Soil pH is a measurement of the acidity or alkalinity of the soil, expressed on a scale from 0 to 14. A value of 7.0 is considered neutral, with numbers below 7.0 indicating increasing acidity. The scale is logarithmic, meaning a soil with a pH of 5.0 is ten times more acidic than a soil with a pH of 6.0. Most common garden plants thrive in a slightly acidic to neutral range, typically between 6.0 and 7.0. When soil becomes too acidic, falling below 5.5, it can prevent plants from accessing necessary nutrients (nutrient lock-up). Low pH also increases the solubility of elements like aluminum and manganese, which can become toxic to plant roots.
Determining Your Soil pH Level
The first step toward correcting soil acidity is obtaining an accurate measurement of the current pH level. Simple home test kits are available, offering quick results, but their accuracy varies widely and they do not provide information on the amount of liming material needed. The most reliable method involves sending a soil sample to a cooperative extension office or a commercial agricultural laboratory.
These labs perform detailed chemical analyses, providing a precise pH reading and a buffer pH reading. The buffer pH measures the soil’s resistance to change, which is essential for calculating the exact amount of material required to achieve the target pH. The target pH for most vegetables and ornamentals is generally between 6.5 and 6.8. If the lab reports a pH below 6.0, an amendment is usually recommended to improve nutrient availability, as elements like molybdenum and phosphorus decrease significantly in strongly acidic soil.
Primary Materials Used for Raising Soil pH
The most common material used to raise soil pH is agricultural limestone, which consists primarily of calcium carbonate. Ground limestone is available in two main forms: calcitic and dolomitic, and the choice depends on the soil’s existing mineral content. Calcitic limestone is mainly calcium carbonate and is suitable for soils that have sufficient magnesium.
Dolomitic limestone, however, contains both calcium carbonate and magnesium carbonate, making it the preferred choice if a soil test indicates a deficiency in magnesium. This dual-action material raises the pH while simultaneously supplying the plant nutrient magnesium. Another option is hydrated lime, which is calcium hydroxide; this material reacts very quickly to raise pH but is caustic and requires careful handling. Wood ash can also be used to raise pH, though its neutralizing power is generally lower than that of agricultural lime. It is best reserved for small, localized adjustments and requires a higher application rate.
Application Techniques and Timing
Successful pH adjustment begins with correctly calculating the required amount of liming material based on the soil test results and the soil’s texture. Clay soils and soils high in organic matter have a greater buffering capacity, meaning they require significantly more lime to change the pH than sandy soils do. Applying the correct amount is crucial, as over-liming can make micronutrients like iron and zinc less available to plants, causing new deficiencies.
The best time to apply liming materials is in the fall or early spring, as the process requires several months to be fully effective. Lime should be spread uniformly over the soil surface using a drop or rotary spreader. For maximum effectiveness, the material must be immediately incorporated into the top six to eight inches of soil using a tiller or spade. Surface application without incorporation is less effective because the lime reacts slowly and primarily affects only the top inch of soil. Since lime’s reaction time can take up to six months, subsequent pH measurement should be taken only after this time has elapsed.
Factors That Contribute to Soil Acidity
Soil naturally tends toward acidity over time due to several environmental and biological processes.
Leaching and Rainfall
One major contributor is heavy rainfall and irrigation, which cause the leaching of alkaline minerals, such as calcium, magnesium, and potassium, from the upper soil layers. The loss of these positively charged base cations allows acidic hydrogen ions to dominate the soil solution, lowering the pH.
Fertilizer Use
The type of fertilizer used in gardening also significantly influences soil acidity. Fertilizers containing ammonium, such as ammonium sulfate and urea, are acid-forming because the process of nitrification converts ammonium to nitrate, releasing hydrogen ions into the soil. Repeated use of these nitrogen-based fertilizers can accelerate the rate of soil acidification.
Organic Matter Decomposition
The natural decomposition of organic matter, including crop residues and plant roots, generates organic acids and releases carbon dioxide. The carbon dioxide dissolves in soil water to form carbonic acid, contributing to the overall acidity of the soil. While organic matter is beneficial for soil structure, its breakdown necessitates periodic pH monitoring and adjustment.