Calcium

Calcium is a critical plant nutrient commonly found in alkaline soil. But that doesn’t mean your plants can get to it. And it doesn’t look the way you might expect.

Pure calcium in a protective argon atmosphere (Matthias Zepper) Public Domain

Calcium inside plants

We all know that calcium makes for strong bones and teeth. It also helps plants stay healthy. Calcium is critical to plant growth and development. Plants use calcium to build strong cell walls, move materials across cell membranes, grow primary root systems, and maintain the cation-anion balance. [Cations and anions are electrically charged atoms of minerals that plants use for food.]

Researchers learned, in 2016, that the movement of calcium molecules in plant root cells triggers proteins that tell the plant that nitrogen-fixing bacteria are nearby. This causes the plant to start building nodules on the roots that will serve as homes to those helpful bacteria. Bottom line, as calcium walks in the door, the welcome mat for nitrogen-fixing bacteria gets unrolled, setting the stage for healthier and more productive plants. [This stuff amazes me.]

Calcium deficiency

​Calcium deficiency is often caused by irregular irrigation. Unlike more mobile nutrients, such as nitrogen, calcium does not move around within a plant easily. Once it stops traveling through the xylem, it pretty much stays where it is. This is why calcium deficiency is rarely seen in older plant tissue. Normally, calcium is moved through a plant by evapotranspiration, which uses a lot of water. Calcium deficiency can also occur when there is too much nitrogen in the soil, causing plants to grow faster than they can move the available calcium. When plants do not have enough calcium, you may see stunted growth, leaf curling, dead terminal buds and root tips, and leaves with brown spots along the edges that spread toward the center. These damaged areas make it easier for pests and diseases to strike. Some crop-specific symptoms of calcium deficiency include:

• apple - bitter pit
• cabbages - tip burn and heart rot
• carrot - cavity spot
• celery - black heart and stunted central leaves
• strawberries, caneberries, lettuce, spinach, artichoke, cauliflower - leaf tip burn
• peppers and tomatoes - blossom end rot

Heart rot of cabbage due to calcium deficiency (Scot Nelson) Public Domain

Blossom end rot

​Every summer, people discover leathery brown bottoms on tomatoes and peppers. This is a condition called blossom end rot. Blossom end rot occurs when plants can't absorb and transport calcium to where it is needed. If calcium is unavailable during the fruiting stage, the cell walls at the bottom of the fruit will collapse. Simply adding calcium to the soil does not correct the problem and foliar sprays have been found ineffective. Water is needed to transport the calcium into and within a plant. Consistent irrigation is key to preventing blossom end rot and other calcium deficiencies.

Blossom end rot on tomato

Drought and minerals

Minerals, such as calcium, are affected by drought in ways that might surprise you. Reduced water supplies often mean we get our tap (irrigation) water from reservoirs that are scraping the bottom of the proverbial barrel. That water already has high salt and mineral contents. The chemical reactions that occur between those salts and plant nutrients can make life difficult for everyone involved.

​California pistachio growers have found that, by adding more calcium to the soil, they can reduce the amount of salt absorbed by plants. This is not something you should attempt in your garden because what you just read is an oversimplification of a complex condition. I only use it to point out the amazing balancing act going on all the time to get you the foods you love. Another factor that involves drought and calcium is drip irrigation emitters. They tend to get clogged by calcium the same way your coffee maker and iron do. If your region has hard (high mineral content) water, you may want to invest in a filter.

Sources of calcium

Before adding calcium to your soil, it is important to find out what it already contains. Most soils west of the Rocky Mountains contain abundant calcium. The optimal range is 1000-1500 ppm. A soil test, conducted by a reputable lab, is the only way to know for sure. Over-the-counter soil tests are not reliable or accurate enough. If you are growing anywhere there used to be an ocean, there’s probably plenty of calcium already present. If you live east of the Rockies, it’s a different story. Agricultural lime and calcium chloride sprays can replenish depleted soils. Sorry, but egg shells do not add calcium to your soil.

Calcium uptake problems

Let’s assume that your soil has plenty of calcium in it and that you are watering regularly and properly. Other problems can interfere with a plant’s ability to absorb this important nutrient. Excessive potassium (K) is one. Too much magnesium (Mg), sodium (Na), iron (Fe), or ammonium (NH4+) can also slow the uptake of calcium. Soil alkalinity or acidity (pH) also plays a role.

The molecular balancing act that occurs between minerals within your soil and plants is mind-boggling, to say the least. Suffice to say, your average gardener (or gardening blogger) only groks the tip of this iceberg. This is not something to guess about. Get your soil tested. Your plants will thank you.