Drinking iron through a straw: the impact of soil acidity (pH) on nutrient availability

Everything I read in the popular press about plants and plant nutrition talks about the macro-nutrients (nitrogen, phosphorus, and potassium) and perhaps some about the micro-nutrients (other stuff).  They are complicating the matter.

If your plants appear to be ill, look at their sunlight, their soil moisture, and their soil pH, in that order. The first two are obvious: not everything likes bright sun, not everybody can tolerate some shade.  Too dry brings obvious problems, too moist depletes oxygen and lets root rotting fungi take hold (clarification: not all fungi rot roots- some fungi are vital to a plant’s health.)

soil ph graph showing which nutrients are available
As long as you are adding compost to your garden, it’s highly likely you have the basic compliment of nutrients your plants are looking for (except maybe phorphorus.  I’ll come back to that.) The real question is, can the plants get at this food?

Soil acidity (soil pH) is a measure of where on a scale of acid to alkaline the soil falls.  Put in terms of common household items: lemon juice is 2 to 3, borax is 9.5, and human blood is within a narrow margin of error at 7.40.   In the garden, there is some variability in plant “preferences”, but far less in what soils your area gifts you.  The acidity (or not) of a soil impacts which nutrients are easily available and which are difficult to get at.  I usually find this information in vaguely helpful graphs like the one above.

What I find more helpful is go through the garden pH “tiers” one by one.

Acid Soils

acidic soil and nutrient uptake capacityAn acid soil (4.0 to just over 6.0), like the east coast of the US enjoys, grows azaleas and rhododendrons with ease.  In particularly acidic areas, blueberries can grow wild.  Why?  I assume they like that ratio of nutrient availability.

By knowing the acidity needs of blueberries, I begin to understand that they must like manganese and zinc quite a lot.  In fact, they like everything and want it all available on demand. Perhaps blueberries aren’t so much picky as lazy.

Onions also like slightly acidic soils; onion grass is a reliable indicator that your soils have slipped acidic.

Potatoes also do well with a somewhat acidic soil, but for a different reason.  One of potatoes worst enemies (scab) can’t take it.  Ta-da!  One disease avoided, and you haven’t even planted yet.

How to lower the soil pH (make the soil more acidic):

Adding high nitrogen raw compost materials like coffee grounds, tea bags, and fresh grass clippings will lower the pH in a specific area.  Note: Do not put green grass clippings against the stems of your plants.

chlorotic rhododendron leaves

Rhododendron leaves with chlorosis. Image courtesy of Glen Doick nursery.

For larger areas, composted pine needles, oak leaves, seaweed, alfalfa hay, urine and manure (without bedding in it) are all very acidifying.  If you have some areas you want to spot treat (like your blueberry shrubs growing in Iowa), you may want a separate compost pile full of acidic stuff that you can top dress the blueberry bed with from time to time.  (Your regular compost will want some of this stuff too, it’s not an either/or situation.)

Plant signals that tell you the soil pH needs to come down: yellow leaves with green veins.  The leaves above are “chlorotic” [klor-ah-tik].

Neutral Soils

neutral garden soil pH
Most vegetable gardens do best on the acidic side of neutral, from about 6.0 to 7.0.  Above 6.5, many soil nutrients start to get “locked up”, making them less available to the plants.  Vegetables seem to prefer it that way, or maybe it’s our taste buds.

Earthworms also seem to like the neutral soils best, and they are marvelously helpful, so that’s another possibility for why closer to neutral works so well.

Two common things that raise a soil’s pH (make the soil more alkaline): too much rain, and concrete very nearby.

Rains will flush the soil nutrients deeper into the soil, effectively lowering the nutrients available to plants without deep roots.  Eventually, too much rain will flush the soil enough to make even oak trees start turning yellow.

I once was asked to help out in a backyard garden in Virginia, surrounded by white pines and oak trees.  The soil should have been pretty darned acidic.  The gardener, however, had planted her blueberries up against the back of the garage for sunlight reasons.

