My SoCal Garden

pH Testing Your Soil

By Steve Thomas-Patel·
Chemistry table setup

There are lots of tests you might want to run on your soil, but perhaps the most important one of all is the pH test

Every plant has a "preferred" pH range that they like to grow in. This is because they are adapted to retrieve minerals from the soil at a given pH range.

Quick refresher for those of us who don't remember our high school chemistry, the pH scale runs from 0 to a max of around 14. 0 is a very strong acid and 14 is a very strong base. 7 is neutral.

A good example of a plant that needs a low pH range is Blueberries. They definitely prefer a low pH, about 4.5-5.5. That is a very low pH for plants. In California, we tend to a have a slightly alkaline soil, with a pH range around 7.3. That will kill your berries.

But, the soil around your house is rarely the soil that was there before your house was built. Expecially topsoil often comes from somewhere else. There is often fill brought in during construction to level out portions of the land. And "topsoil" added over that in order to grow a nice lush lawn to sell the property in the first place. Then many types of store bought soils could easily have been added over the top. And who knows what chemicals were added before you moved in?

For these reasons, you may want to test your soil. And especially your pH!

pH Test Types

There are 4 ways you might test your soil pH.

1. Household Supplies

This method will just give you a general idea of your pH. It doesn't provide any real accuracy beyond telling you whether your soil is acidic or alkaline. To do this, take a sample of your soil. You shouldn't take it from the top, but an inch or two under the surface. You need about 2 tablespoons for each of the test we are going to do.

The first test is to check whether your soil is alkaline. Mix the two tablespoons of soil with about a half cup of distilled white vinegar (the cheap stuff used mainly for cleaning). If it bubbles, you have alkaline soil.

If it doesn't bubble, mix equal parts baking soda and water, about a quarter cup. I buy baking soda from Costco and it's super cheap when you buy it in bulk (and also a great cleaning product). Mix this with your remaining soil sample. If it bubbles your soil is acidic.

If neither test causes your soil to bubble, you have neutral soil.

2. Litmus Test

Litmus testing is the traditional "scientific" way to test your soil. This method will make you feel like a chemist (or like you are back in Chemistry class). pH Litmus Test strips are cheap and readily available. You should buy ones that have a limited range. Since you are testing soil, you can assume what you find is not an extreme in either direction. And the limited range will help with accuracy.

Take a sample core about 6 inches into your soil. Mix it in a cup of water. Then dip in your test strip. The color will tell you your pH. The package should come with a color chart to help you figure this out.

Or you can do what I do, and buy a soil testing kit from Home Depot. For 20 bucks you get 10 pH test, plus 10 Nitrogen, 10 Phosphorus and 10 Potassium tests. If those elements sound familiar, it's because they are the same elements you find in fertilizer: NPK -- the top the "macronutrients" for plants. The pH test in the kit I got is a litmus test, but instead of dye on a strip, the dye is in a powder that mixes with the soil in water and colors it to indicate your pH.

3. Digital Soil Tester

Put simply, I'm very skeptical of digital soil testers. At least the ones in the affordable cost range.

Digital soil testers range in cost from about $20 to $500, so you can guess the accuracy of them based around that. I've got one, mainly because it's an extra feature of a moisture meter I have. It actually reads pretty close to neutral (about pH 7.1), whereas my soil tends toward about a 6.5 according to the litmus tests. It's hard to know if this is accurate or not without having a range of values to try out.

The nice thing about digital meters is their convenience. But I'm skeptical. Mainly because my soil is very clay heavy and the probes do not penetrate very well. Also, acidity is measure of free hydrogen in an aqueous solution, and these tests are claiming to know based on a mixture with relatively little liquid involved.

4. Send it to a Testing Lab

Undoubtedly this is the most accurate way to do it. Let the professionals handle it. Having your soil lab tested is illuminating. Or so I hear. It is way too expensive for me. The simplest tests cost about $40, but you are going to want to test lots of different parts of your garden. Around my house, I've got a minimum of 6 places I'd like to test regularly on a quarter of an acre lot, plus the two garden beds. That's $320 I'd have to lay out every year, or even twice a year. That's why I prefer the DIY kits at 10 for a $20.

Or you may choose both. Use the professional lab to check yourself. Maybe test ten locations of your own and have a couple of redundant tests sent out and see hw you did. Or plunk down the money this year to get an accurate baseline and work off of it every year hereafter.

How to Fix Alkaline Soil

Maybe fix is the wrong word. Who decides nature is wrong for giving you alkaline soil? Let's just state the problem, you have slightly alkaline soil and you want to grow blueberries. What do you do?

The first thing to do is to recognize the problem with alkaline soil. When your soil is alkaline, certain nutrients that are in the soil may not be reachable by your plants. Think about the minerals that are reactive with acid. I once left a pitcher of organic weed killer (strong vinegar) next to some metal brackets. By the time I noticed my error, they were covered in rust and a layer of white salt.

Plants that love acidic soil and are grown in alkaline soil quckly suffer from iron deficiency, a condition called "Iron Chlorosis". Their leaves turn yellow and they fail to thrive. So how do you treat iron chlorosis? You can either a) add iron, or b) add acid. More iron will mean even in alkaline soil, more iron is available to the plant. Adding acid will not increase the iron supply, but will increase the availability of the iron. Option a can only work up to a point.

Blueberries are super important in my family. In fact, in spite of having almost no unsponken for land left, I'm considering installing about 4 more bushes this year. But our soil is only slightly acidic, about pH 6.5. What's the solution?

You can add acid. Natural sources of acid for soil are pine needles and peet. Peat, unfortunately, is a not-very-renewable resource. It is formed incredibly slowly on Canadian marshland and harvested much faster than it can regenerate. A pine needle based mulch is a great idea.

Another approach is sulfur. Most "acidifying fertilizer" use sulfur. Sulfur isn't an immediate cure-all though. It takes a long time for soil to absorb any modifying chemicals, such as sulfur. And soil will revert back to whatever properties it had before you started modifying it. That is because you are probably only going to apply sulfur to a small area of soil and very slowly over time the chemical compounds in the neighboring soil will work its way back over. It's a very slowly occurring, but losing fight. If you have to add acid to your soil, you will always have to add acid to your soil.

While blueberries notoriously demand acidic soil, some plants just do better with slightly acidic soil, like tomatoes. So it's good to know what alkaliity your plants prefer, and arrange like near like.

How to Fix Acidic Soil

If your soil pH is well below 7, depending on what you want to grow, you may have to raise the alkilinity of your soil. And some plants just love alkaline soil, such as honeysuckle, beets and lima beans.

Raising the alkalinity of youru soil isn't much different than lowering it. Instead of adding an acid (sulfuric acid) add a base. The most common add here is lime or wood ash. Think of things that add a soapy feeling.

But, just like with adding acid, any changes you make will take time to be absorbed, and will fade away with time. If you are making changes to your soil pH, you will want to test your soil often.

What Determines Soil Alkilinity?

After testing your soil, you might be wondering why your soil is alkaline or why your soil is acidic. If you are hoping to use that information to solve a problem, you might be disappointed.

It's usually very large factors that determine the alklinity of your soil, things you have little or no control over. In Southern California, we tend to have fairly alkaline soil because of the aquaduct. Before our water gets to us, it has travelled many miles in concrete. Cement, the binding agent in concrete, is very alkaline. As the water passes down to us, it picks up some of this alkalinity. We then water our plants day in and day out, each time slightly alkalizing our soil.

But most of the processes that are happening in our soil are acidifying. When bacteria and fungus decompose wood, or leaves, they leave a weak

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