Gardening Gardening Basics

How to Add Calcium to Soil: 9 Methods

Using Lime, Bone Meal, Gypsum, and More

Calcium pellets being added to garden with gloves

The Spruce / Almar Creative

Calcium is one of the secondary macronutrients in soil. While not required in the quantities of nitrogen, phosphorus, and potassium, calcium is crucial for plant growth and makes plants less susceptible to diseases and pests. But when it comes to calcium, more is not always better. Too much calcium in your garden soil can go hand in hand with a high pH, which means the soil is too alkaline. This affects the absorption of the macronutrients contained in other fertilizers.

The way to determine whether you need to add calcium to your soil is through a professional soil test. It is different from a simple pH test that you can do at home. A professional soil test analyses both the calcium level and the pH of your soil. Professional soil tests such as those offered by a state cooperative extension office also will tell you exactly what to add to the soil, and in which amounts.

Keep in mind that in a living plant, calcium moves from the root tips upwards throughout the plant with water via transpiration. That’s why sufficient watering is always important. Once calcium has reached its destination, such as new, young tissue or tips, it will stay put. 

How to Measure Calcium in Soil

The calcium level in your soil does not say anything about how much of it can actually get absorbed by plants. Add calcium to your soil when it is low in organic matter and nutrient-absorbing properties like clay, which can be determined by its Cation Exchange Capacity (CEC). This measures the calcium absorption of the soil.

Understanding Calcium Absorption

Imagine the soil like a storage tank of plant nutrients for calcium and magnesium. CEC is the relative ability of the soil to absorb and hold a particular nutrient in the form of cations or positively charged ions. Whether your plants are getting enough calcium—and what you can do about it—is directly connected to the CEC level. It is also linked to the pH of your soil; soils with a higher pH level usually contain more available calcium.

Soil Testing for Absorption

A professional soil test will determine the CEC of your soil. The higher the CEC, the more organic matter and clay are in your soil, which holds water and nutrients better than sandy soil. A low CEC indicates a sandy soil which is more likely to lack nutrients because they leach out of the soil quicker.

Based on the CEC, the soil test might tell you to add organic matter to the soil. It also indicates whether you need to make your soil less acidic by adding lime, or make it more acidic by adding sulfur. The combination of increasing the CEC and adjusting the pH will increase the availability of calcium to your garden crops.

Blossom end rot on tomato
Miyuki-3 / Getty Images

Signs of Calcium Deficiency

While the soil test is the surest way to determine if you need to amend your soil, there are also some telltale signs of calcium deficiency in plants.

Signs of calcium deficiency include stunted or weak growth, curling of young leaves or shoots, scorching or spotting on young leaves, inhibited bud growth, stunted or dead root tips, cupping of mature leaves, chlorosis (yellowing of leaves), burnt leaf tips, and fruit damage such as blossom end rot on tomatoes or bitter pits in apples.

Calcium Fertilizers

There are numerous calcium sources. Which one is right for you, how much to apply, and when depends on the pH level of your garden soil, the timing, and also the crops you are growing.

Calcium sources in three multi-colored boxes

The Spruce / Almar Creative

  • 01 of 09

    Lime

    Garden lime
    Garden lime gives soil a strong calcium boost

    annick vanderschelden photography / Getty Images

    Adding lime to your soil is the biggest calcium booster you can give your soil but it also raises your soil pH, making it less acidic. Lime is also known as calcium carbonate and other forms of mined limestone.

  • 02 of 09

    Gypsum

    Front view of sprinkling gypsum into clay soil

    The Spruce / Jacob Fox

    This is a fast-acting calcium supplement (calcium sulfate) with low CEC that does not lower nor raise the pH.

  • 03 of 09

    Bone Meal

    Bone meal poured on soil under shrub with shovel

    The Spruce / Almar Creative

    This high-phosphate fertilizer is slower released than lime and less soluble. Use it for soil where you want to moderately raise the pH. Bone meal is especially beneficial for bulbs and root crops.

  • 04 of 09

    Egg Shells

    Egg-shell mulch protects edible planting in 'A Child's Garden in Wales', RHS Chelsea Flower Show 2011. Designed by Ysgol Bryn Castell & Heronsbridge School Horticulture Students with Anthea Guthrie
    L Alfonse / Getty Images

    For a more effective calcium fertilizer, grind up the eggshells before adding them to the soil. If you do not grind them up, the calcium cannot be released from the shells and into the soil.

    Continue to 5 of 9 below
  • 05 of 09

    Dolomite Lime

    Adding dolomitic limestone to garden soil

    Helin Loik-Tomson / Getty Images

    In addition to adding a good amount of calcium to your soil, dolomitic lime (calcium carbonate) also contains magnesium carbonate and it’s therefore used for raising the pH on low-magnesium soils. If a soil test shows that magnesium levels are already high, choose another calcium product.

  • 06 of 09

    Foliar Spray

    spraying on foliar fertilizer

    The Spruce / Letícia Almeida

    Commercial foliar calcium sprays (calcium acetate, calcium nitrate, and calcium chloride) are the quickest remedy for acute calcium deficiency, as plants absorb nutrients more efficiently through leaves than through roots. It is often used as a remedy for container plant issues, especially for seedlings and transplants. Foliar sprays do not amend the soil and should be considered a supplement to proper soil nutrition management.

  • 07 of 09

    Ground Oyster/Clamshell Flour

    close up of heaping pile of discarded empty oyster shells

    Lucy Ryan / Getty Image

    Calcium carbonate is a good source of calcium. But the shells have low solubility and take several years to be effective. They will also slightly raise soil pH over time.

  • 08 of 09

    Wood Ashes

    Wood ash in garden; hand holding shovel mixing in wood ash

    Helin Loik-Tomson / Getty Images

    Hardwood ashes (calcium carbonate)–unlike softwood ashes, which are not recommended–add a good amount of calcium to your soil but they also raise its pH. If you need to raise your soil pH, keep in mind that hardwood ashes are half as effective as lime.

    Continue to 9 of 9 below
  • 09 of 09

    Soft Rock or Colloidal Phosphate

    Brown fertilizer pellets being added to potting soil

    The Spruce / Kara Riley

    Also called rock phosphate, calcium oxide, or colloidal phosphate, soft rock releases calcium to the soil slower than lime and is less soluble. It moderately raises the pH.

FAQ
  • How can I increase calcium in my soil naturally?

    There are plenty of natural options to increase the calcium in your soil with methods that range from adding lime to clamshell flour and wood ashes to bone meal.

  • What is the best source of calcium for plants?

    Gypsum and lime are two of the best sources of calcium for plants. It's important to ensure the soil itself is of good quality, however, so the plants can absorb the calcium.

  • Can plants recover from calcium deficiency?

    Most plants can recover within a week or so from calcium deficiency but only if the deficiency is not too severe. Very slow-growing plants with extensive brown and yellow discoloration and curling leaves are the signs of acute calcium deficiency.