Understanding Soil pH: Acid vs. Alkaline Soils or Why My Sister's Rhododendrons Died

Understanding Soil pH: Acid vs. Alkaline Soils or Why My Sister's Rhododendrons Died

Part of Ep. 2001 Curing a Brown Thumb

Soil scientist Doug Soldat takes the mystery out of soil pH and shows how to acidify soil, essential to growing rhododendrons and other acid loving plants in alkaline soils.

Premiere date: Mar 03, 2012

TRANSCRIPT+
Wisc Gardener Transcript: 

Shelley Ryan:

You're looking at the beautiful prairies at the UW-Madison Arboretum. I'm with UW-Extension Soil Scientist Doug Soldat. We are talking about soil pH. Sometimes this is one of the biggest mysteries whether you're a novice gardener or an experienced one. You know, Doug, soil pH can be the kiss of death if you don't plant the right plant in the right soil pH.

 

Doug Soldat:

Right, pH is a really something abstract. We can't see it. We can't touch it. We can't feel it. In the native plant garden at the UW Arboretum, these plants are not only adapted to Wisconsin summers and winters, they're also adapted to the chemical properties of this particular soil.

 

Shelley Ryan:

Right, and we have more alkaline soils in southern Wisconsin and more acid in northern parts, and the native plants are happy.

 

Doug Soldat:

They're adapted to it. They evolved in that situation. If you drew a line from Marinette down to Baraboo, and over to LaCrosse, below south of that line you'd have neutral to alkaline soils. Above, you would have acidic soils.

 

Shelley Ryan:

Let's talk about what that actually means. Again, soil pH is, like you said, it's invisible. Acid, I think vinegar.

 

Doug Soldat:

Yeah, vinegar, things that are sour, citric acid. Orange juice is acidic.

 

Shelley Ryan:

Okay, what about alkaline?

 

Doug Soldat:

Alkaline is things like baking soda. Baking soda has a pH of 8.2. It's the opposite of acidic.

 

Shelley Ryan:

Acid tends to be a lower number.

 

Doug Soldat:

Yes.

 

Shelley Ryan:

Five?

 

Doug Soldat:

Yeah, on the pH scale, 7 is neutral.

 

Shelley Ryan:

Okay.

 

Doug Soldat:

So, if you're below 7, we consider that acidic. If you're above 7, we consider that alkaline.

 

Shelley Ryan:

Why do the plants care?

 

Doug Soldat:

Well, pH affects nutrient availability.

 

Shelley Ryan:

Okay.

 

Doug Soldat:

Most plants evolved with a certain soil pH. They're used to that to maximize their nutrient uptake.

 

Shelley Ryan:

The larger group of plants actually prefer alkaline. So, it's not an issue for the larger group of plants.

 

Doug Soldat:

Yes.

 

Shelley Ryan:

Seven and above, they're happy.

 

Doug Soldat:

Most plants do fine. Even 6 and above, they tend to do fairly well.

 

Shelley Ryan:

Where we get into trouble, especially in an alkaline soil is if we want to plant rhododendrons, blueberries, I think Pieris japonica falls into that group. If we're down here in southern Wisconsin where it's usually alkaline, we have a problem.

 

Doug Soldat:

That's right.

 

Shelley Ryan:

We're back to number one, read the plant label, then...

 

Doug Soldat:

Have your soil tested.

 

Shelley Ryan:

Right.

 

Doug Soldat:

We're going to show you how to do that. You have your soil probe. You definitely don't need this specialized of a device. A garden trowel or a shovel will do fine.

 

Shelley Ryan:

Yeah, but this looks cool.

 

Doug Soldat:

It makes you look smart. Push it down to about the depth to about the root zone. I've gone a little too shallow there probably for most plants. We take that soil drop it into a soil sampling bag which is available at your local County Extension office. And we submit it to the laboratory on the bag.

 

Shelley Ryan:

You want to pick some samples from all over from where you're going to plant.

 

Doug Soldat:

Right.

 

Shelley Ryan:

It's going to come with this form, when you pick up the kit.

 

Doug Soldat:

Exactly.

 

Shelley Ryan:

If you don't see the plant you're going to grow there's a blank, fill it in. if your growing Pieris japonica, for instance write it down so they know what you're testing for.

 

Doug Soldat:

Right. So there is no good or bad pH. It's just the right pH for the plant that you try to grow.

 

Shelley Ryan:

You just need to know what it is.

 

Doug Soldat:

You have to tell the lab what plants you're trying to grow so they know what the right pH is for your plants.

 

Shelley Ryan:

In two weeks, it comes back with a pH number. Down here again, we're getting maybe a 7 and I want to grow blueberries, so I'm in trouble.

 

Doug Soldat:

So, we're going to have to bring that down somehow.

 

Shelley Ryan:

Frequently people will ask me, "Can I use pine needles?"

 

Doug Soldat:

Pine needles will produce some acidic affect but it's just not strong enough for what you need. We're going to recommend an application of sulfur. Sulfur is this yellow material that when you put it in the soil micro-organisms convert it into a different form which acidifies the soil.

 

Shelley Ryan:

Okay, dig in it, don't sprinkle it on top.

 

Doug Soldat:

Exactly. It's easiest to correct the soil pH when we do it at the time of planting.

 

Shelley Ryan:

Do it before the plant is in the ground.

 

Doug Soldat:

Yeah, right, exactly. So we put this down, about two cups per 100 square feet till it in, mix it in with a shovel throughout the root zone. Then about every three months we're going to test the soil again for pH.

 

Shelley Ryan:

Okay.

 

Doug Soldat:

We're going to add more sulfur based on the test results.

 

Shelley Ryan:

And this is--? Oh, cool, let me see that.

 

Doug Soldat:

This is just something really pretty accurate and inexpensive you can pick up at a garden center. Follow the directions to get a reading of your pH instantly.

 

Shelley Ryan:

We're testing every three months, because the soil is always going to want to revert back to it's natural state.

 

Doug Soldat:

Exactly.

 

Shelley Ryan:

Okay, this is for acid. If we want to make our soil more alkaline that's a lot easier?

 

Doug Soldat:

Yeah, it's more conventional. So, in northern Wisconsin, let's say we have a pH of 5, and we need to get that back up to 7. You submit your soil sample to the lab. The lab, if you select the plants will tell you how much lime you need to add to get your pH to that target value.

 

Shelley Ryan:

As you said, lime is easier. That's lime?

 

Doug Soldat:

This is lime, yeah. It comes in a variety of shapes and sizes. The kind I'm going to recommend is going to have a very fine consistency because it reacts quicker in the soil.

 

Shelley Ryan:

Again, we just dig it in, like you did with the sulfur.

 

Doug Soldat:

Dig it in, till it up and then have your soil retested the next year.

 

Shelley Ryan:

Every year is a good idea. Read the label, test your soil. Thanks, Doug. 

 

 

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