Gardeners in North Carolina know that our Old North State soils are naturally too acidic to grow a vegetable garden without lime.
While there is great benefit in applying lime, too much lime can be harmful, whether applied at one time or over multiple occasions.
Dr. David Hardy, Soil Testing Section chief with the N.C. Department of Agriculture and Consumer Services Agronomic Services Division, offers the following review of lime benefits, plant requirements and tips to help home gardeners determine lime application needs.
Lime supplies plants with calcium or magnesium nutrients, depending on the type. Calcitic lime provides only calcium, while dolomitic lime provides magnesium and calcium. Dolomitic lime is preferred in more sandy soils where magnesium may be low without its addition.
Lime, regardless of type, when mixed in soil performs similarly to old fashioned Tums in your stomach, neutralizing acid when it is too great, Hardy said. Most gardeners relate acidity to the pH scale of 0 to 14, with 7 being neutral; pH below 7 is acid while that above 7 is referred to as alkaline or basic. Lime is added to soil to raise pH and decrease acidity, he said.
A proper pH for a vegetable garden is 6.5, although most vegetable plants, with the exception of tomatoes, grow well at a pH of 6.0. Tomatoes need more calcium than other vegetables, which can be supplied through a higher pH level with a greater addition of lime. By increasing pH, the availability of many nutrients is increased and the detrimental effects of too much acidity on plant root growth and function is eliminated. A proper pH also helps beneficial bacteria and microorganisms function.
If lime is good, then many people think more must be better, but that is incorrect, Hardy said. If lime is applied every year, or at rates much higher than recommended by a soil test, the benefits can be lost. Some nutrients are needed in very minute amounts and are referred to as micronutrients. Two examples are manganese and iron. If pH becomes too high, micronutrient availability can become limited and poor or abnormal growth can result. When availability is severely limited, yellowing of plants, often between the veins in leaves occurs.
Once lime has been applied, soil pH should be stable for two to four years depending on soil type. Sandy soils with light color become acidic faster than fine clay textured soils. To increase pH, sandy soils generally require lower rates of lime, Hardy said.
“The best rules of thumb are to not apply lime yearly to a vegetable garden and never apply lime without a current soil test,” Hardy said.
More detailed information about lime needs for homeowners is available at content.ces.ncsu.edu.