How Does Soil pH Affect Plant Germination?

Definition of pH

• Soil pH ranges from 0 to 14 with 7, being a neutral pH. Soil pH is defined as the negative logarithm measure of hydrogen ion concentration: The increase of hydrogen ions in soil increases soil acidity, bringing the pH to a negative number. Acidic soil has a reading of pH 7.0 to 0 on the scale, and alkaline soil has a reading of 7 to 14 pH. Acidic soil is sometimes referred to as sour and alkaline as sweet. Most plants germinate, grow and thrive in the pH range of 6 to 7.

Seed Germination

• A seed protects a new plant in a resting, or dormant, condition. Growth begins again at germination and continues until a sprout reaches through soil. Seeds need water, oxygen, correct temperature and soil nutrients to germinate. Extreme high and low temperatures affect the success of germination. If the seed is planted before germinating, then planting depth, watering and availability of nutrients in the soil, also play leading roles. The pH factor in soil directly affects nutrient and mineral availability. Extreme pH levels can adversely affect plants; for example, the calcium, phosphorus and magnesium are less available to seeds in low pH soils, while aluminum and manganese are available at toxic levels in high pH soils.

Soil Testing

• Soil fertility changes throughout the growing season and year to year. Test soil with a home test kit or at a local county agency each year in the fall after growing season. This gives an accurate picture of what nutrients need to be added before next year’s garden when seed germination is important again. Test soil when it is dry. Standard soil tests measure levels of available plant nutrients and possible toxic elements, as well as pH and lime levels. Other soil tests measure soil structure, organic matter content and microorganism biomass populations in addition to pH.

Adjust pH Levels

• Achieving the near-neutral neutral pH level of 6 to 7, which is best for seed germination, may require adding amendments to the soil. Limestone is most frequently added to soil to make it less acidic, raising its pH. Wood ashes reduce soil acidity because they contain fairly high amounts of potassium and calcium. Soil acidity can be increased by the addition of elemental sulfur, but the effect is temporary. Mature compost, whether acidic or alkaline, brings soil to a neutral pH level.