Soil structure is one of the most important factors in growing a healthy garden. It affects drainage, plant nutrition and soil pH. Clay in soil has an important impact on pH. An understanding of how clay affects soil pH will help the gardener in improving soil quality and plant health.
The mineral content of soil is made from clay, silt and sand. Clay is the smallest particle of the three, making clay soils particularly compact and water-retentive. This is a problem for most garden plants, which require moist, well-drained soil rich in organic matter.
Effect on pH
Clay has the tendency to store acidity, releasing it slowly in soil. This is called buffered acidity. Active acidity is the acidity immediately available to plant roots. Because of this buffering, it takes greater quantities of alkaline amendments to raise the active pH of clay soil than it does to do the same for sandy soils.
Always get a soil test done before modifying soil pH. Your sample should consist of a mix of several samples taken from different parts of the garden or yard. Many state cooperative extension services provide soil testing services. Request a test of soil structure and pH.
Amend acidic clay soils with pulverized limestone (lime) or dolomitic limestone (dolomite). Modify clay soils with iron sulfate or aluminum sulfate. Soil test results often include recommendations for the amount of material you should use to modify soil to your target pH.
Soil pH changes over time as plants consume nutrients, water washes through soil, and organic materials are added or depleted in the soil. Maintain acidity by mulching annually, which adds acidic material as it breaks down. Maintain alkalinity by mulching sparingly to minimize acidification. Test your soil every three to five years and amend pH as recommended.