of a chemical-only process so it will work slower to achieve its results and will be very temperature dependant. It will hinge on the biological and microbial life of your soil to work so it is very seasonal.
If you put this down before soil temps get up into the 50s then it’s not going to do a thing… and when soil temps drop in the late fall below 50 then biological process grinds a halt.
When the sulfur combines with water and oxygen sulfuric acid forms and H ions are released… but only when the microbes are active.
This process is more effective in well aerated soil, an aerobic environment, as well. Compacted, anaerobic, soil will have less oxygen exchange and will slow the effect of Sulfur down.
For many yard owners who fertilize regularly season after season and year after year their fertilizers alone have a slow and gradual effect on pH in the top soil. If you have a very small pH correction you want to make and expect to be fertilizing your grass regularly over the coming years then choosing your fertilizers based on their ability to acidify the soil can be a great option.
Synthetic fertilizers like ammonium nitrate and ammonium sulfate can stimulate top growth on your lawn through readily available nitrogen that doesn’t have to be digested by soil microbes. As the plant uses the ammonium for nitrogen the surrounding soil is acidified by the release of.
Of all the ammonium fertilizers ammonium sulfate works to acdify the soil most effectively but you have to be careful to not apply too much to a lawn at any one time because the grass can burn easily when too much quick release nitrogen is applied at any one time.
Because the ammonium sulfate doesn’t rely on microbes to work this is an option for lawns that are not dormant but still cold. When the grass is growing with soil temps below 50 then this can be effective with small application rates.
As soil temps warm up then Urea nitrogen becomes an option for slowly acidifying the soil while still feeding the lawn.
Urea nitrogen isn’t plant available until microbes in the lawn break it down into ammonia. Only then does it enter the root system of the plant and release acids into the soil.
Urea for acidification is best used in mid spring when soil temps are high enough to support microbial activity or in the early fall while the plants are still capable of pushing top growth without excessive stressors like heat or drought.
In My Lawn
Because my lawn is slightly acidic 6.8, and because I plan on fertilizing regularly throughout the year I’ll be avoiding the aluminum sulfate altogether.
In the very early spring, when soil temps are around 45 and the grass is greening up I’ll be applying a very light application of ammonium sulfate fertilizer to the lawn… not for the nitrogen, but for the very quick acidifying effect. Getting an immediate drop in pH, even a slight drop, will help my grass uptake all nutrients more efficiently throughout the year. I’ll just have to be mindful of keeping the grass cut regularly as growth accelerates due to the extra quick release nitrogen.
3-4 weeks later, as soil temps get up into the mid 50’s, I’ll apply a very small application of urea nitrogen, again, only to get the slight acidifying effect on the lawn as early in the season as possible. This application will give m a smaller effect than the ammonium sulfate and a slightly slower release of nitrogen and top growth to the grass plants.
By late Spring I’ll then apply a very small app rate of elemental sulfur to the lawn expecting this to have an effect on my soil pH by late Fall or early Spring in the following year.
I don’t want to overcorrect and I’m happy with this plan. I would expect these actions to only drop my pH slightly by next year, possibly moving the needle from 6.8 to around 6.6 at best but only time will tell.