Conversion rates for liming. What is lime?
Lime is calcium or magnesium compounds ground into powder. Usually the calcium and magnesium come from limestone rock. They could come from marl, oyster shells, or industrial byproducts. Lime makes soil pH higher.

How do I know if I need lime? The best, simplest, and surest way is to take a soil test. Then compare the pH on the soil analysis report with the ideal pH for the crop you want to grow.
There are indicator plants that grow better in acidic soils. When present, these plants indicate lime is needed. Broomstraw and chickweed are local examples. High organic matter will make some indicator plants less reliable. Even if the indicator plants are reliable, you still need to take a soil test to determine the rate of lime needed.
At one time farmers literally tasted the soil. They described soil as "sour" when the pH is too low, "bitter" when the pH is too high and "sweet" when the pH is suitable for good crop growth. If you wish to use this method, you will still have to rely on soil test until you have calibrated your taste buds.

Why do I need lime in acidic soils? Lime will raise the pH which improves the availability of essential plant nutrients. When soils become too acidic, beneficial bacteria are less active, certain non-essential elements like aluminum become more toxic, and some essential elements are chemically unavailable for the plant.

Is lime an essential plant nutrient? While calcium or magnesium is essential for plant growth, lime is applied at higher levels than will be used by the plant for the purpose of raising the pH.

How much lime should I use?
The only way to determine the amount of lime needed is through a soil test. The amount of lime will vary depending on initial soil pH, other soil characteristics, and crop you are planning to grow. Sometimes local experts will have developed rules of thumbs. On clay soils in Piedmont North Carolina 2 tons lime per acre is a good starting point. In sandy soils on North Carolina Coastal Plains, 1/2 ton per acre is a good guess.

Should I use pelletized lime or powdered lime?
Adding water soluble resin to powder lime forms pellets. This is called pelletized lime Pellets are easier to apply and less messy while powdered lime is cheaper. Use powder when tilling the lime under. For other applications, choose between low cost and less mess.

What is agricultural lime?
In common usage, agricultural lime refers to limestone rock ground into powder and doesn't include burnt lime (a.k.a. quicklime) or hydrated lime (a.k.a. slaked lime).

What is burnt lime?
When lime is heated, it becomes burnt lime. This increases the reactivity of the lime. This material can be used when plants are not present but can burn plant roots. It is also more disagreeable to handle. Most gardeners will use agriculture lime instead.

What is quick lime?
The same thing as burnt lime.

What is hydrated lime?
When water is added to burnt lime, the lime forms hydroxides. Gardeners calls this hydrated because we don't know if hydroxidated is a real word. This material is bulkier than agriculture lime. It can be used when plants are not present but can burn plant roots. It is more caustic than burnt lime. Most gardeners will use agricultural lime instead.

What is slaked lime?
The same thing as hydrated lime.

What is bog lime?
A soft impure calcium carbonate found under organic soils. It may have formed when the area was underwater from the shells of mollusk or when certain aquatic plants caused calcium to precipitate. It can supply lime but is more slowly available than pulverized limestone.

What is marl?
Often used to refer to bog lime. Technically, a lime/clay deposit. It can supply lime but is more slowly available than pulverized limestone.

What is shell marl?
A high calcium clay containing shells. It can supply lime but is more slowly available than pulverized limestone.

Can I use burnt lime or hydrated lime for gardens or landscape?
It can be used when plants are not present but can burn plant roots. Both are more caustic and disagreeable to handle than agricultural lime. Most gardeners will use agricultural lime instead.

What is feed grade lime?
This is calcitic lime used for animals. It can be used as agricultural lime but may not react as rapidly as lime specifically ground for adding to the soil.

Will the limestone rock used as filler in fertilize act as lime?
The large size of these particles prevent any meaningful soil reaction.

What does calcitic and dolomitic mean?
This refers to the major compounds found in the original rock. Limestone rock is either calcitic or dolomitic. Calcite is a calcium compound. Calcitic lime is mostly calcium. Dolomite rock contains at least 6 percent Magnesium oxide. So dolomitic lime contains calcium and magnesium.

Should I use calcitic or dolomitic lime?
You normally use dolomitic lime on sandy soils. Sandy soils don't have any way to hold magnesium or calcium. Both are needed for plant growth and should be added regularly. The only way to tell which lime you need to use on clay soils is with a soil test. Clay soils with high magnesium levels perform poorly. They will develop more cracks and have a tighter structure. These soils don't need additional magnesium. Look for the magnesium base saturation (Mg BS) percentage on a soil test. The ideal Mg BS is 10%. Clay soils with Mg BS over 20% should get calcitic lime. Local experts may have developed a rule of thumb for certain soils.

When should I lime?
Lime anytime the soil needs it.

Can I lime and fertilize at the same time?
Yes.

