What is Lime?
While there are several types of products that can be used to provide Calcium and Magnesium to the soil that go under the general heading of “lime”, this article is going to concentrate on two varieties of one type – “ground limestone”. Other types, such as “ag lime”, “slaked lime”, hydrated lime” and “burnt lime” are difficult to handle, caustic, and hard-to-find. It has been estimated that 95% of the lime sold in the US for lawns is ground limestone, anyway.
Ground limestone is mostly Calcium Carbonate, and in areas where it is mined ground limestone contains various amounts of Magnesium Carbonate. Those ground limestones that contain a low percentage of Magnesium Carbonate are known as “calcitic lime” and those that contain a higher percentage of it (sometimes almost as much Magnesium as Calcium) are known as “dolomitic lime”. Later in this article we’ll see why this is important.
Lime isn’t the racehorse of lawn supplements – some studies have shown that it can take lime that is spread on top of the soil up to two years to move a mere 2 inches into the soil in some situations.
Why do we add Lime to the soil?
The soil in many parts of the US is naturally acidic, and the presence of acid rain and synthetic fertilizers worsen the degree of acidity over time. A pH of 7.0 is neutral, and grasses tend to like the soil slightly acidic – the optimal range is between pH 6.5 and ph 6.8, although some variance to either side is not generally an issue. As soils start getting below pH 6.2 however, the availability of some nutrients degrades and certain other metals in the soil begin to pick up a toxicity. The chemistry of the soil begins to change. It is not unheard of to have soils that can get down to pH 4.0, and soils that are at ph 5.0 (or just above that) are reasonably common. As acidic rains continue to fall, more and more Calcium and Magnesium (the common components of alkalinity in the soil) leach out over time, and the situation gets downright hostile to growing grass.
When we add lime to the lawn, Calcium Carbonate and Magnesium Carbonate being to separate and attach themselves to other things in the soil, like the organic material that is present and the soil particles themselves. For various reasons too technical to explain here, Calcium (Ca) has a tendency to loosen the bonds in the soil and make more room for water, air and nutrients, and Magnesium tends to have the opposite effect – the soil “tightens” and has less space for water and air. The carbonates bind with other items in the soil. The smallest particles in the soil are the clay particles, and if there isn’t Calcium present they bind with each other, making an impenetrable type of soil.
So, what does the lime do? Well, first of all it adds the alkaline components to the soil, reducing its acidity. Metals are less toxic, and nutrients are more available. And Calcium and Magnesium are critical themselves to plant biology – Calcium is a regulator of plant growth and a big part of the cell walls of plants. Magnesium is a critical part of chlorophyll. Lastly, it improves the soil structure and encourages good soil microbe activity.
The varieties of lime
As I mentioned earlier, lime is either Calcitic or Dolomitic, depending on the proportions of Calcium Carbonate and Magnesium Carbonate. These percentages are important, as the ratio in the soil of these items can cause the soil and plants (that includes grass!) to either be a dominant plant or a secondary (struggling to survive) plant. Great soils have a ratio of Calcium to Magnesium ranging from 7:1 up to 10:1. If the ratio is lower, the soil will begin to “tighten” and “specialist plants” that are experts in bringing more Calcium from deep in the soil to the surface will begin to flourish instead of the grass. We know these “specialist plants” as dandelions and curly dock (among others). In other words, we encourage weeds to become the dominant plant when the ratio is too low.
If your Calcium number is less than 7 times your Magnesium number, you should consider the use of a calcitic lime instead of a dolomitic one - the dolomitic one is going to push the balance even further away from 7:1. If you're above 10:1 and the Calcium isn't excessive, use dolomitic. Always remember though - when you add any type of lime, the pH will move up.
So, why don’t we see both varieties at the big-box stores and most garden centers? Because most lawn owners don’t know the difference and don’t get to see both products side-by-side to ask. Big-box stores keep their prices low by buying lots of one product that doesn’t require long-distance shipping. If there is lots of dolomitic lime to be mined within 200 miles of you, you will see dolomitic lime everywhere. If it’s an area with lots of Calcitic lime, the reverse will be true. You can, however, find Calcitic lime in areas that are dolomitic areas if you search – the national “Jonathan Green” product line contains “Mir-A-Cal” and “Mag-I-Cal” products that are calcitic. It is important to note that calcitic products work faster and go 10 times farther than most dolomitic products, even though they are more costly per bag. Estimate the total cost of a treatment – not the cost of a bag.
How do you know when the ratio is right? You do a good soil test. High-quality soil tests always measure the amount of Calcium and Magnesium.
Forms of Lime
Most lime comes in either a white powder or a pelletized product. The pelletized products are much easier to work with, and are the only ones that can be used with a broadcast spreader.
How much should I use?
As a rule of thumb, never apply more than 50 lbs of lime per 1000 square feet of lawn at one time. Leave 2-3 months between applications. Lime does not enter the soil quickly and you’ll make a situation where the top of the soil is hostile to things that can live in the lower soil (and the reverse) if you go with too many applications in too short a period of time. Also, it is not a good idea to apply lime and fertilizer at the same time – you can cause chemical interactions between them that you don’t want. Leave at least a week between a lime treatment and fertilization.
When should I apply lime?
Although you can apply lime at any time, the later fall is a good time because the lime will work into the soil throughout the winter when you’re not trying to add other products. A good soil test in the Spring will tell you how you’re doing. If you apply in the Fall, just remember to time the application so that it doesn’t interfere with that all-important last fertilizing application.
What happens if I apply to much lime?
Applying lime “because I do it twice every year” without soil tests is a recipe for an “overshoot” – a condition where the soil becomes too alkaline. In severe cases, it can lead to Lime-Induced Chlorosis, where the grass becomes suddenly very yellow. It can be a very difficult situation to correct.