Gardeners often read the recommendation to check soil pH, but what does that mean and why is it important? The pH is simply a measure of how acid or alkaline a substance is. When I say substance, this can mean swimming pool water, cheese or beer — all is influenced by pH.
Soil acidity or alkalinity (soil pH) is important because it influences how well plants take up nutrients through their roots. Many references list the preferred pH for specific plants. For example, blueberries thrive in acidic soil, while carrots like it on the sweet (alkaline) side. The good news for gardeners is that, for the most part, the majority of plants will tolerate a fairly wide range of soil pH.
Here’s how nutrient uptake and pH are linked: Plant roots absorb mineral nutrients such as iron and nitrogen when the nutrients are dissolved in water. If the soil solution (water and nutrients in the soil) is too acid or alkaline, some nutrients won’t dissolve easily, so they won’t be available for uptake by roots. It’s like having money in the bank, but you can’t access it.
Most nutrients that plants need dissolve easily when the pH of the soil solution falls between 6.0 to 7.5. Below pH 6.0, as in Kodiak’s volcanic ash-based soils, some nutrients, such as nitrogen, phosphorus, and potassium, are less available. This explains why it’s so important to add organic materials to our local gardens, not only to balance the pH, but to improve the soil tilth, or texture.
Many environmental factors, including amount of rainfall, vegetation type, and temperature, can affect soil pH. In general, areas with heavy rainfall and forest cover have moderately acid soils. Soils in regions with light rainfall and prairie cover such as the Midwest tend to be near neutral. And dry areas of the western United States tend to have alkaline soils.
Most garden plants grow well in slightly acid to neutral soil (pH 6.0–7.0). Some common exceptions include potatoes and rhododendrons, which prefer moderately acid soil. You can make small changes to soil pH by applying soil amendments. However, you’ll have better success if you choose plants that are adapted to your soil pH and other soil characteristics. Adding organic matter such as compost to the soil is one of the best ways to buffer the pH, in that it tends to bring both acid and alkaline soils closer to neutral.
If you have your soil analyzed by a lab, the lab report will include soil pH. You can also test soil pH yourself with a home soil test kit or a portable pH meter. Both are available locally. Home kits and portable meters vary in accuracy but they can be helpful in assessing the general pH range of your soil.
The quantity of material needed to change soil pH depends on many factors, including current pH, soil texture, and the type of material. A soil lab report will contain recommendations on types and quantities of amendments to use. In Kodiak, nine times out of 10, you’ll need to sweeten the acidic soil by raising the pH.
If your soil is too acid, you must add alkaline material, a process commonly called liming. The most common liming material is ground limestone. There are two types: calcitic limestone (calcium carbonate) and dolomitic limestone (calcium-magnesium carbonate). In most instances, you’ll use calcitic lime. Apply dolomitic lime only if your soil also has a magnesium deficiency.
Spread liming materials with a garden spreader or by hand for small areas. If hand spreading, be sure to wear heavy gloves to protect your skin and do it on a calm day.
Ground limestone breaks down slowly in the soil, so it’s best to apply it to the garden and lawn in the fall to allow time for it to act on soil pH before the next growing season. A rule of thumb for slightly acidic soils is to apply 5 pounds of lime per 100 square feet to raise pH by one point. In general, sandy soils will need less limestone to change pH; clay soils will need more.
Applying wood ashes also raises soil pH. Wood ashes contain up to 70 percent calcium carbonate, as well as potassium, phosphorus and many trace minerals, making it a valuable commodity in our local gardens. Because it has a very fine particle size, wood ash is a fast-acting liming material. Use it with caution, because applying too much can create serious soil imbalances. More is not better. Limit applications to 2.5 pounds per 100 square feet (or 25 pounds per 1,000 square feet). If you don’t have a scale, use a coffee can. Also, apply ashes only once every two to three years in any particular raised bed or area. At this rate, your soil will get the benefits of the trace minerals without adverse effects on pH.
When I started my first garden in Kodiak, back in 1986 or so, I mailed a soil sample to the state lab. It came back reading like a bad report card. My soil pH was 5.0, very acidic, and it lacked organic material. Neither calcium carbonate nor dolomite lime were available, and I didn’t want to use the chemical-based “quick lime,” so I opted to correct my soil’s pH by adding compost and wood ashes. The combination proved to be ideal.
Why is it called pH?
You can thank chemists for this odd abbreviation. It stands for “potenz Hydrogen” (“potenz” means “the potential to be”). But what does hydrogen — much less the potential of hydrogen — have to do with soil acidity or alkalinity?
Well, the activity of hydrogen ions in solution — and soil is considered a solution at the microscopic level — determines the acidity or alkalinity of the solution. Acidic solutions (soil) have a high concentration of hydrogen ions; alkaline solutions have a low concentration. This description might seem old-fashioned, but here’s a fun fact to share with your friends: The inventor of the pH scale developed it to determine the acid content of his beer.
Get growing indoors even without lights: dahlias, gladioli, tuberous begonias should flourish.
Window box geraniums (pelargoniums): Start at the tops of plants and cut back the stems until you hit the live, green tissue. Pot up plants if they need it.
Vegetable seeds to start: celery, onions, leeks, green onions
Flower seeds to start: lobelia, carnation, verbena (all three may take 21 days to germinate; do not cover with soil as seeds need light), snapdragons (10 days and seeds need light, cool), pansy, salvia.
Herbs to start: oregano, lovage, mint, parsley, lemon balm
Kodiak Garden Club: Soil 101
“How to prepare your soil for spring planting” is the topic of this meeting. Rule No. 1: Not all soil is created equal and to have a successful garden you need good soil. Never mind that there is still loads of white stuff on the ground. Sooner or later you’ll have bare ground to play in. Come to the Marian Center on today at 7 p.m. to get the dirt on dirt. You don’t have to be a member to attend, so bring a friend. Always good treats on hand, too.
Know how to grow! Marion Owen will be teaching a 5-week Organic Gardening class at Kodiak College starting April 5. You can read Marion’s latest blog postings at http://marionowen.wordpress.com. She also has her own Facebook page. Archived copies of her columns are posted at www.kodiakdailymirror.com. Contact Marion at firstname.lastname@example.org.