Blossom end rot
Blossom end rot of tomatoes, caused by calcium deficiency

Calcium plays two main roles in the health of plants. First, it keeps plants healthy indirectly by maintaining soil health. Second, it keeps plants healthy directly by regulating key processes in plant tissues.

In the soil, calcium regulates pH and salinity. This affects the electrical charges in the soil, which in turns alters the soil structure. Therefore, calcium is important in promoting the proper aggregation of soil particles, key for soil aeration and drainage. Also, calcium in the soil plays a role in the health of beneficial bacteria.

Inside the plant, calcium is a structural component of cell walls, but it does so much more than give plants strong stems. It helps transport nearly all substances into and out of the cells. For example, it regulates the movement of carbohydrates and other nutrients throughout the plant, but it also prevents enzymes from harmful fungi and bacteria from entering.

For all of these reasons, plants depend on calcium to properly grow, photosynthesize, absorb nutrients, and resist stress.

Natural Sources

Calcium is widely found in nature. Therefore, most compost contains calcium. Many water wells in inland parts of North America contain this mineral, as well.

On a large scale, supplemental calcium is typically supplied in the form of calcitic or dolomitic lime to correct pH. Gypsum can also be used if a pH adjustment is not desired.

On a smaller scale, a quick boost of calcium can usually be supplied using one of the following ingredients:

  • Seaweed.
  • Eggshells.
  • Chamomile.
  • Epsom salts.
  • Pulverized oyster shell.


Bitter pit
Bitter pit of apples, caused by calcium deficiency

Calcium deficiency is frequently undetected or untreated in plants unless there is an associated pH imbalance. That said, it is not an uncommon disorder because plants depend on having calcium readily available to them in soluble form.

While calcium can be leached out of the soil by heavy rainfall, particularly if the soil is sandy, relatively few soils are actually deficient in calcium. The problem is typically with availability and transport throughout the plant tissues (in Kansas, assume that this is the case). Examples of conditions that can be problematic include:

  • Insufficient soil moisture.
  • High humidity.
  • Cold ambient temperatures.
  • Rapid plant growth.

Excess phosphorus, potassium, magnesium, or sodium can also interfere with calcium absorption.

Symptoms of calcium deficiency in plants include:

  • Fungal disease.
  • Stunted, distorted growth.
  • Weak stem.
  • Tip burn.
  • Death of terminal growing points.
  • Leaf curl.
  • Unusually dark foliage.
  • Reddish-brown color along margins of grass blades.
  • Death of leaf margins on new leaves.
  • Internal browning of cabbage and Brussels sprouts.
  • Premature blossom or bud drop.
  • Blossom end rot.
  • Bitter pit in apples.
  • Reduced shelf life of tomatoes and muskmelons.
  • Short, brown roots.
  • Club root in brassicas.
  • Potato scab.
  • Cavities or black lesions on carrots.

Because calcium deficiency issues are typically related to availability rather than soil mineral levels, these symptoms can be addressed by making sure plants can more easily absorb calcium from the soil. The most important step to take is to make sure that your plants get enough water in summer. Water deeply to encourage good root development, and consider using mulch. If you are still concerned about the amount of soluble calcium available to your plants, consider amending the soil with one of the natural calcium sources listed above.


Calcium toxicity is very rare in plants. However, in alkaline soils, an overabundance of calcium can bind to other minerals and prevent plants from absorbing them. In this scenario, excess calcium will likely present as a deficiency in one of the following nutrients:

  • Potassium.
  • Phosphorus.
  • Boron.
  • Magnesium.

Should this occur, correct the pH to bring it closer to neutral. Obviously, you will want to stop applying amendments high in calcium, such as dolomite or gypsum, as well.

Complete Series

Minerals in Plants

Minerals in Plants

Improving Your Garden Soil