Everything A Gardener Needs to Know About Lime
By Gil Medeiros
If you are a gardener or homeowner who maintains a lawn, it is very likely you have used lime, but how much do you know about it? Let’s see if we can expand your knowledge.
1. What is lime?
There are a lot of materials with the word “lime” in the name: slaked lime, burnt lime, builders lime, key lime pie, to mention a few. Except for the juices from the tart citrus fruit, these materials hold in common that they are compounds of the element calcium.
The topic of this piece is agricultural lime. It is made from limestone, a rock composed mainly of a mineral called calcite. Calcite is calcium carbonate. The rock is ground-up or pulverized to a very fine powder — the finer, the better (more explanation given under question six).
Limestone is a grainy rock formed by the sedimentation of marine animal skeletons from eons ago. It is not really a renewable resource, but no one is claiming we will run out of it any time soon.
Limestone is also an important building material, the same stuff used to build the Lincoln Memorial, the Empire State Building, cathedrals, pyramids and many more public structures. Indiana is famous for its production of limestone slabs for construction.
2. Why do we use agricultural lime?
If you are kind to your plants, you want their soil to be in the proper pH range. pH is a measurement of the acidity or alkalinity of your soil. If the measurement is below 7, the soil is acidic. (For more on pH see Phenomenal pH Phacts, Fairfax Gardening, April 2017.)
If the pH is above 7, the soil is said to be basic or alkaline. Most plants like the pH slightly acidic –between 6 and 7. This is the happy zone. When the pH is in this zone, the plants can absorb the vital nutrients necessary for their health.
In our area it is not unusual to find soil pH between 5.0 and 5.9. Unless we are growing acid-loving azaleas or blueberries, we need to raise that number above 6 but below 7. We do that by adding agricultural lime. Calcium ions in the lime displace the hydrogen ions that make the soil acidic.
How does the soil become so acidic? Often our urban soils have diminished the ability to retain nutrients because the soil is so low in organic matter. Rain leaches out the calcium carbonate, which is moderately soluble in acidic water, a little bit at a time. We need to add calcium back to the soil occasionally to replace what has been lost.
Calcium not only adjusts pH upward, it is a plant nutrient. Calcium is important to the formation of cell walls in plants and therefore to cell growth.
Calcite-based agricultural lime contains some magnesium, so lime raises the magnesium level in soil, too.
3. How do we know how much lime to add?
Not by the calendar… it is a mistake to say, “It’s October! Time to add lime to my lawn!”
We ALWAYS perform a soil test before adding agricultural lime. Too much alkalinity in the soil is just as bad as too much acidity, and it is a more difficult problem to correct.
The soil testing lab not only measures the pH, they measure the buffering capacity of the soil. The latter measurement tells them how much lime this soil needs to change the pH. In combination with information you provide about what you want to grow in that space, the lab can tell you how much lime to add to reach the target pH level.
4. What’s the difference between agricultural (calcitic) lime and dolomitic lime?
Dolomitic lime contains the mineral dolomite, which is calcium magnesium carbonate. By law, agricultural lime sold in the Commonwealth of Virginia must contain a minimum of 85 percent by weight calcium carbonate. The remaining 15 percent is clay, magnesium carbonate and other minerals. To be labeled as dolomitic lime, the material must contain a minimum of 15 percent magnesium carbonate; the remainder is mostly calcium carbonate.
dolomitic(left), calcitic (center), and pelletized (right)
You should be aware, however, that the use of dolomitic lime as a soil amendment is a bit controversial in gardening circles. Dolomitic lime neutralizes slightly more acidity than calcitic lime and provides the added benefit of increasing the magnesium level. There is a school of thought that says dolomitic lime should be used whenever there is an acidity problem to be corrected. The proponents contend that the magnesium is essentially free. However, there is a competing theory that the ratio of calcium to magnesium affects soil structure. The proponents of the competing theory say that the ideal ratio is 7 : 1 calcium to magnesium. As the magnesium level gets too high relative to calcium, the soil particles tend to stick together and the soil becomes prone to compaction. The problem, once caused, is difficult to correct.
The safe approach is to follow the guidance of the soil testing lab. The addition of lime — whether calcitic or dolomitic — should not be done without a soil test. If you need magnesium, the soil test will tell you that and the best way to add it to your soil. Thus, the consequences of adding minerals that are already in abundance will surely be avoided.
5. Is lime the same as gypsum?
No, there are similarities in the way they are used as soil amendments, but they are not the same, and they are definitely not interchangeable. Gypsum is calcium sulfate, the main component of plaster. It is used as a soil amendment, usually in combination with organic matter, to loosen very heavy clay soil. Gypsum is not effective at raising soil pH so it is not used for that purpose. However, gypsum is used to correct for calcium deficiency when the pH of the soil is already in the desired range.
6. How fast does lime work?
The speed of raising pH is governed by several factors, but the important factor that is a characteristic of the lime itself is the particle size distribution of the crushed limestone. Remember this is mined from the earth as rock. It must be ground up or crushed to a powder so that it can be spread and dissolve in a reasonable period of time. The finer the particles, the more surface area is exposed to water, and the faster the lime dissolves.
For pulverized (crushed) agricultural lime, which is commonly sold to consumers for backyard and garden use, Virginia law specifies that 95 percent of it must pass through a coarse 20-mesh (20 wires per inch) screen and 70 percent must pass through a fine 100-mesh (100 wires per inch) screen. Research has shown:
The 5 percent that does not pass through the 20-mesh screen may take more than four years to dissolve completely. This portion of the lime is essentially useless as a soil amendment.