Why do so people say you shouldn't lime and fertilize at the same time?
They are concerned about the reaction between nitrogen and Calcium. However, lime reacts for up to three years so the reaction will still occur even if the applications are separated a week or a month.

What is gypsum?
Gypsum, is calcium sulfate. It doesn't change the soil pH. Applying gypsum to an acid soil (pH less than 5.5) can have adverse effects on certain crops by displacing soil aluminum, which is toxic to plant roots. Use lime until your pH is at the desired level. Then use gypsum to add more calcium.

Can gypsum be used instead of lime?
No. It doesn't change the soil pH. Use lime until your pH is at the desired level. Then use gypsum to add more calcium.

Does gypsum improve the soil?
On high sodium soils, gypsum will improve the structure of the soil without changing the pH. Most soils in North Carolina don't contain high sodium levels.

What is landplaster?
The same thing as gypsum.

Why is landplaster used on peanuts?
To provide calcium without changing the pH. Peanuts don't have the ability to translocate adequate calcium to the fruit so the pegs must touch calcium to form properly. Sandy soils don't hold calcium so land plaster (gypsum) puts the calcium there when it is needed. Clay soils have the ability to hold calcium so landplaster isn't needed on peanuts grown in clay soils.

Can you overlime?
Yes. On sandy soils it is easy to overlime. On clay soils it is so hard to overlime that many professionals consider it impossible. Rates higher than 12 tons per acre has been applied on heavy clay soils without causing problems. What are the symptoms of overliming?
Deficiencies of iron, zinc or manganese will create interveinal yellowing. Deficiencies of boron will kill the terminal bud and cause heart rot or hollow heart in vegetable stems. Copper deficiency varies but sometimes includes wilting of terminal leaves.

Why would a crop turn yellow after lime is applied?
If plants appear chlorotic after a lime application, low manganese is usually the problem. Applying lime raises the soil pH and reduces the availability of manganese. Apply manganese if the soil test indicates low manganese. Soils with adequate manganese levels should not show symptoms following lime application. A local expert may know if low manganese is a common problem for that location. Most soils in the piedmont and mountains have very high levels of manganese.

How often should lime be applied?
Apply lime only when recommended by a soil test. Sandy soils need to be limed more frequently than clay soil. Test sandy soils annually. Test clay soils annually until they reach the target pH, then every three years. Potting media also needs frequent liming.

How fast does lime react?
Lime starts reacting immediately if moisture is present but it takes several months for the total benefit to occur.

Can wood ashes be used as lime?
Wood ashes will adjust the pH upward like lime but will not provide calcium. In addition, wood ashes should be limited to no more than 20 lbs per 1000 square foot. This equal the neutralizing value of 10 lbs of lime. Wood ashes are more caustic than lime.

Can oyster shells or other sea shells be used for lime.
Yes. They will work fine if finely pulverized. The typical oyster shells used for chickens are so large that only limited reactions occur. The lime can also be released by burning the shells

Can phosphorus sources be used as lime?
Phosphorus will adjust the pH upward like lime but will not provide calcium. Phosphorus is normally not used to adjust pH because it is relatively expensive and high phosphorus can also limit beneficial fungi.

What is pH.
The negative log of the concentration of Hydrogen ions. In practical terms it measures acidity on a scale of 1 to 14. 7 is neutral. Less than 7 is acidic while over 7 is alkaline. Soil acidity varies due to parent material, amount of rainfall, type of plants and agricultural practices. Soils in hot and humid areas are typically acidic. Some dry climates are alkaline. Local experts should know if the soil is normally acidic or alkaline.

What pH is ideal?
Different crops require different pH's for optimum growth. The majority of plants prefer soil between 6.0 and 7.0 pH. A major exception is the Ericaceae plants such as azaleas, rhododendrons and blueberries.

What does sweet soil and sour soil mean?
Sweet means a low acid level. There is no technical definition but this would correlate with a pH of 6 to 7. The term sour is used two ways. A low pH soil is called a sour soil. In addition, poorly drained soil with anaerobic decomposition is also called sour.

What does it mean when lime flocculates the soil?
This is a physical change when the calcium molecules help separate the layers of silicon clay. Flocculation is more obvious on soils with high cation exchange capacity. Magnesium molecules nest inside the silicon layers and doesn't flocculate the soil.

Should I lime the compost pile?
Most gardeners don't because lime, in the presence of organic acids, drives off nitrogen and reduces the nitrogen value of the compost. Lime may increase bacteria action by adjusting the pH but is not necessary for proper composting. Some gardeners like the convenience of spreading only one product instead of spreading both lime and compost but a mixed product reduces your flexibility to varying rates of lime.

Conversion rates.
1 acre = 43,560 square feet
1 ton = 2000 lbs
1 ton per acre = 45 lbs per 1000 square feet
2 ton per acre = 90 lbs per 1000 square feet

If you have additional questions please contact David Goforth

Return to Master Gardener Diagnostic Guide