What is Sulfur, and Why is it Important for Plant Growth?
Sulfur is a naturally occurring element commonly found near volcanos and hot springs, and also in decaying organic matter such as compost and manure. Sulfur is an essential mineral for all living things; plants need it to make enzymes, proteins, and amino acids—fundamental building blocks for their healthy development and growth.
Sulfur also plays a critical role in adjusting soil pH. When soil is too alkaline for plants to absorb nutrients effectively, adding sulfur lowers the pH, making it more acidic. As TOH has covered extensively, most plants—including lawn grasses—thrive in slightly acidic to neutral soil with a pH between 6 and 7. Without the proper pH, the microorganisms that break down organic matter and release nutrients to plants are not as active, which means even nutrient-rich soil can starve your plants.
Sulfur and Soil pH
Sulfur can raise soil’s acidity—a benefit to gardeners, especially if the soil is too alkaline. The acidity and alkalinity of soil is determined by its pH level, measured on a scale of 0 to 14, with 7 being the neutral point.
Anything below 7 is considered acidic, and anything above 7 is alkaline. Most plants thrive in neutral pH levels of 6.5 to 7.5, but some, like blueberries and azaleas, thrive in acidic environments, preferring a pH level of 4.5 to 5.5.
How to Check Your Soil’s pH: Before adding sulfur, you need to know where your soil stands. A complete soil test from your state’s cooperative extension service or university soils lab will tell you not only your pH but also levels of nitrogen, phosphorus, potassium, micronutrients, and even heavy metals like lead—especially important in urban settings where lead paint may have leached into the ground. As TOH has covered, you can also use a home test kit: add water, a reactant agent, and soil to the test chamber, shake it up, let it settle, and compare the color of the solution to the kit’s color-coded chart. A reading above 7.5 means your soil is too alkaline and sulfur may be needed to bring it into the ideal range.
Identifying a sulfur deficiency in your garden
There are various ways to augment sulfur in the soil to benefit plants and crops. But first, you need to determine if additional sulfur is actually needed. Indications that plants may be suffering a sulfur deficiency include stunted growth, poor crop yield, and the yellowing of new leaves (as opposed to the yellowing of older leaves which may indicate a nitrogen deficiency).
If you suspect a sulfur deficiency in your soil, purchase a soil testing kit. Make sure to test various areas of your garden, not just one spot, for the most comprehensive results. Once you receive the results, you can augment your soil’s sulfur content (and add any other elements needed) to resolve deficiencies and enhance plant growth.
Go Beyond the DIY Kit: A do-it-yourself soil test kit from a garden center will give readings on pH and levels of crucial nutrients like nitrogen, potassium, and phosphorus. But for the most accurate and complete results, have your soil tested by your county extension service or state university. They’ll analyze a sample from your yard and send you a detailed report covering pH, texture, nutrient levels, and recommendations for correcting any deficiencies. A good complete soil test will also tell you how much organic matter you have in your soil and check for all the micronutrients essential to plant growth — plus it can test for heavy metals like lead, which is especially important in an urban setting.
Keep in mind that soil pH directly affects whether roots can absorb minerals in the soil. If your test reveals the pH is too high (alkaline), adding sulfur will lower it into a more ideal range. Conversely, lime is used to raise soil pH. Once you have your test results, convert the nutrient recommendations into a usable ratio: divide the weight of each nutrient (nitrogen, phosphorus, and potassium) by the nutrient with the lowest weight. For instance, if the recommendation calls for 1.5 pounds of nitrogen, 0.5 pound of phosphorus, and 1 pound of potassium, dividing each by 0.5 gives you a 3-1-2 ratio — then purchase a fertilizer with a label that most closely matches, such as 9-3-6.
3 Ways to Add Sulfur to Your Garden
If you have a sulfur deficiency in your garden, here are some cost-effective ways to augment the soil’s sulfur content:
- Amend your soil with compost. Since sulfur naturally occurs in decaying materials, adding compost to your garden will introduce more of this essential nutrient as well as other beneficial ones. If you aren’t already composting at home, starting to compost your food scraps is an easy DIY that not only benefits your garden but has the added plus of keeping waste out of landfills. You can also use mulch or manure for similar results.
