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, which then affects the absorption of the macronutrients contained in other fertilizers.
The way to determine whether you need to add calcium to your soil is 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 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.
When to Add Calcium to Soil
The calcium level in your soil does not say anything about how much of it can actually get absorbed by plants.
A key term for the calcium absorption of soil is Cation Exchange Capacity (CEC). Imagine the soil like a storage tank of plant nutrients for calcium and magnesium. CCEC is the relative ability of the soil to absorb and hold a particular nutrient in the form of cations or positively charged ions. Knowing this, the questions whether your plants are getting enough calcium, and what you can do if they aren't, are 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.
A professional soil test will determine the CEC of your soil. The higher the CEC, the more organic matter and clay is in your soil, which is good, because that’s a soil that holds water and nutrients such as calcium 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.
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.
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, and bitter pits in apples can all be signs of calcium deficiency.
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 acetate, calcium nitrate, calcium chloride)
Commercial foliar calcium sprays 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.
(Calcium carbonate and other forms of mined limestone)
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.
Dolomite Lime (Calcium carbonate)
In addition to adding a good amount of calcium to your soil, dolomitic lime also contains magnesium carbonate and it’s therefore used for raising pH on low-magnesium soils. If a soil test shows that magnesium levels are already high, choose another calcium product.
Gypsum (Calcium sulfate)
This is a fast-acting calcium supplement with low CEC that does not lower nor raise the pH.
Ground Oyster/Clam Shell Flour (Calcium carbonate)
While it is a good source of calcium, the shells have a low solubility and take several years to be effective. They will also slightly raise soil pH over time.
Wood Ashes (Calcium carbonate)
Hardwood ashes – 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.
Soft Rock or Colloidal Phosphate (Calcium oxide)
Also called rock phosphate, it releases calcium to the soil slower than lime and is less soluble. It moderately raises the pH.
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.
It’s a myth that ground eggshells prevent blossom end rot. While they decompose too slowly to be effective as a calcium fertilizer, they are still a welcome addition to compost as organic matter.