Soil Acidity and Liming: The Key to Successful Planting

Have you ever wondered why some plants thrive while others struggle? The answer lies in the soil’s acidity level. Soil acidity plays a crucial role in determining the health and growth of plants, and as gardeners and farmers, it is essential to understand the impact of pH levels on different crops.

The Optimum pH for Plant Growth

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It’s important to note that the optimum pH for plant growth varies depending on the crop and the type of soil. For instance, in most Midwestern soils, crops grow best at a pH of 6.5 to 7.0. However, these values can cause micronutrient deficiencies in certain regions like North Carolina. As pH increases, many micronutrients become less soluble, reducing their availability to plants. This often leads to manganese deficiencies in North Carolina soils after overliming.

To achieve the best results, farmers and gardeners in North Carolina target a pH of 6.0 for most commonly grown field crops. However, it’s crucial to consider that the state also has a significant acreage of organic soils in the eastern region. Organic matter ties up aluminum, allowing plants to thrive at lower pH levels compared to mineral soils. For mineral-organic soils, the target pH is 5.5, and for organic soils, it is 5.0. The criteria used to determine these soil class distinctions are the amount of humic matter and soil density.

The Importance of Accurate Soil Testing

It’s worth noting that different soil laboratories may use various testing methods tailored to their specific soil conditions. In North Carolina, the NCDA&CS laboratory reports both the soil pH and the “Ac value.” The “Ac value” measures the exchangeable acidity, which includes exchangeable aluminum and hydrogen cations. Both the soil pH and the Ac value are necessary to calculate lime applications accurately.

While portable soil test kits provide a rapid pH measurement, it’s insufficient for making accurate lime recommendations. If you submit soil samples to other soil test laboratories, be sure to inquire about their methods and target pH assumptions for lime recommendations. Taking this extra step will ensure that you receive the most reliable and tailored guidance for your specific needs.

Understanding Plant Tolerance to pH Levels

Different plants have varying levels of tolerance to low pH levels. Optimum pH values for plants can range from 4.5 to 6.5. For example, blueberries, azaleas, and native ornamentals thrive in highly acidic soils, while crops like alfalfa, cotton, and tomatoes prefer lower acid soils with higher pH levels.

To help you with this, we’ve compiled a list of target pH values for various North Carolina plants on mineral soils:

  • Field crops: 6.0
  • Vegetables: 6.0-6.5
  • Small fruits: 4.5-6.0
  • Forage grasses: 6.0-6.5
  • Forage legumes: 6.0-6.5
  • Lawns/gardens: 5.0-6.5
  • Nursery: 5.0-6.5
  • Trees/Orchards: 5.5-6.5

Please note that these target pH values can differ for mineral-organic and organic soils. In such cases, the pH targets are lower, at 5.5 and 5.0, respectively, for crops that have an optimum pH of 6.0-6.5 on mineral soils.

Calculating Lime Applications

The NCDA&CS soil test uses a specific equation to calculate the amount of lime needed to achieve the target pH for a particular soil class and crop combination. The equation is as follows:

Lime (tons/acre) = Ac * [(target pH - current pH) / (6.6 - current pH)] - RC

To convert the results to pounds per 1,000 square feet, divide the recommended number of pounds of lime per acre by 43.5. The Ac value and target pH have already been discussed, while the current pH refers to the pH of the analyzed sample. “RC” stands for “residual credit” given to recently applied lime that may not have fully reacted. The RC value depends on the soil class and the timing of lime application.


Understanding soil acidity and proper liming practices are essential for farmers and gardeners alike. By considering the specific needs of different crops and accurately testing soil pH, you can create an optimal growing environment for healthy and thriving plants. Remember, achieving the right balance is key to successful planting!

For more expert gardening tips and insights, visit Tips Tree Planting. Let us help you embark on a journey to productive and flourishing gardens!

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