Dolomite lime is used everywhere.
A search through both conventional and organic gardening websites reveals that most garden experts happily pass on this information.
Sometimes using dolomite garden lime is warranted, but the truth is it often makes things worse, sometimes just a little, and sometimes a lot. Let’s look at why...
Dolomitic lime is an attractive rock. It's calcium magnesium carbonate. It has about 50% calcium carbonate and 40% magnesium carbonate, giving approximately 22% calcium and at least 11% magnesium.
When you buy garden lime, it has been ground into granules that can be coarse or very fine, or it could be turned into a prill.
Dolomite lime fertilizer is certainly allowed in organic gardening. It is not inherently bad, but how it is used in the garden is often detrimental.
I touched on this before when I talked about pH. The belief is that minerals in your soil are continuously being leached by rain and consequently your soil is always moving towards more acidic.
Dolomite limestone is used to counteract this, to “sweeten” the soil. It can do that, but that doesn’t mean it’s good.
Minerals may or may not be leaching from your soil. If they are, it could be partially because of rain, but there are other reasons, too.
If your soil is low in organic matter, which is often the case, it probably can’t hold onto minerals very well, especially if it is low in clay and high in sand and silt. If you have lots of clay, you probably don’t have much to worry about.
(Chemical fertilizers can cause a lot of acidity, so if you use them, that is part of the problem, too.)
Whatever the cause, dolomite lime fertilizer is not the answer. Let’s look at why garden lime is probably not what you want.
The main point I want to make is that even if minerals are leaching from your soil, it doesn’t make sense to blindly go back adding just two of them (the calcium and magnesium in dolomitic lime) without knowing you need them. You might already have too much of one of them. We need to think a little more than that when organic gardening.
Many biological and organic soil consultants would say your soil needs a calcium to magnesium ratio of somewhere between 7:1 (sandier soils) and 10:1 (clayier soils). Outside of this range, your soil will often have drainage problems, your plants will often have health problems and insect and disease problems, and you will have weed problems.
One of your most important goals in the garden is to add specific mineral fertilizers to move the calcium to magnesium ratio towards this range.
Of course not everyone agrees that this is the ratio to go for, but it's what has worked for me and most of my mentors.
The problem with dolomite lime? It has a calcium to magnesium ratio of 2:1. That’s way too much magnesium for most soils. Magnesium is certainly an essential mineral. Too much of it, however, causes many problems, compaction being one of the most common, but also pest and weed problems.
So if you add dolomitic lime to your lawn every year, chances are you’re just causing more compaction and weed problems.
You should only use garden lime when you have a soil test showing a huge deficiency of magnesium in your soil.
Even then, calcium carbonate (calcitic lime) is generally the way to go because it has a small amount of magnesium and often a calcium to magnesium ratio of about 6:1, with a calcium content of 30% to 40% or more.
Instead of dolomitic lime, I use calcium carbonate regularly in my garden, but even then, only when I need it. A soil test is the main way to find out if you need it and I talk about soil nutrient testing often on my website.
Adding fertilizers based on the results of soil pH kits just doesn't make any sense (that's a good article that will show you why).
If you have any thoughts on dolomite lime, I'd love to hear them below.