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Doctoring Your Soil: Testing pH With a Kit, or Red Cabbage


Supplies (additional supplies listed below for the Red Cabbage activity):


pH test kit



distilled water


In the garden, it's all about chemistry. Just as our bodies use chemistry to break our food down to the point where we can use it for nourishment, plants need the soil to be "Chemically Correct" for them to grow optimally.


Before you sow any seeds or plant any plants, you should make sure you know the nutritional status of your soil. That way, you can "prescribe" the soil amendments - or changes - that you need to add, to create the best-possible growing environment for your seeds and plants.


Although they will be very busy in the spring, you can contact a county extension agent to have your soil tested. The cost may exceed $25. That may be the best way to ascertain your soil pH (the measurement of the soil's chemistry, acid vs. alkaline), and how much salt and lime are in your soil. These professional tests also can tell you how sandy or clay-textured it is, how much organic matter is there, and how high or low it is on nitrogen, phosphorus, potassium, zinc, iron, manganese and copper.


That might be more than you really want or need to know, though. The most important factor is your soil pH. If the soil is too acidic or too alkaline, the nutrients in the soil won't dissolve in water, and then they won't be available to the plant roots. So you need to get your soil into proper pH balance.


The pH scale runs from 1 to 14. A measurement of 7 means the soil is neutral. Below 7 means the soil is acid, and over 7 means it is alkaline.


An ideal pH for most garden plants is about 6.5 - slightly acidic. If you have clay soils or a lot of organic matter, it may take more lime to raise your soil's pH than average.


A few plants do better with slightly different levels. Carrots, eggplant, sweet corn and potatoes like acidic soil, about 5.5, while cabbage and cauliflower will grow fine at that level and also as alkaline as 7.5.


To find out where your soil stands, you can purchase an inexpensive pH test at a nursery center and do the soil test with the kids in the garden club. There are electronic kits available, but the chemical kits are likely to be more affordable.


Try not to do this shortly after a rain, nor within two weeks of applying fertilizer to your garden spot.


Dig up a couple of cups of soil from about 6 to 8 inches under the surface and lay it on a newspaper. Take samples from several different spots in your garden. Break up any clumps and remove stones and other litter. Follow the test kit instructions after that, using distilled water and the color chart that should have been provided with the kit.


You can use the results to fine-tune different parts of your garden for different plants. For example, hydrangeas need extra-acidic soil in order to produce those gorgeous blue blossoms. Such acidic soil would not be good for most plants. So if there's a spot in your garden where you are going to plant a hydrangea bush, you can work bone meal or fireplace ashes in to that particular spot and not affect surrounding plants.


The pH results are easy to respond to:


Too acidic (below 7): add lime according to package directions. That's called "sweetening" it. Wood ashes are a good source, although you can buy lime by the bag.


Too alkaline (above 7): add organic matter such as peat moss, pine needles, leaf mold (decomposed tree leaves, especially from oak trees), aged sawdust or wood shavings.


Note: it's best to do this in the fall, but if you're starting your garden in the spring, do this at least three weeks before you plant or sow seeds, to give the pH level time to change.


Safety note: wear a dust mask when you add lime or ashes to protect your lungs.


Make sure to keep a record of your pH test results for future reference. You should have a sketch of your garden plan in a notebook to keep from season to season. You probably should repeat the pH test every fall after the growing season, to chart how various plants might be drained their part of the garden of nutrients. It's best to add soil amendments in the fall and let them settle in over the winter to be ready to go by spring planting time.







red cabbage

blender or knife

boiling water

coffee filters

One large glass container

Six small glass containers

household ammonia

baking soda

washing soda

lemon juice


cream of tartar


seltzer water

muriatic acid or masonry's cleaner


safety goggles and gloves

clothespins or string


Red cabbage juice contains a natural pH indicator that changes colors according to the acidity of the solution. You can make your own pH paper strips using this everyday vegetable.


Red cabbage contains a pigment, or color-containing, molecule called flavin. It dissolves in water and is also found in apple skin, plums, poppies, cornflowers, and grapes. If what you're testing is very acidic, it will turn a red cabbage solution a red color. Neutral solutions result in a purplish color. Basic (alkaline) solutions appear in greenish-yellow.


Therefore, it is possible to determine the pH of a solution based on the color it turns the pigments in red cabbage juice.


First, chop the red cabbage into small pieces until you have about 2 cups of chopped cabbage. Place the cabbage in a large glass container. Add boiling water to cover the cabbage.


Allow at least ten minutes for the color to come out of the cabbage. It'll be purple, not red.


Another way to do this is to put the 2 cups of red cabbage into a blender, cover with boiling water, and blend.


Strain out the plant material to obtain a red-purple-bluish colored liquid. This liquid is at about pH 7.


Pour a little out into each of the small glass containers. Then add white vinegar to one glass, drop by drop. Mix a teaspoon of baking soda into another glass. How many colors can you make by adding different amounts of each ingredient? Can you end up with bright blue, shocking pink, and purple?


This works because red cabbage water reacts with either acids like vinegar, or base (alkaline) substances, like baking soda, and changes colors.


Acids tend to turn it pink, and bases turn it blue or green.


You can also try adding lemon juice, clear soda water, or laundry detergent.


Then you can tell which are acids and which are bases. Make a chart.


Finally, put coffee filters in a shallow container and soak them in a concentrated red cabbage solution for a few hours. Hang by a clothespin on string to dry. Cut the filter into strips and use them to test the pH of various solutions.


By Susan Darst Williams www.KidsGardenClub.org Planning 10 2010


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