Liming Your Soil and Soil pH

— Written By and last updated by Leeann Crump
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If you are a gardener – growing roses, a lush green lawn, or a vegetable garden – you have probably heard the admonition to “lime your soil” to raise the soil pH. What is lime and what exactly is happening in the soil that raises the pH? And what the heck is pH anyway?

Soil pH is a way to express soil acidity, which refers to the number of hydrogen and aluminum ions in a soil. On a scale of 1 to 14, pH below 7 is acidic and above 7 is basic or alkaline; 7 is neutral. It is a logarithmic scale, meaning that soil with a pH of 5 is ten times more acidic than soil with a pH of 6, and 100 times more acidic than soil with a pH of 7. Aluminum and hydrogen are acidifying to the soil and by applying limestone (“liming”), aluminum ions are knocked off the soil and allowing basic ions such as calcium, potassium, and others that support plant growth to attach to the soil.

I once taught a gardening class and was explaining the importance of liming the soil. A participant asked “how many limes did it take” and how were they applied to the soil? It was a great moment – it reminded me not everyone is familiar with gardening concepts. Unlike making a gin and tonic, “liming the soil” requires no limes: it is the process of taking a limestone product (many different forms – more on that in a bit) and applying it to the soil to reduce soil acidity. On a basic level, there are two types of liming materials: one is calcium carbonate, the other magnesium carbonate, often called dolomitic lime. Dolomitic lime also provides magnesium, and should be used if magnesium levels are low in the soil. Most sandy soils are low in magnesium so generally dolomitic lime should be used in our area. The effect of liming is temporary, and so should be done every two or three years (more frequent in sandy soil, less in clay). Factors to consider when choosing a liming material include the fineness of particles, indicated by mesh size (smaller particles are better: 40-50 mesh works better than 8-20 mesh) and the purity of material, called Calcium Carbonate Equivalent (CCE). Materials that claim to be “twice as effective as lime – only need half as much!” should be looked at closely. Liming materials are required to list the “Effective Neutralizing Value (ENV)” and CCE on the label – verify this effectiveness before using the material. Any number less than 100% means the material is actually less effective than pure lime, and the amount of material for application should be adjusted accordingly.

Generally speaking, soil in NC is acid, due to the age of the soil (very old) and the natural weathering process (calcium, magnesium and other nutrients leach from the soil). In the Sandhills, our native soils pH can be quite low, in the 4s; blueberries like acidic soil, around this level, and plants like azaleas and rhodendrons also like a lower pH, around 5 – 5.5. Centipedegrass likes a lower pH, around 5.5, but other turfgrasses perform better around 6.5. Most other garden plants including fruits and vegetables prefer 6.0- 6.5 soil pH. With the exception of those plants adapted to low soil pH, such as blueberries, plants are unable to access nutrients in the soil if the pH is too low, and a plant’s failure to thrive is often related to low soil pH.

There are many benefits of proper liming. Nutrient availability is improved for plants when the correct pH is achieved. Phosphorus is tied up by low soil pH; conversely, manganese is less available to plants if the soil pH is too high, and may be toxic if levels are too low. Plant root development is inhibited by high levels of aluminum and manganese in acidic soils, so plant growth is improved as soil pH is raised. Lime is also a good source of calcium, an essential plant nutrient. Calcium unavailability causes Blossom End Rot in tomatoes and other fruits. How to be sure you are applying the proper amount of lime? Take a soil test, to ensure you don’t apply too much or not enough. The Richmond County Extension office can provide information on taking your test ($4 sample charge through NCDA&CS lab through March). Fall is the best time of year to lime, but to paraphrase the old saying about the best time to plant a tree, if you didn’t lime in the fall the next best time is now. So, if icy winter weather has got you dreaming of your summer garden, plan to soil sample and lime according to recommendations to get the best start possible.