It’s been quite a while since I tested soil samples from my lawn, but it’s clearly time to do so. The turf is looking a bit thin and the weeds seem to be gaining ground. I suspect that soil pH is the source of both problems. When the pH is out of kilter, plants can’t use the nutrients in the soil around them. It’s like standing at an all-you-can-eat buffet with your jaw wired shut. Even when the soil contains plenty of phosphorus, magnesium, iron, and other minerals, plants can’t use it them if the pH is too high or too low. No matter how much fertilizer you pour on, the plants will starve.
Lawn grasses in most northern areas thrive in a pH range of 6 to 7. Most ornamental and garden plants grow best when the soil pH is between 5.5 and 7.5. On the pH scale of 0 to 14, 7.0 is neutral. Soils that have a pH higher than 7 are considered alkaline and soils that are lower than 7.0 are in the acidic range.
Soil pH is essentially a measure of the chemical “charge” in your soil. The more positive ions, the more acidic your soil; the more negative ions, the more alkaline your soil. These chemical charges cause minerals to bind more or less tightly to the soil particles, creating a tug-of-war between plant roots and soil particles.
A soil’s natural pH level is primarily determined by the makeup of the native soil and the amount of rainfall in the area. In general, soils in arid regions tend to be alkaline, while those in higher-rainfall areas are acidic.
The pH of your soil determines whether certain nutrients are available to your plants or not. It also affects the activity of soil microbes, and that’s especially important to organic gardeners. Microbes are the little guys that break down compost and other organic matter into nutrients that plants can absorb. When the soil is too acidic, microbial activity slows down, which can cause a nitrogen shortage.
Weeds can be a helpful indicator as to the pH of your soil. I live in acidic-soil territory and the weeds in my lawn reflect this condition. The turf is full of dandelion, plantain and sorrel, which all thrive in acidic soil. White clover, a plant that takes nitrogen from the air and transfers it to the soil, rivals grass in much of the yard. It’s a dead give-away that there’s a slowdown in microbial action and our soil lacks sufficient nitrogen to feed nitrogen-hungry turf grasses. Up the road, our neighbor’s sandy lawn has large patches of yellow and orange-flowering hawkweed or devil’s paintbrush, which along with cinquefoil, is a common weed on dry, acidic, infertile soil.
I’ll be using a home test kit to check the pH this spring, but I recommend that less-experienced gardeners send their soil samples to a testing lab. For a nominal fee, you can mail a small sample of your soil to a state and county extension service or a private soil-testing lab. They’ll send back a report that gives you pH and nutrient levels and recommendations for how to correct any problems. The kits are available either directly from the testing lab or often from local garden centers. Some labs will also test levels of organic matter content and for heavy metals. Not a bad idea if you are planning to raise vegetables on reclaimed urban or industrial soils.