Soil pH is a measure of how acidic or alkaline the soil is. It affects the availability of nutrients to plants, as well as the activity of soil microorganisms and the growth of weeds. Different plants have different preferences for soil pH, and some may not thrive or even survive in soils that are too acidic or too alkaline. Therefore, it is important for gardeners to know the pH of their soil and how to adjust it if needed.
What is Soil pH and Why Does It Matter?
Soil pH is a scale that ranges from 0 to 14, with 7 being neutral. The lower the pH, the more acidic the soil is, and the higher the pH, the more alkaline the soil is. Soil pH affects the solubility of minerals and nutrients in the soil, which in turn affects how much plants can absorb them. For example, iron, manganese, zinc, and copper are more available to plants in acidic soils, while calcium, magnesium, phosphorus, and molybdenum are more available in alkaline soils.
Soil pH also influences the activity of soil microorganisms, such as bacteria, fungi, and earthworms. Some microorganisms prefer acidic conditions, while others prefer alkaline conditions. Microorganisms play a vital role in decomposing organic matter, recycling nutrients, and improving soil structure.
Additionally, soil pH affects the growth of weeds. Some weeds are more tolerant of acidic or alkaline soils than others and may outcompete desirable plants if the soil pH is not optimal. For example, dandelions, plantains, and clovers tend to grow well in acidic soils, while chickweed, bindweed, and thistles tend to grow well in alkaline soils.
How to Test Soil pH
Before attempting to raise or lower soil pH, it is essential to test the current pH level of the soil. There are several methods to test soil pH, such as using a commercial kit, a digital meter, or a laboratory analysis.
- A commercial kit usually consists of a color chart and a chemical solution that reacts with a soil sample. The color of the solution indicates the approximate pH level of the soil.
- A digital meter measures the electrical conductivity of a soil sample mixed with water or a buffer solution. The meter displays the exact pH value of the soil.
- Laboratory analysis involves sending a representative soil sample to a professional lab that can provide an accurate measurement of soil pH as well as other parameters such as nutrient levels and organic matter content.
The best time to test soil pH is in the spring or fall when the soil is moist but not wet. To collect a representative soil sample, take several subsamples from different locations and depths (usually 4 to 6 inches) in the area you want to test. Mix them together in a clean container and remove any stones, roots, or debris. Use about one cup of this mixture for testing.
How to Raise Soil pH
If your soil test indicates that your soil is too acidic for your plants’ needs, you can raise the pH by adding a liming material. Liming materials are substances that contain calcium or magnesium carbonate, which react with acid in the soil and neutralize it. The most common liming materials are:
- Agricultural lime: This is finely ground limestone that contains mostly calcium carbonate. It is widely available and relatively inexpensive.
- Dolomitic lime: This is similar to agricultural lime but also contains magnesium carbonate. It is recommended for soils that are low in magnesium.
- Wood ash: This is a by-product of burning hardwoods that contains calcium carbonate as well as potassium and other trace elements. It is an organic alternative to lime but may be less consistent in quality and availability.
The amount of liming material you need to apply depends on several factors, such as:
- The current pH level of your soil
- The target pH level for your plants
- The type and quality of liming material you use
- The depth and texture of your soil
- The frequency and intensity of rainfall in your area
A general rule of thumb is that you need about 5 pounds of agricultural lime per 100 square feet to raise the pH by one unit in sandy soils, 10 pounds in loamy soils, and 20 pounds in clayey soils. However, this may vary depending on the specific conditions of your soil. Therefore, it is advisable to follow the recommendations from your soil test or consult an expert before applying any liming material.
The best time to apply liming material is in the fall or winter when the soil is moist but not frozen. This allows enough time for the lime to react with the soil before planting season. To apply liming material evenly, use a drop spreader or a rotary spreader and follow the manufacturer’s instructions. After spreading, lightly till the soil to incorporate the lime into the top 4 to 6 inches of soil.
How to Maintain Soil pH
Once you have raised the pH of your soil to the desired level, you need to monitor and maintain it regularly. Soil pH can change over time due to factors such as:
- Leaching of nutrients by rainfall or irrigation
- Decomposition of organic matter by microorganisms
- Application of fertilizers, pesticides, or compost
- Crop removal or residue incorporation
To prevent soil pH from dropping too low, you can:
- Test your soil every 2 to 3 years and apply liming material as needed
- Use fertilizers that are low in nitrogen or contain ammonium sulfate, which tends to acidify the soil
- Add organic matter such as compost, manure, or mulch, which can buffer the soil and provide nutrients
- Grow crops that are tolerant of acidic soils, such as potatoes, beans, peas, or blueberries
- Rotate crops with different pH preferences to balance the soil
Raising soil pH is not a difficult task, but it requires some planning and patience. By testing your soil, choosing the right liming material, applying it properly, and monitoring your soil regularly, you can create a healthy and productive environment for your plants.