In late fall and early winter, before the ground is frozen solid in most of the country, it’s the perfect time to brush up on knowledge that can help us build a better garden come spring. One way we can help ensure our garden crops grow to their maximum potential is to gain an understanding of our soil’s pH. The pH describes the relative acidity or alkalinity of the 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.
What is Soil pH?
All pH is measured on a scale from 0 (most acidic) to 14 (most alkaline, or basic); 7.0 is neutral. Most food crops prefer a pH of 6.0 to 6.5, but you can have a productive food garden at pH levels between 5.5 and 7.5. 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 a pH 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, have alkaline soil (7.0 to 7.8).
However, local differences can cause huge variations within these general patterns—for example, weathered limestone can create alkaline patches in otherwise acidic landscapes. Soil is often severely disturbed during construction; sometimes native topsoil is completely lost. Some synthetic fertilizers—mainly those high in ammonium or sulfur—can also acidify soil, as can tillage methods that reduce soil’s levels of organic matter. Acid rain caused by 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. Finally, some organic matter, such as peat moss and pine needles, acidifies during decomposition.
Alkaline soil occurs 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 topsoil with accumulated salts. Many plants can still thrive when grown in alkaline soil that has been generously enriched with organic matter, which also improves soil’s ability to retain water. Mulches also will slow buildup of salts in plants’ root zones by reducing surface evaporation.
How to Test Soil pH
Soil chemistry is complex, so how can we boil it down to help us in our gardens? If your crops seem to be thriving, you probably don’t need to worry about your pH. But if you find that plants just don’t seem to be growing as well as you’d like, you should probably have your soil checked with a pH test. The cost for a basic soil evaluation by a state soil-testing lab ranges from free to $25, depending on the state, 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. When a team of Missouri extension experts submitted soil samples to 82 soil-testing laboratories and compared the labs’ results with those from do-it-yourself pH-measuring kits, the $20 LaMotte Soil pH Test Kit—available at Peaceful Valley Farm & Garden Supply (grow organic.com)—earned high accuracy ratings. Personally, I like pH color kits because they’re fun to use, and a practiced eye can detect small variations in the shades of orange (acid) and green (neutral to alkaline) in the results.
Start by Adding Compost
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 the primary method used to improve soil with extreme pH issues should be regular infusions of compost. 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.
Correcting Acidic Soil
The standard treatment for overly acidic soil is lime, an inexpensive soil amendment made from ground limestone that slowly raises pH over a period of months. Dolomitic lime is usually preferred because it contains both calcium and magnesium. But if you have dense soil and a soil test indicates excess magnesium, try low-magnesium, calcium-rich powdered crab or oyster shells as your liming material. Follow the label, as products differ in application rates, which 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 amendments, 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. 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. In larger spaces, apply no more than 20 pounds of ashes per 1,000 square feet of garden bed.
If you avoid 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 never apply lime unless a pH test indicates it is needed, and never use it where you’re growing plants that prefer acidic soil, such as blueberries and azaleas.
Correcting Alkaline Soil
If you have exceptionally alkaline, high-pH soil, you can often tame it by adding organic matter and powdered sulfur. But sulfur may do little good in alkaline soil that is rich in free lime, or calcium carbonate. 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 your primary goal should be improving soil quality with organic matter—not lowering pH. Alkaline soil can be stubborn about releasing phosphorus to plants, so amend it every chance you get with composted manure. The humic acids in both composted manure and vermicompost help make phosphorus available to plants, 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; other mulches have little effect on soil pH.
The garden soil I’ve nurtured organically for years tests at a near-neutral pH. My newer beds that have tight clay, on the other hand, test acidic. Monitoring soil pH helps me gauge how well I’m improving the soil, reducing effects of acid rain and maximizing the soil’s microbial activity—which all add up to a better garden.
Ideal Acidity for Various Plants
Most fruits and vegetables like slightly acidic soil with a pH between 6.0 to 6.5, but there are a few exceptions. For example, berries and potatoes like acidic soil, while asparagus loves alkaline conditions.
Apple: 5.5 to 6.5
Blueberry, highbush: 4.5 to 5.5
Cherry, sweet: 6.5 to 8.0
Grape: 5.5 to 7.0
Pear, common: 6.5 to 7.5
Plum, American: 6.5 to 8.5
Raspberry, black: 5.5 to 7.0
Raspberry, red: 6.0 to 7.5
Strawberry: 5.5 to 6.5
Asparagus: 6.0 to 8.0
Bean: 6.0 to 7.0
Beet: 6.5 to 8.0
Broccoli: 6.0 to 7.0
Cabbage: 6.0 to 7.5
Cantaloupe: 6.0 to 7.5
Carrot: 5.5 to 7.0
Corn: 5.5 to 7.5
Cucumber: 5.5 to 7.0
Eggplant: 5.5 to 6.5
Lettuce: 6.0 to 7.0
Onion: 6.0 to 7.0
Pea: 6.0 to 7.5
Pepper: 5.5 to 7.0
Potato: 4.8 to 6.5
Sweet potato: 5.2 to 6.0
Radish: 6.0 to 7.0
Rhubarb: 5.5 to 7.0
Spinach: 6.0 to 7.5
Squash: 6.0 to 7.0
Tomato: 5.5 to 7.5