In addition to the fact that the hill there was made by the dirt that had been moved when the house was built (all B horizon, no A or O), the cement used in the foundation of the garage was doing what cement does.  It was very very slowly leaching into the surrounding soil, giving her an excellent site for lilacs, but really irking the heck out of her blueberries.

She is an avid coffee drinker.  I suggested adding coffee grounds around the blueberry plants every week or two.  I came back a month later and the shrubs looked quite happy and healthy.  I love a cheap fix.

Alkaline Soils

basic pH garden soils alkaline high pHCheck out how wacky the nutrient balance gets in these higher pH ranges.  While lilacs and buckeyes and a few other plants seem to like more alkaline (basic) than truly neutral soils, too basic a soil moves quickly out of healthy and into toxic.  The most common causes of a baseline basic region are long standing history of fire in the area, various salts in plentiful supply, and a high calcium bedrock under the soil (like limestone).

Several of the most popular herbs trace from Italy, Greece, and Turkey. These are all regions known for having some heavy swaths of limestone.  Rosemary, lavender, and oregano all do well with higher than neutral pH soils.  So do lawns, which is why some people lime their yard every third fall.

To raise the soil pH (make the garden more alkaline or ‘basic’):

Aside from the excessive rainfall mentioned above, the two best ways to raise a garden’s soil pH are to add calcium sources  or to add the ashes from a fireplace. The worst way, and the most common accident, is to pile up a deep bed of wood mulch.

Calcium is generally available as either fine crushed eggshells (crushing was a coveted chore when I was growing up- I’d then go sprinkle the grinds under the marigolds) or as limestone or gypsum. Gypsum will not impact your pH, so I won’t cover it here except to say that it will bind up some soil salts, so folks living along the seaboards might like it.

The “garden lime” that’s on the market is pretty purely calcium.  It is definitely possible to add too much, so read the directions.  Also, err toward larger pellet sizes.  These will take longer to break down, creating a mechanical time-release buffer that eases the garden along.

Dolomitic limestone has magnesium and calcium together.  While magnesium in excess can lead to soil compaction and soil toxicity, that’s moderately difficult for a gardener with some common sense to achieve. Also, Dolomitic limestone will stop raising the pH at 7.0, which is darned convenient.

While an eastern garden may appreciate some wood ash (which is always composted separately from the main pile) to bring the soil into the neutral zone, the Great Plains and California’s central basin both tend toward basic.  Wood ash in those areas is somewhere between redundant and toxic.

The final method of achieving soils with pHs in the 8.0’s (I have honestly seen this result in professional soil test results) is to put down a thick layer of big fat wood chip mulch.  Wood will decompose by adding nitrogen to its carbon.  The nearest nitrogen source is the underlying soil.  Once the wood is thoroughly decomposed, it will give all that nitrogen back, but it locks it up for the short term nice and tight.  While logs in the forest rot fairly slowly, those wood chips are rotting much faster, exacerbating the effect.

Spread wood chip mulch thinly (an inch or two), and then let it break down for several years before you add another layer of mulch.  Mix it up and lay down something different next time.

I haven’t forgotten phosphorus, but I wanted to tell the story of the guano craze of the late 1800s, so I’ll come back with that (soon!)  In the meantime, get your soil tested: test only dirt in the top 3-6 inches, test from several spots.  Cooperative Extension will do this very a very low cost.

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One response to “Drinking iron through a straw: the impact of soil acidity (pH) on nutrient availability

  1. Fascinating post — heavy rains making the soil alkaline makes total sense and helps explain our soil trending in that direction even in the areas that aren’t over limestone.

    By the way, I’m finally getting my book into a stage to send to the publisher, hopefully by the end of next week. I promise I won’t nag again, but wanted to check in and see if you’d had a chance to get that image for me. If you’re too busy, it’s no big deal! But if you do have a chance, I think it’d really help some readers out.

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