While many vegetables, fruits and ornamentals grow well in a fairly wide range of neutral soils (6.5 to 7.5), some plants, like blueberries, azaleas and hydrangeas, won't tolerate alkaline soils. To grow them, the soil pH must first be lowered, by amending the soil with elemental sulfur.
To find out your soil type, first you have to test the pH levels of the soil. A general, scientific classification for soil type is based on pH, which measures the levels of acid and alkali present. The pH scale uses numbers 1 to 14 with soil testing below 7 as acidic. Test results above 7 indicate an alkaline soil.
Lowering your soil pH is often a long term project. When the alkalinity is at 8.5 or higher, it may take several years and repeated applications to achieve ideal pH for most plants. Sulfur levels can also be depleted so it's a good idea to test your soil pH every few years and adjust if needed.
How to Test Your Soil
Your cooperative extension office can perform a basic analysis with a soil sample from your garden for a minimal fee. Results include pH as well as primary and secondary nutrients and micronutrients that your sample may be lacking.
You can also purchase a test kit at retail outlets, garden centers and online, or you can perform a basic test using ingredients from your kitchen cupboard.
When to Lower Soil pH
If you are planning a new garden bed, it's helpful to have the soil tested as early as possible. Sulfur reacts with soil bacteria to lower pH, but soil temperatures must be above 55 degrees Fahreneheit for it to be effective. You should allow about a month before planting following a sulfur application. Adding sulfur in fall or winter will not give the desired result.
Making adjustments to soil composition also is easiest and most effective before you plant. This isn't always practical but take care to use the best methods and materials to avoid root damage.
Always take precautions when working with chemical substances. Gloves, safety glasses and protective clothing can prevent unintended burns and other skin and eye irritations.
Equipment / Tools
- hand or wheeled spreader
- rototiller, shovel or hoe
- hard rake
- protective glasses
Lower Soil pH With Sulfur
Sulfur for amending soil is sold at retail centers and gardening outlets. It is often labeled as sulfur but may also be called soil acidifier. It's usually sold by the pound in five to 50 pound bags as dust or small particles. Your extension agent can help you calculate how much you need based on the size of the planting area, how much the pH should be lowered, and your soil type.
A general rule of thumb for lowering the pH by 1 point is as follows:
- Clay soil-1/2 lb of sulfur per square yard
- Sandy soil-1/4 pound of sulfur per square yard
- Loamy soil-6 ounces of sulfur per square yard
Use the hand or wheeled spreader to distribute sulfur over the planting area in the correct amount determined by the results of your soil test.
Work Sulfur Into the Soil
Use the shovel, hoe, or a rototiller to work sulfur into the soil to a depth of 6 inches.
Water the Treated Planting Bed
Tilling and watering in the sulfur gives faster results. Wet the soil but avoid drenching. Too much water dilutes the application with less benefit.
Wait to Plant
Give the sulfur application about a month to work before planting. This insures more even soil distribution and prevents plantings from exposure to concentrated acidity.
What Can Affect Soil pH?
The natural pH of soil is determined by the type of rock from which it formed. It also is affected by dry or wet climates, vegetation, topography and time. Fertilizers with ammonium sulfate and nitrogen fertilizers with ammonium reduce pH over time. Some organic fertilizers can also reduce pH, slowly. Minerals and salts in hard water and large amounts of wood ash raise the pH in soil.
How to Lower pH Naturally
Lowering pH with natural materials takes time and repeated applications but is one of the easier methods for existing flowerbeds and gardens. You can add organic materials like compost and manure to vegetable gardens before planting, or as side dressings raked or hoed in once crops are established. Chicken manure works best for acidifying soil but all types should be aged before added to garden soil. Pine products including mulches, barks and needles lower pH but break down very slowly.
How to Lower pH After Planting
Soil acidity is depleted over time which causes the pH level to rise. This makes maintaining the best balance for plants an ongoing process. Ammonium nitrate works quickly but is also depleted quickly, creating a need for repeat applications. It has to be worked into the soil around the plants and adding too much can damage roots and nutrient uptake. The best way to maintain a neutral pH for your flowers, fruits and vegetables is to add well-composted material and organic mulches annually.