How does the alkalinity of concrete affect surrounding plants or soil pH?

The alkalinity of concrete can significantly influence the pH of surrounding soil, potentially harming nearby plants. Fresh concrete has a high pH (often 12-13) due to calcium hydroxide and other alkaline compounds. When rainwater or moisture interacts with concrete, alkaline runoff can leach into the soil, gradually raising its pH.

This pH shift creates unfavorable conditions for many plants, particularly acid-loving species. High alkalinity reduces nutrient availability (especially iron, manganese, and phosphorus) while increasing toxic elements like aluminum. Symptoms in plants include chlorosis (yellowing leaves), stunted growth, and root damage.

The effect depends on factors like concrete age (new concrete is more alkaline), rainfall patterns, and soil composition. Sandy soils are more vulnerable to pH changes than clay soils. Over time, carbonation reduces concrete's alkalinity as it reacts with atmospheric CO₂.

Mitigation strategies include:

- Creating physical barriers between concrete and soil

- Using acidifying fertilizers

- Selecting alkaline-tolerant plants near concrete structures

- Allowing concrete to weather before planting nearby

Regular soil testing helps monitor pH changes, and amendments like sulfur can counteract alkalinity when necessary. Understanding these effects is crucial for landscape planning near concrete structures.