Testing and Amending Garden Soil
by Fern Ness
Different types of plants prefer different degrees of soil acidity or alkalinity. Some plants are very sensitive to soil pH and will not grow properly if planted in the wrong type of soil.
The acidity or alkalinity of the soil is measured by pH (potential Hydrogen ions). A soil with a pH lower than 7.0 is an acid soil and one with a pH higher than 7.0 is alkaline. Generally, soils in moist climates tend to be acid and those in dry climates tend to be alkaline.
In order for a plant to grow optimally, the soil must be adjusted to suit it’s preferred pH if it is not already within that plant’s requirement range.
Importance of Testing Your Soil
A quote by Edward C. Smith, from "The Vegetable Gardener’s Bible" tells just how important it is to determine the pH of your soil. He says, "Many years ago my wife, Sylvia, and I started a new garden in the Berkshire hills of western Massachusetts. It was the worst garden we have ever had. Most of the seeds didn’t germinate at all, and many of those that did gave up quickly. The tomato plants grew to be 9 inches high and bore no fruit at all. The corn never got more than a foot tall. All this trouble in soil that looked beautiful! It had wonderful tilth, a deep, dark brown color. Soil tests showed no significant nutrient deficiencies. But what the tests did show was a pH of 4.5. With soil that acidic, none of the rest mattered."
Testing your soil pH
To have your soil tested you can usually take or send a sample to either a private, provincial or state soil testing lab. See the Resources section (link to resources soil testing page) for a center in your area. For a fee your soil will be analyzed and amendments suggested if necessary. Some good garden centers may test a soil sample for you, or you can buy an inexpensive pH testing kit at most nurseries or hardware stores. These test kits generally consist of a test tube, some testing solution and a color chart. A sample of your soil is placed in the tube, a few drops of test solution are added, then it is shaken up and left for an hour or so to settle. The solution in the tube will change color according to the pH of your soil. Compare the color of the sample with the color chart that came with the kit and match the color to determine the pH of your sample. Some kits contain a booklet which tells you how to interpret the result and what to consider for amendments.
The Science Behind pH
Ever wondered exactly what is checked when you have a pH test? A pH test determines the level of positively charged and negatively charged hydrogen molecules. Molecules with a positive charge are acidic and ones with a negative charge are basic. When the positively charged molecules outnumber the negatively charged ones, the soil is acidic. The greater the difference between them, the more acidic the soil. When the negatively charged molecules are greater, the soil leans towards alkalinity.
Adjusting your soil pH
Once you have determined the soil pH you can determine if amendments are necessary. If the soil is within .5 of the number preferred by the plants you hope to grow, you don’t have to change anything but if it beyond that range you should consider making changes to it. The materials required are commonly available at your local garden center or hardware gardening center. The best way to adjust pH is gradually, over several seasons.
Raising the soil pH to make it more alkaline
Generally, it is easier to make soils more alkaline than it is to make them more acid. The most common way to raise the pH is to add lime to the soil. Because different soil types react in different ways to the application of lime you will have to add more lime to clay soils and peaty soils than you will in sandy soils to achieve the same result.
To increase your pH by 1.0 point and make your soil more alkaline.
- Add 4 ounces of hydrated lime per square yard in sandy soils
- Add 8 ounces of hydrated lime per square yard in loamy soils
- Add 12 ounces of hydrated lime per square yard in clay soils
- Add 25 ounces of hydrated lime per square yard in peaty soils
Lime should only be applied when tests show it to be necessary and correction of an overly acid soil should be done gradually over the long term, rather than trying to accomplish it in one year. It is better to test your soil each year and make your adjustments gradually over time. The addition of hardwood ash, bone meal, crushed marble, or crushed oyster shells will also help to raise the soil pH.
Fall is the best time to add lime to your soil. If you find it necessary to adjust it in the spring, make sure to do it as early as you can, at least three weeks before you want to plant.
Lowering the soil pH to make it more acid
The best way to improve too-alkaline soils is to add acidic organic matter such as peat moss, pine needles, cottonseed meal, leaf mold, or even aged sawdust or shavings. Sulfur may be used to lower the pH and make the soil less alkaline if it is available. To reduce the soil pH by one point, mix in 1.2 oz of ground rock sulfur per square yard if the soil is sandy, or 3.6 oz per square yard for all other soils. The sulfur should be thoroughly mixed into the soil before planting. As with lime, sulfur should be added in the fall but if you have to use it in the spring, do it as early as possible.
For excessively alkaline soil, you may find that you are better off to build a raised bed using topsoil purchased from a nursery rather than attempting to amend the soil.
Important Safety Points to Keep in Mind
Always read and follow the manufactures safety recommendations when using chemical products.Use appropriate protection such as a dust mask, goggles, and gloves.