Agricultural lime is a soil amendment made from pulverized limestone or chalk, containing calcium carbonate or a combination of calcium and magnesium carbonate. While tomato plants do not inherently “like” lime, they benefit when soil conditions are overly acidic. Lime’s primary function is to neutralize acidic soil, and it also provides calcium. Application should always be based on soil test results, as adding it unnecessarily can be detrimental to plant health.
Optimal Soil pH for Tomato Growth
Tomatoes thrive in soil that is slightly acidic to neutral, with a preferred pH range between 6.0 and 6.8. This specific range is where the chemical balance of the soil is best suited for nutrient availability and uptake by the plants. When the pH is maintained within this narrow window, essential macronutrients like nitrogen, phosphorus, and potassium are easily accessible to the tomato roots.
A pH below this range causes nutrients to become less available, hindering growth. Conversely, a pH that is too high also leads to nutrient deficiencies. Optimal soil acidity fosters a healthy environment for beneficial soil microbes, which convert organic matter into usable plant food.
Determining the Need for Lime
The principal reason for applying lime is to raise the soil pH when testing reveals it is too acidic, meaning below 6.0. Soil tests provide the specific data needed to calculate the exact amount of lime required to adjust the pH into the optimal 6.0 to 6.8 range. This adjustment helps to unlock nutrients that would otherwise be unavailable in acidic conditions.
Lime is also a valuable tool for addressing calcium deficiencies, which manifest in tomatoes as Blossom End Rot (BER). BER is a physiological disorder where the bottom of the fruit turns black and leathery due to insufficient calcium. The choice of lime depends on other nutrient levels; calcitic lime supplies calcium, while dolomitic lime provides both calcium and magnesium. Dolomitic lime should only be chosen if the soil test indicates a concurrent magnesium deficiency, as excess magnesium can interfere with the uptake of other nutrients.
Potential Harm from Excessive Liming
Applying lime when the soil pH is already neutral or alkaline (above 7.0) is detrimental to tomato plants. Over-liming drives the soil pH too high, leading to a condition known as nutrient antagonism or “lock-up.” This high alkalinity renders many essential micronutrients chemically insoluble, preventing them from being absorbed by the roots.
Micronutrients such as iron, manganese, and zinc become trapped in the soil structure, leading to deficiencies despite their presence. These deficiencies often present as interveinal chlorosis, a yellowing between the veins of the leaves, particularly on new growth. Since tomatoes require a slightly acidic environment for optimal nutrient absorption, excessive liming can severely stunt growth and yield.
Applying Lime Safely and Effectively
The most effective application of lime is strictly guided by the recommendations provided in a soil test report. This report dictates the type of lime to use and the exact application rate needed to achieve the target pH. Because lime is slow-acting, the ideal time for application is in the fall, allowing several months for the material to react with the soil chemistry before spring planting.
The lime should be evenly spread over the soil surface and thoroughly incorporated into the top six to eight inches of soil. This mixing ensures the lime contacts a large volume of soil particles, promoting a uniform change in pH throughout the root zone. If the soil pH is optimal but the plant suffers from BER, an alternative is to use gypsum, which supplies calcium without significantly altering the existing pH level.