The 25 percent that passes through the 20-mesh screen but not the 100-mesh screen takes one to four years to dissolve.
The 70 percent that passes through the 100-mesh screen is the best stuff; it will dissolve in under one year. Although the law does not require further screening, some of the particles that pass through the 100-mesh screen will also pass through the 200-mesh screen. This fraction of the lime will begin to work immediately if it is watered in.
So, the answer is that agricultural lime — AKA Aglime — sold in Virginia will begin to work immediately, assuming it is watered in. It will complete most of its work in raising pH to the desired level within one year. It will continue to work slowly for the following two years or so. This explains, at least in part, why we recommend that soil tests be conducted every three years.
7. I notice there are a few choices at the store. What kind of lime should I buy?
Generally, the home improvement centers offer two or three choices:
- Pulverized agricultural lime: 50 to 60 lb. bags, costing about $6 per bag;
- Pelletized agricultural lime (sometimes called lawn lime): 40 to 50 lb. bags, costing about the same as agricultural lime;
- Pelletized lime: labelled as “fast acting” or “plus AST” costing 5 to 10 times more per pound than the pulverized lime but claiming that you need only one fifth as much as other pelletized lime products.
The best choice for garden applications is the inexpensive agricultural lime. Virginia law mandates that this is a quality product. The lime prescriptions in your Virginia soil test are based upon the specifications in Virginia law.
The best choice for lawn application is the pelletized agricultural lime because it provides ease of spreading with either a rotary spreader or a drop spreader. It is finely pulverized lime that has been converted to pellets that are held together by an organic binder. The pellets are well-behaved in the spreading process; they will not produce a dust storm even on windy days. The binder breaks down after spreading and releases the fine particles to the soil. The pellets work slightly more slowly than the pulverized lime.
The more expensive pelletized products offer the advantage of faster action because essentially all of the lime passes through a 100-mesh screen. Although research on this speed advantage conducted by several states has produced inconsistent results, some farmers use it in spite of its higher cost, a very strong endorsement. For the home gardener, ease of spreading with an inexpensive rotary spreader is also an advantage.
The claim that these expensive products can significantly reduce the quantity of lime that is used is a bit of a stretch. Because these products are purer in terms of calcium carbonate than Virginia requires for Aglime, there can be a savings of up to 18 percent by weight. For example, if the soil test shows that you need 100 pounds of Aglime to raise your pH from 5.5 to 6.5, you may need only 82 pounds of the expensive stuff, hardly worth the difference in cost.
8. What is the best way to apply lime to a garden bed?
For gardens, apply the lime as uniformly as you can. After measuring the area where I am applying the lime and calculating how much I need to meet the prescription of the soil test, I measure out the amount. Then I usually spread it by hand and rake it to improve the uniformity. I have always been a soil tiller, so I mix the lime into the top six inches of the soil by digging or cultivating. This gets the benefits of the lime to the level of the soil where the plant roots will be, and it increases lime-to-soil contact, which will speed the process of altering the pH.
For those of you who adhere to “no till” gardening, you might rake the lime into the top inch of the soil and leave it at that. Understand that it will take longer for the effects of the lime to reach the roots of your plants.
Finally, add water. The lime does nothing without water. It will begin to dissolve the calcium carbonate and free up calcium ions to raise the pH or address a calcium deficiency.
9. What is the best way to apply lime to a lawn?
A drop spreader works best for pulverized lime. A rotary spreader works very well for the pelletized lime.
10. What is the maximum amount of lime I should apply at one time?
Do not apply more than 50 pounds per 1,000 square feet in a single session. If your soil test calls for 80 pounds per 1,000 square feet, apply 50 pounds in the first application. Wait at least 30 days, and apply the remaining 30 pounds.
11. When can I put down lime?
You can spread lime any time as long as the ground is not frozen or covered by snow. Therefore, winter is not an optimum time for spreading lime. Neither is summer because summers tend to be dry in our region. Lime needs water to dissolve and neutralize soil acidity.
Consider when you want the soil pH to reach a target level. Add lime about six months prior to that date. That gives it time to do its work. The pH will not be at the target level for several more months, but it will be on its way. Fall is a great time to apply lime because it will have done most of its work before the Spring growing season. However, if a soil test shows you have a severe acidity problem, it is wise to start as soon as possible.
If the forecast calls for heavy downpours later in the day, you would be wise to avoid putting lime on the surface of the soil. The downpours may wash away the material. If the lime is worked into the soil, application should not be a problem ahead of storms unless the area is prone to erosion.
References
Calcitic Lime vs, Dolomite Lime, Baker Lime Corp
The Myth of Gypsum Magic, Linda Chalker- Scott
Changing the pH of Your Soil, Clemson Cooperative Extension
Soil pH and Lime Requirements for Home Grounds Plantings, Rutgers Soil Testing Laboratory
Soil pH and the Home Landscape or Garden, University of Florida IFAS Extension publication # SL 256
Sources of Lime for Acid Soils in Virginia Virginia Cooperative Extension # 452 510
Pelletized Lime: How Quickly Does It React? University of Kentucky Cooperative Extension
“Pelletized Lime in Michigan” Michigan State University Cooperative Extension CSSIS#1
“Facts about Soil Acidity and Lime” Michigan State University Extension Bulletin E 1566