How to use Dolomite Lime for house plants
It's rare to find fertiliser with calcium included, let alone enough to meet the demands of indoor plants, especially calcium lovers like Peperomia. That's because calcium is NOT a mobile nutrient. You know when an old leaf goes yellow? When there's a nutrient deficiency, yellowing leaves are your plant shifting mobile nutrients from old leaves. Sacrificing old leaves to support new leaves, until you (hopefully) correct what they're deficient in. But our plants can't do that with calcium. Once there's a calcium deficiency, it's deficiency for the life of that root, leaf, stem or bud, because Calcium is not a mobile nutrient.
What are the signs of a calcium deficiency?
Unlike many other nutrient deficiencies which tend to impact old growth first, when it comes to calcium, the signs will appear on new and younger growth first. Common symptoms are smaller leaves, deformed or misshapen growth, and crinkled, split or torn leaves. New buds may fall off before flowering, and new buds may turn out to be rotted inside. What you won't often see however is the impact not enough calcium has on inhibiting growth of root tips also.
What plants need calcium?
Calcium is an essential nutrient. That means all plants need calcium. However some are better known as 'calcium lovers' because they show the signs of a deficiency more than others. Others need lime added to keep substrates like bark from rotting over time. These include Orchids, African Violets, Hoya, Peperomia, Kalanchoe, Spider Plants, Ivy, Tomatoes, Apples, Lemons, Peppers, Pears and more.
What is Dolomite Lime?
Dolomite is the most commonly used limestone. It contains both calcium and magnesium. It also helps neutralise acidic soil, improving soil pH to a range where plants can more easily absorb nutrients from potting mix. Considered an 'organic slow-release fertiliser'. The magnesium in dolomite lime assists in photosynthesis, improving light absorption for growth.
What about using other calcium sources like milk or eggshells?
Milk or 'milk water' is often suggested, but not recommended for indoor plants. It attracts fungus gnats (who love rotting organic material), as well as fungus and mould. Eggshells are also often suggested as a natural way to increase calcium, however used indoors they aren't likely to break down. In compost and outdoor gardens, eggshells take around 3 to 5 years to break down.
Boiling water does help release the calcium from eggshells faster, however it isn't recommended to water plants with boiling water (and boiling water has been found to release only around 0.2% of the calcium in eggshells). If dried (air-dried or in the oven), then ground very fine (mortar and pestle recommended), and the soil is artificially made more acidic, eggshells can release calcium, however the pH needed for eggshells to break down would harm house plants.
How do you use dolomite lime to increase calcium for indoor house plants?
As dolomite lime is more often used outdoors to prevent or correct calcium and magnesium deficiencies in fruits and fruiting trees (like Tomatoes and Apple Trees), you'll see the directions on the bag is based on square metres of soil. Directions are not given for indoor plants on the bag, however ask and the internet provides...
How much dolomite lime to add to potting mix:
Add about one tablespoon of lime per 5 litres of dry potting mix. Mix in thoroughly before potting up. Dolomite lime is slow release over 2 to 3 months, so can be reapplied about quarterly, three to four times year, if needed.
How much dolomite lime to add to an established plant:
If you're already dealing with a calcium or magnesium deficiency, you can also add dolomite lime to an established plant without repotting. A little goes a long way. For seedlings and baby plants its best to mix lime in with the potting mix (see above). For smaller pots around 11cms to 14cms: Wearing gloves, sprinkle about 1 teaspoon of lime evenly over the surface of dry potting mix. Use a fork to mix in well, into the top layer of the potting mix. Water as usual. Reapply every 2 to 3 months if you're still seeing signs of a deficiency or need to adjust your soil pH. For medium to larger pots, around 15cm to 20cm, follow the same directions as smaller pots, however increase to around 1.5 to 2 teaspoons of dolomite lime.
How often should you add dolomite lime to houseplants?
Dolomite lime is a slow-release source of calcium and magnesium, gently increasing levels available to the plant while adjusting soil pH to a healthy range. You can reapply dolomitic lime every 3 months or so depending on watering. The more often you water, the faster the lime will be used up and washed out . It's advised to test your soil before each application to check pH level. If needed, apply about two monthly in higher-watering times of year or for frequently watered plants, otherwise every three months is standard if needed.
Dolomite or dolomitic lime is not as harsh as other forms of lime, but should still be handled with care. Best to wear gloves, a dust mask and safety glasses as the powder form is quite fine. Mix in well with soil for best results. Avoid breathing in dolomite lime. Avoid contact with eyes and skin. Wash hands after use.
Where can you buy dolomite lime in New Zealand?
Right here at Love That Leaf! I sell the popular Yates Thrive Natural Organic Dolomite Lime