How Soil pH Impacts Plant Health

When a plant is struggling, the problem often lies beneath the surface—in the soil. Specifically, the soil’s pH, which measures its acidity or alkalinity, plays a crucial role in plant health. Unfortunately, this is often one of the last things gardeners check—when it should be the first.

Why Soil pH Matters

According to Bob Denman, co-owner of Denman & Co., “Many plant health problems are not caused by disease, insects, or nutritional deficiencies, but rather by soil that is too acidic or too alkaline.”

Soil pH affects nutrient availability. Research plant physiologist Darren Haver explains: “If the pH is too low or too high, many nutrients cannot be released to the plants.”

• Phosphorus is only available near neutral pH. Without it, plants cannot perform key functions like photosynthesis or grow healthy roots and flowers.
• Iron deficiency (chlorosis), especially in citrus, can occur in alkaline soil even if iron is present.
• Other nutrients such as calcium, magnesium, and nitrogen can also become unavailable due to incorrect pH.
• Microorganisms vital to soil health may decline with extreme pH levels.
• Toxins like aluminum become more potent and absorbable when pH is off.

Joe Sweazy of Environmental Test Systems adds that “burn” on plants could be caused by pH imbalance—not just watering or fertilizing issues.

Understanding the pH Scale

• The pH scale ranges from 0 (very acidic) to 14 (very alkaline). A reading of 7 is neutral.
• Most plants prefer pH levels between 6.0 and 6.5.
• Some plants, like azaleas, thrive in acidic soil, while others, like certain California natives, need alkaline conditions.
• Each whole number represents a tenfold change in acidity or alkalinity. For example, pH 6 is ten times more acidic than pH 7.

As Denman states: “A full point change can mean the difference between life or death for certain plants.”

Soil pH in Orange County and the Western U.S.

In Orange County and much of the western U.S., soils are generally alkaline due to low rainfall, which is needed to flush out naturally occurring alkaline salts.

In contrast, regions with more rainfall, like the Pacific Northwest and Southeast, tend to have more acidic soils.

How to Test Your Soil’s pH

• Use a pH test kit (good for occasional use; $6–$30 depending on features).
• Invest in a pH meter for long-term monitoring (accurate models cost $50–$70).
• Take 3–4 samples from 2–6 inches deep for accuracy.
• Mix soil with neutral-pH water (distilled is best) to avoid skewed results.
• Note that pH varies across your yard and throughout the year, especially near concrete or where fertilizers are used.

Adjusting Soil pH

Lowering Alkaline pH

• Use soil sulfur for best results—it’s fast, long-lasting, and pure.
• General rule: 1 lb per 10×10 ft area lowers pH by one point.
• Clay or organic-rich soils resist pH change; sandy soils adjust more easily.
• Re-test soil 2 weeks, 1 month, and 2 months after treatment.
• Apply sulfur cautiously during hot weather—start with 1/4 dose weekly. In cool seasons, apply full dose.

Raising Acidic pH

• Use finely ground agricultural lime following package instructions.

Always test the pH of any fertilizer or amendment before use.

For nutrient deficiencies (e.g., calcium, potassium, magnesium), consult a certified nursery professional.

Planting with pH in Mind

Group plants with similar pH needs together. For example, don’t plant acid-loving orchids next to alkalinity-loving alyssum.

Resources

• Denman & Co., 401 W. Chapman, Orange. (714) 639-8106
• Accugrow Test Kits: (800) 589-5551 or Accugrow.com
• Soil and Plant Laboratory, 1594 N. Main St., Orange. (714) 282-8777

What Can You Grow?

Plants that Prefer Alkaline Soil:

• Alyssum
• Asparagus
• Baby’s breath
• Bean
• Beet
• Cabbage
• Carnation
• Cauliflower
• Celery
• Cosmos
• Cucumber
• Coral bells
• Dianthus
• Iris
• Leek
• Melons
• Mint
• Nasturtium
• Okra
• Parsnip
• Pea
• Peach
• Peonies
• Phlox
• Rhubarb
• Salsify
• Spinach
• Squash
• Sweet pea
• Swiss chard
• Walnut

Plants that Prefer Acidic Soil:

• Azalea
• Basil
• Blackberry
• Blueberry
• Butterfly-weed
• Camellia
• Chrysanthemum
• Clematis
• Fir
• Flax
• Gardenia
• Heather
• Hydrangea
• Lupine
• Magnolia
• Marigold
• Oak
• Orchid
• Pine
• Potato
• Radish
• Raspberry
• Rhododendron
• Rose
• Strawberry
• Viola
• Yew