Secondary Nutrients

Secondary plant nutrients are calcium (Ca), magnesium (Mg), and sulfur (S). 

The nutrients plants use the most are called primary nutrients. Nitrogen, potassium, and phosphorus are primary nutrients. On the other hand, only tiny amounts of boron, copper, iron, chloride, manganese, molybdenum, and zinc are needed. These micronutrients used to be called trace elements. In the middle are the secondary nutrients. Secondary nutrients rarely need to be supplemented, but they are very important to plant health.

Most soils already contain high enough levels of these secondary nutrients, but you don’t know for sure without a lab-based soil test. The effects of not enough or too much of any one nutrient can create a domino effect that is difficult to diagnose. Simply adding more fertilizer can often makes problems worse, rather than better.

Why are these secondary nutrients important and what are some signs of toxicity or deficiency? Let’s find out!

Calcium

Plants use calcium to build strong cell walls, to move materials across cell membranes, to grow primary root systems, and to maintain the cation-anion balance. [Cations and anions are electrically charged atoms of minerals that plants use for food.] Optimal levels of calcium range from 1000 to 1500 parts per million (ppm).

Calcium is relatively immobile inside a plant. It takes a lot of water to move a calcium molecule around inside a plant. That’s why blossom end rot is more of an irrigation problem than a calcium deficiency problem. Calcium deficiencies, whether caused by real lack or insufficient irrigation, are rare in nature. When they do occur, they can cause bitter pit in apples, cavity spot in carrots, and leaf tip burn in several different plants.

Too much calcium is also rare, but it can interfere with the absorption of magnesium and potassium, causing deficiencies in those nutrients. Bottom line with calcium: irrigate adequately, regularly and consistently.

Magnesium

Magnesium is essential for plant health. Ideal levels of magnesium range from 50 to 120 ppm. Magnesium stabilizes cell membranes, making plants better able to withstand drought and sunburn. Magnesium is found in enzymes that plants use to metabolize carbohydrates. Most important, magnesium is contained in the chlorophyll molecules that convert the sun’s energy into food. This process, the Calvin Cycle, is what makes photosynthesis possible. 

Too much magnesium in the soil makes it difficult for plants to absorb calcium and other nutrients, which can lead to blossom end rot, bronzing, and many other problems. This is a common problem in areas with alkaline soil. The opposite is true in areas with acidic soil. 

Insufficient magnesium symptoms look like potassium toxicity symptoms. Older leaves, at the bottom of the plant, start turning brown, between and alongside the leaf veins, working upward through the plant. Magnesium deficiencies in stone fruits often start out as slightly brown areas along leaf edges (margins) that expand inward, causing cracking, necrosis, and leaf loss. Magnesium deficiency in California is extremely rare.

Sulfur

Plants use a surprising amount of sulfur. This secondary nutrient is used in making chlorophyll and certain proteins and enzymes. Sulfur is also part of the arrangement between legumes and rhizobia bacteria that allow them to make use of atmospheric nitrogen.

Sulfur deficiency is seen first in new growth. Leaves are pale and growth is spindly. If sulfur levels become toxic, leaves will be smaller than normal and have scorched edges. Sulfur is commonly used as an organic fungicide and to acidify the soil.

Do not use horticultural oil within 2-4 weeks of applying sulfur. Sulfur and horticultural oil are phytotoxic (poisonous to plants) when combined. Also, it is better to use fixed copper, rather than sulfur, on apricot trees.

Your plants may not need as much of these secondary nutrients, but they are just as important to plant health. Get a lab-based soil test to find out what is in your soil.