Science Says

Please Stop Liming your Soil Based on the pH!

Astra16 comments457 views
Spread the love


Guest Rant by Phil Nauta, author of Building Soils Naturally: Innovative Methods for Organic Gardeners

Soil pH is talked about a lot in the gardening world, but most people don’t understand it, so it’s generally misused.

I’m here to rant about it.  To simplify what pH is, it’s basically a measurement comparing how much hydrogen we have in our soil versus a handful of other nutrients — mainly calcium, magnesium, potassium, sodium and aluminum.  The more hydrogen we have, the lower our pH is – the more “acidic” it is. The more of the other nutrients we have, the higher our pH is – the more “alkaline” it is.

The scale goes from 0 to 14, with 7 being neutral, but most soils are between 4 and 9.  It’s usually best to have a pH somewhere in the middle. Actually, between 6 and 7 is generally considered ideal, which is often be true, but this is where a mistake is often made.  If your soil pH is 5.5, the common advice would be to add lime to raise the pH of our 5.5 soil, usually dolomite lime.

Dolomite is calcium carbonate and magnesium carbonate. The calcium and magnesium in the lime will probably knock some of the hydrogen out of the way. That will give us less hydrogen and more of these minerals, therefore raising the pH, at least in the short term.

So the problem is not that dolomite lime won’t raise the pH, but that our pH test did not tell us if we actually needed calcium and magnesium. Perhaps we already have too much magnesium, or too much calcium.  It’s almost certain that we don’t need both in the ratio that dolomite lime gives us. Adding more of the wrong nutrient is just going to make things worse. For example, too much magnesium causes some major compaction, among other things.

The reason I’m ranting today is because I don’t like to see my friends slowly destroying their soil with annual applications of lime, as recommended in in so many of their gardening books.

Looking at the other end of the scale, some high pH soils are due mostly to sodium and potassium, and they actually still need calcium and perhaps magnesium. We wouldn’t know that if we just used the pH number as our basis for liming.

The pH does give us a clue that we may have a nutritional and microbial imbalance in our soil, but this gives us no information as to why that may be so. As such, it’s of very little use to us.

It is not that pH isn’t important to plants and microbes. For the most part, we’re happy to have it be between 6 and 7 to have the healthiest plants.

Knowing the pH value, however, doesn’t help us much with soil management decisions, and it certainly shouldn’t be used to determine how much lime to add to the soil.  pH is the result of the elements in our soil, not the cause.

Now, the reason we’re happy with a relatively neutral pH is that most nutrients, particularly the most essential nutrients, are most readily available to plants somewhere in the 6-7 pH range, gradually decreasing as the pH gets further up or down the scale.  And a potential problem is that some micronutrients become more available outside this range, especially in low pH soil, sometimes to toxic quantities.

So it’s not that the acidity of a 4.5 pH soil is harmful in of itself; it’s that most nutrients aren’t as available to plants, and a few may be too available. Further, many microbes can’t live at an extreme pH, so the soil food web will be lacking. But plants that are considered “acid-loving” don’t actually love hydrogen. Instead, there are various benefits they may get out of a lower pH soil. They may just need certain trace minerals in abundance, and those trace minerals are more available in acidic soil. Or they may just need a fungal-dominated soil – fungi decrease soil pH, so it may be that these plants don’t care at all about the pH, and they just want their fungi.

Rhododendrons, for example, are often thought of as acid-loving. In reality, they love magnesium, which is sometimes more available at a low pH, and they aren’t particularly fond of calcium. They’ll grow just fine in a high pH soil if they have sufficient magnesium and lots of organic matter. I’ve seen huge blueberry harvests on high pH soils.

Trying to make your soil acidic by applying peat moss or chemicals doesn’t give the plants the nutrients they need or the biology they need. And trying to make it more alkaline by applying lime will often give the wrong nutrients, causing serious problems.

In my view, what we need to do is focus on a more holistic approach to soil management, such as creating high quality compost and using things like rock dust and seaweed in order to give the plants the chelated minerals they need.

And then the other important step is soil testing. A soil test will not only tell you your pH, but also which minerals need to be added back. It will rarely be dolomite lime.

When all of these factors are brought in line, the pH will follow.

WIN A COPY OF PHIL’S NEW BOOK

Just leave a comment, preferably telling us what you’ve done to improve your soil. The winner will be chosen by Random.org.  Entries close Friday, August 3 at midnight EST.

Posted by

Phil Nauta

on July 28, 2012 at 8:34 am, in the category Guest Rants, Science Says.

16 Comments

  1. Hallelujah!! Yes, let’s please talk about soil health, and not simply lean on industry touted cures for symptoms. A good gardeners manifesto should always include the words “Soil Test”! Thanks for this rant!

  2. pH is NOT a measurement of nutrients! It is a measurement of H+ ions and OH- ions! An acid pH = great H+ ions and an alkaline pH = greater OH- ions! Lime raises pH NOT by the calcium alone, it is the combination of CaCO3 (lime) and water (H2O) which produces Ca, CO2, and OH-. So voila! You produced OH- ions, so your pH shall raise!!! I’ve analyzed countless soil reports, and rarely will you find a soil that has “too much Ca and Mg” and an acidic pH. If you’re pH is low, I seriously doubt that Ca is anywhere near “too high”.

  3. Soil health really is about more than pH. Unfortunately, soil tests aren’t always easy to come by – in some parts of Canada, figuring out where you can send your soil for testing can be a nightmare!

  4. Ya, I’d actually send down to the U.S. I’ve never seen anyone in Canada doing the kind of test I would recommend. If you really want to, A&L at least does a base saturation test, but they’re not focused on organic recommendations. I prefer Crop Services International in Michigan.

  5. I use leaf mold and composted manure in a community garden space that was used by others for the last 30 years. My soil test said that I have a ph of 6.6 and excessive amounts of Phosphorus and Calcium with sufficient amounts of everything else. I need your book to figure out what I should be doing.

  6. I recommend Mr. Nauta’s book unreservedly – solid science, but highly readable. FYI, I will be reviewing it on my site within the week. A fine addition to any gardener’s bookshelf.

  7. What a great post, very clear and helpful. I would love to read more on this topic! Knowing what’s going on in your soil is one thing, figuring out how best to make it better is another, and more info in this area is useful.

  8. I am a new home gardener, in farm country in Indiana. I don’t trust the soil around my place, and have started with a raised bed, and a vermiculture box. I have also located a raiser of grass-fed beef, who follows organic practices. He will be selling his herd’s manure soon, and I plan to get some.
    I need to learn more about how to put it all together to improve the soil in my beds.

Leave a Response