Learn what causes acidic soil and alkaline soil, plus how to apply the results of a soil pH test in your organic garden.
To ensure that your garden crops make the most of the rich, organic soil you create, you need to understand your soil’s pH. The pH describes the relative acidity or alkalinity of your soil’s makeup, and it has important implications for plant health and growth. Soil pH impacts beneficial fungi and bacteria in the soil and influences whether essential minerals are available for uptake by plant roots.
A solution’s pH is a numerical rating of its acidity or alkalinity. All pH is measured on a logarithmic scale from zero (most acidic) to 14 (most alkaline, or basic); 7.0 is neutral. The pH scale is used by chemists to measure the concentration of reactive hydrogen ions (H+) in a solution.
Most food crops prefer a pH of 6.0 to 6.5, but you can have a productive food garden as long as your pH is about 5.5 to 7.5 (see chart in slideshow). A difference of just 0.5 may not seem like much, but the pH scale is logarithmic, which means, for example, a pH of 7.0 is actually 10 times less acidic than a pH of 6.0. Potatoes and most berries, which grow best in more acidic soil, are the main exceptions to the average preferred pH range.
A soil’s pH results from interactions among native rocks, plants and weather conditions over many years, and it varies with climate and physical surroundings. In moist climates that support dense forests, such as those east of the Mississippi River and along the Pacific Coast, soil tends to be acidic, with pH ratings usually between 4.0 and 5.5. The grasslands of the comparatively dry Midwest often have slightly acidic soil (6.0 to 6.5), while most arid regions, such as the Rocky Mountains, are dominated by alkaline soil (7.0 to 7.8). Local differences in rock can cause huge variations within these general patterns, however — for example, when weathered limestone creates alkaline patches in otherwise acidic landscapes, or when elevation leads to more or less rainfall. Plus, soil is often severely disturbed during construction, and sometimes native topsoil is completely lost.
Some synthetic chemical fertilizers — mainly those high in ammonium or sulfur — can make soil more acidic, as can tillage methods that reduce soil’s levels of organic matter. Acid rain caused by air pollution from coal combustion began to acidify streams and soil during the late 1800s, and continues to push soil in some regions into the acidic range every time it rains. In addition to outside influences, some types of organic matter, such as peat moss and pine needles, acidify naturally during decomposition.
Alkaline soil occurs naturally in places where soil is formed from limestone or other calcium-rich minerals, and high water-evaporation rates common in arid climates aggravate the problem by loading the topsoil with accumulated salts. Many garden plants can still thrive when grown in alkaline soil that has been generously enriched with organic matter, which also improves the soil’s ability to retain water. Mulches also will slow the buildup of salts in plants’ root zones by reducing the amount of surface evaporation.
Soil chemistry is complex, so how can we boil it down to help you in your garden? If your crops seem to be thriving, then you probably don’t need to worry much about your pH. But if you find that plants just don’t seem to be growing as well for you as they do for your neighbors, then the problem could be related to pH and you should probably have your soil checked with a pH test. The cost for basic soil evaluation done by a state soil-testing lab ranges from free to $25, depending on the state in which you live, and typically includes a pH test along with results for major and sometimes minor nutrients. Soil-test kits with detailed instructions are usually available at extension service offices, or you can order them by mail.
If one bed or small section of your garden goes wonky, you might try a home pH test kit rather than waiting on lab results. When a team of Missouri extension experts submitted soil samples to 82 soil-testing laboratories and compared the lab’s results with those from do-it-yourself pH-measuring kits, the $20 LaMotte home color kit (available at Peaceful Valley Farm & Garden Supply ) earned high accuracy ratings. Personally, I like pH color kits because they are fun to use, and a practiced eye can detect the small changes in color between shades of orange (acid) and green (neutral to alkaline) in the test results.
Raising the organic matter content of soil will usually move the pH of both acidic and alkaline soils toward the neutral range. This is because organic matter plays a buffering role, protecting soil from becoming overly acidic or alkaline. Finished compost usually has a near-neutral pH, so regular infusions of compost should be the primary method you use to improve soil with extreme pH issues. If your pH readings are only slightly acidic or slightly alkaline, compost and organic mulches may be the only amendments you need to keep your crops happy and your garden growing well.
The standard intervention for overly acidic soil is to amend it with lime, an inexpensive soil amendment made from ground limestone that slowly raises the pH over a period of months. Products labeled “dolomitic lime” are usually preferred because they contain both calcium and magnesium. But if you have dense soil and a soil test indicates excess magnesium (which can tie up nitrogen), you should use low-magnesium, calcium-rich powdered crab or oyster shells as your liming material. Read and follow the label, because products differ in application rates, which, in turn, vary with soil type. You can’t apply a correct amount of lime unless you know your soil’s pH first, and if you apply too much, it will be extremely difficult to correct. Err on the cautious side by applying too little lime at first.
After the pH of acidic soil is raised above 6.0 using organic amendments and dolomitic lime, I’ve found it can be maintained with a light, yearly application of alkaline woodstove ashes. In addition to containing enough calcium and magnesium to have a liming effect, wood ashes contain an array of micronutrients, too. The key is to use them sparingly, in small, dispersed amounts, and to never add wood ashes or lime to soil with a pH higher than 6.5. A quart of wood ashes (1 pound) is about right for 50 square feet of cultivated space. When you have a lot of ashes to spread, apply no more than 20 pounds of ashes per 1,000 square feet of garden bed.
If you are not using acidic chemical fertilizers, a normally acidic soil may not require liming again for several years, if ever. Then again, if your soil is porous sand in a high-rainfall area, pH testing may show a need for liming every other year. Just be careful to never apply lime unless a pH test shows it is needed, and never use it where you are growing plants that prefer acidic soil conditions, such as blueberries and azaleas.
If you have exceptionally alkaline, high-pH soil, you can often tame it by adding organic matter and powdered sulfur. However, sulfur may do little good in alkaline soil that is rich in free lime, also known as calcium carbonate. You can test for free lime by covering a soil sample with vinegar; if it bubbles, you have free lime and should consider gardening in beds filled with non-native soil.
Extension experts in places where alkaline soil predominates emphasize that most plants will grow well in organically improved soil with a pH as high as 7.5, and improving soil quality with organic matter — rather than lowering the pH — should be your primary goal. Alkaline soil can be stubborn about releasing its valuable phosphorus to plants, so amend it every chance you get with composted manure, which has been found to solve several problems associated with high pH levels. The humic acids in both composted manure and vermicompost help make phosphorus available to plants grown in alkaline soil, as does the presence of rotted plant tissues from both regular compost and cover crops. Acidic mulches, such as pine needles, can help lower soil pH slightly, but other mulches, such as bark or wood chips, have little effect on soil pH.
Check out our complete Garden Know-How series online for articles about seed starting, composting, companion planting, season extension and much more.
Contributing editor Barbara Pleasant gardens in southwest Virginia, where she grows vegetables, herbs, fruits, flowers and a few lucky chickens. Contact Barbara by visiting her website or finding her on Google+.