What’s the (soil) scoop on ‘creeping acidity’?


Now that planting season’s afoot, what’s the single (most controllable) factor that decides whether you’re destined for success or heartache? Hint: Maybe heartburn is more appropriate — jalapeño spicy.

The answer, of course, is pH. It’s the unsung hero of ag productivity. It governs everything we know about soil chemistry and how crop roots interact with nutrients and microbes (both friend and foe).

In other words, pH determines what nutrients are available for uptake and plant growth. Are they perpetually hungry, with metals being the only snack available at low pH (that they don’t need in large quantities anyway, if at all)?

Additionally, is the root environment filled with friendly, helpful citizens or predatory (fungal) thugs eager to infest your crops? The pH level arbitrates the good/bad neighbor dynamic. (There’s also a host of other issues, like nutrient toxicity, poor nitrogen fixation (for legumes), and changes in herbicide effectiveness.)

Luckily, an out of whack pH is a deceptively cheap and easy fix. It’s the ultimate soil-quality of life hack. And both the pathology and cure share many similarities with those saintly souls who tend the land — the farmer.

For the soil, there’s no prescription strength Zantac or the incessantly advertised “purple pill” (Prilosec), but there is something over the counter — lime. It’s like buying Tums to soothe your acid churning stomach. Pop a couple, and it brings everything back into balance. In fact, the active ingredient of Tums is lime (aka calcium carbonate). So, from a medicinal point of view, field and gut ecosystems strangely have something in common.

Image by fizkes, Shutterstock

The difference is that humans tend to suffer from acid spikes triggered by diet. A cup of coffee, an innocent nip of chocolate, a sip of OJ, or a generous serving of mom’s patented tomato sauce. You know when you erred, and you’ll pay for it upfront.

With soils, it’s also caused by “diet” (read: fertilizer and other inputs), but the symptoms are much more drawn out: its [menacing drum roll] creeping acidity.

Without a course correction mid-flight, you’re already hurtling toward the earth in a pH nosedive, albeit at a snail’s pace. Problem is, you don’t realize the trajectory of your impending demise until it’s too late. And it prompts a series of cascading in-field failures. If only you had kept up with liming, all this discomfort could have been prevented.

But what is this notion of pH, and why is it so mission critical?

The idea of pH and acidity isn’t unique to ag, it’s also embedded in environmental messaging like ocean acidification. Additionally, it’s made the human health rounds on social media, like blood pH — and dubious efforts to “alkalinize” blood through diet and supplements [never mind that blood is keyed to stay in a very narrow peak operating range].

Appropriately, plants also have comfort ranges — and this is essential to understand the operational concept of pH.

pH stands for the power of the hydrogen (some say potential of the hydrogen). Basically, it’s a measure of how many H+’s are floating around in a solution. This includes water tightly wrapped around soil particles. But there’s also another party in the water vying for attention: OH-.

If H+ is the yin, then OH- is its yang. No need to be an .edu masochist and dive into the dry math, but a scale of 0-14 is used to measure pH. Think of it a simple number line with tickmarks at each whole number. Each tick is a tenfold difference between its neighbors.

If there’s:

  • More H+’s, it’s acidic (<7)
  • The same quantity of H+’s and OH-‘s floating around, it’s neutral (=7)
  • More OH’s, it’s basic (or alkaline) (>7)

Some crops prefer it very acidic, like blueberries (as low as 4.5). Others don’t mind it on the higher side of the scale, like alfalfa (7.5). This preference is stark: at the most extreme ends of the spectrum, blueberries prefer it up to 1000x more acidic than alfalfa (3 whole hops on the number scale, at 10x a hop = 10x10x10=1000). Or put another way, alfalfa likes it 1000x more alkaline (basic).

But these are extremes. Peak operating efficiency for most crops is between 6.2 and 6.8.

The profound effect of pH is easily demonstrated in a hydroponic setting. Depending on the source, tap water can have a pH of around 7.7. After applying a fertilizer solution, I’d lower it to about 6.8. Still not quite at the target. Then I’d add phosphoric acid to further inch it down to the desired level. And it’s very easy to over (or under)shoot if you’re not careful. And then you’re mired in a “pouring contest” trap, alternating laughably tiny quantities of acids and bases (like some sort of alchemist) in a seemingly never-ending quest to find your happy medium.

Luckily, soil isn’t quite so prone to huge fluctuations, because it’s buffered. It resists major changes. But beyond that, life is generally acidifying. With that biologically propelled momentum in play, what can we do to temper soil’s natural proclivity toward heartburn?

Enter this idea of “creeping acidity.” The phenomenon is especially bad in the Northern Plains of the U.S. The main culprit is fertilizer, which is naturally acidifying. In general, for every 100 pounds of fertilizer applied, 500 pounds of ag lime must be applied to counteract the effect and maintain equilibrium.

Lime potentiates yield and overall plant health. Consider it preventive medicine: an apple that keeps the (plant) doctor away and prevents heartburn and ulceration (complications) down the road.

Liming isn’t even Ag 101; it’s a remedial class. Don’t skimp on the fundamentals. Religiously take a representative soil sample at the start of every season.


Tim Durham’s family operates Deer Run Farm — a truck (vegetable) farm on Long Island, New York. As an agvocate, he counters heated rhetoric with sensible facts. Tim has a degree in plant medicine and is an Associate Professor at Ferrum College in Virginia.

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