Soil Amendments and their Impact on pH

Not all soil is created equal. Many environmental factors can swing your soil’s pH out of balance one direction or the other. “pH” refers to potential hydrogen, or the hydrogen ion concentration of your soil. Using a scale from 1 to 14, it is a measurement of soil acidity. Neutral pH is 7.0. A soil with a pH lower than 7 is an acidic soil. A soil with pH higher than 7 is an alkaline soil. Most food crops prefer a pH of 6.0 to 6.5, but you can have a productive food garden as long as your pH is about 5.5 to 7.5.

Soil acidity determines the availability of mineral nutrients for your vegetables. In alkaline soils, phosphorous, iron, and zinc are limited. In acidic soil, calcium and magnesium are less available to plants.

Many areas of the country where the native soil is influenced by an abundance of rainfall have acidic soils because alkaline minerals like calcium are leached out. Also areas dominated by soils derived from weathered granite will be more acidic in nature. On the other hand, limestone and shale based soils are much more alkaline. Heavy yield crops can also turn soil more acidic by consuming the minerals keeping it in balance.

Unbalanced soil pH can limit what plants will be successful in your gardens. Some plants thrive in acidic soils while others prefer an alkaline environment. Ideally, though, we want to create a balanced soil pH so that our plants and soil are healthy. After testing your soil and determining its pH level, there are several ways of amending the soil to bring the pH back into balance.

List of Soil Amendments and Their Impact on pH:

Potash

Both people and plants need potassium. In plants it is essential for water uptake and for synthesizing plant sugars for use as food. It is also responsible for crop formation and quality. Commercial bloom foods contain high amounts of potassium to promote more flowers of better quality. The addition of potash in soil is crucial where the pH is alkaline. Potash fertilizer increases the pH in soil so should not be used on acid loving plants such as hydrangea, azalea and rhododendron. Excess potash can cause problems for plants that prefer acidic or balanced pH soils. It is wise to do a soil test to see if your soil is deficient in potassium before using potash in the garden

Potash doesn’t move in soil more than an inch so it is important to till it into the root zone of plants. The average amount for potassium poor soil is ¼ to 1/3 pound of potassium chloride or potassium sulphate per 100 square feet.

Compost

In general, the addition of organic material to soil will work to buffer your pH regardless of which direction your soil leans. Organic material like compost, natural mulch (wood chips) or cover crops plowed under will slowly decompose and introduce carbon and other trace minerals to your soil which will balance your pH levels.

However, the addition of organic matter to soil is an indirect form of pH balancing. In reality, what you’re doing by increasing the organic matter in your soil is giving your plants a higher tolerance for poor pH by increasing the bioavailability of nutrients they need.

Lime

The easiest, most common and most economical material to correct acidic soil is agricultural limestone. Liming the soil will bring up your soil’s pH from an acidic state by working to bring calcium to the forefront. As limestone dissolves it in the soil, calcium is brought to the surface of soil particles. The calcium carbonate reacts with the acidity producing carbon dioxide and water. The result is that the soil’s pH comes up and becomes less acidic.

Lime works slowly to correct pH in your soil and is generally part of a long-term soil management plan related to high-yield crops or naturally acidic environments. If using lime to correct your soil, be sure to test regularly over the span of a year or so to determine if the pH is heading in the right direction.

Sulfur

Elemental Sulfur reacts slowly to adjust pH in your soil. Soil bacteria convert the sulfur to sulfuric acid lowering your soil pH. Since it is a biological process it is slower than a chemical reaction. The process occurs when the bacteria are most active in moist warm soil and are not active in the winter. It has more acidifying capability than the other products and lasts for years. However because it reacts slowly and may take several months to work, it is recommended to add sulfur in the warm season before planting.

Sulfur should be worked into the soil at a depth of at least 6 inches to allow the soil sub-surface bacteria an opportunity to do their work.

Aluminum Sulfate

Aluminum sulfate can be used to lower pH. However, it is not recommended as a soil-acidifying agent because it can produce aluminum toxicity in plant roots.