For beds that aren’t yet planted, top-dressing with several inches of well-aged compost in both spring and fall will not only introduce sulfur but also increase the soil’s ability to hold nutrients and water, make it “fluffier,” and stimulate microbial action. Once you’ve improved the soil structure, targeted amendments — as indicated by your soil test — will fill in any remaining deficiencies. - Choose sulfate-based fertilizers. Take a look at the fertilizer you’re using. Most already contain sulfur, so you may be adding it to your garden already. If not, the next time you’re at the store, look for and purchase fertilizers that are sulfate-based.
- Add sulfur-rich material to your soil well before planting your garden. If your garden is seriously lacking in sulfur or you want to dramatically lower your soil’s pH, elevating its acidity, you’ll need to plan in advance. Increasing soil acidity takes time and ideally begins at least six months to a year before any plants are placed in the ground.
Start by tilling sulfur-rich material into the ground, at least 6 to 10 inches deep. If plants are already in the ground, it will be harder to penetrate the soil without damaging the plants and their root systems.
You can sprinkle sulfur powder or pellets on top of the soil and use a rake to distribute the material (being careful not to damage stems, leaves, and roots), but this method is slow going as you’ll have to wait for the surrounding area to absorb the element. And adding too much sulfur too quickly can cause plants to die.
How much sulfur do you need in your garden? Use this handy calculator.
Don’t Guess — Test: Before amending with sulfur, invest in a soil test. As TOH Magazine advises, at about $15 through a university extension service (such as soiltest.umass.edu), it’s the smartest garden investment you can make. The analysis will provide exact recommendations for nutrients, including sulfur, so you know precisely how much to add. Retest every five years or so to track your soil’s progress.
Tips for Acid-Rich Soil:
- Add mulch and compost to your garden regularly. This will help replace nutrients that may be lacking and keep toxic levels of other elements at bay.
- Test your water’s pH level before you water your garden. If you want acidic soil but the water you’re using is too alkaline, it may impair the efforts you’ve made to increase your soil’s acidity.
- Grow acid-loving plants. Alkaline soil makes it harder for plants to absorb the nutrients they need to thrive. Thankfully, it’s easier to grow plants in slightly acidic soil than in more alkaline-based soil. See the list below for which plants thrive in acidic soil.
Expert Insight: TOH landscape contractor Roger Cook notes that regional soil conditions vary widely: “The soils in New England are notoriously acid. Vegetables don’t like acid soil, they like a pH of about 6.8. So what the limestone does is raise that pH up to a more acceptable level for the vegetables.” If your soil is on the opposite end of the spectrum and too alkaline, sulfur works in reverse — lowering pH to create the acidic conditions that plants like blueberries and hydrangeas prefer.
Flowers and Plants That Thrive in Acidic Soil
- Hydrangeas (certain cultivars will turn from pink to blue, and the blue ones will turn a deeper shade of blue)
- Camellias
- Gardenias
- Magnolias
- Zinnias
- Foxglove
- Blueberries
- Cranberries
- Gooseberries
- Evergreens
- Peppers
- Tomatoes
- Potatoes
- Squash
- Rhubarb
- Azaleas
- Rhododendrons
- Most evergreen shrubs and trees
- Dogwood
- Magnolia
How Soil pH Affects Bloom Color: Bigleaf and mountain hydrangeas are a vivid example of why soil acidity matters. Most varieties flower blue in very acidic soils (pH 5.5 and lower), purple or a mix of shades in less acidic soils, and pink in more alkaline soils (pH 6.5 and higher). Hydrangeas require aluminum to produce blue flowers, and plants can easily extract metals from acidic soils. Under alkaline conditions, metals become insoluble and flowers bloom pink instead.
Shade-Loving Acid Lovers: If your acidic-soil garden is also shaded, pair your azaleas and rhododendrons with tough companions like fothergilla, drooping leucothoe, clethra, and oakleaf hydrangea. For perennials, astilbe, bleeding heart, foxglove, ferns, and Japanese forest grass all thrive in partial shade on acidic ground—and most are deer-resistant, too.
Edible Plants for Acidic Soil: Beyond the well-known blueberries and cranberries, consider ornamental blueberry varieties like Vaccinium corymbosum x ‘Perpetua,’ a self-pollinating cultivar that sets fruit twice in the same year. A wave of bell-shaped white flowers in April is followed by sweet berries, and the cycle recurs in late June—giving you both ornamental value and a harvest from your acidic beds. Lingonberry (Vaccinium vitis-idaea), a creeping evergreen shrub hardy in Zones 2–6, is another acid-lover whose bright-red fruit is often used to make preserves or sauces.
