The power of (Ca)

Calcium (Ca) is an important element for plant growth, and plays a critical role in the plant cell and in cell membranes. It is considered to be a secondary macronutrient. Calcium is important for its various structural roles in plant growth and increasing cell wall strength. It is also involved with cell elongation, as well as cell division. Cell wall thickness and strength are increased with increased Ca levels.

Calcium is also an element that influences cell pH and acts as a regulatory ion in carbohydrate translocation. It has been shown to increase a plant’s resistance to disease, salinity tolerance and drought protection.

If calcium is deficient in the media, there will be cell wall stability problems. Calcium levels will vary by species and even cultivars in greenhouse crops, but typically are between 0.5 to 3 percent dry-weight of shoots in most plants.

A look at growing media

Typically, most growing media contain calcitic and dolomitic limestone, and frequently gypsum (CaSO₄). In most cases, these media-incorporated materials will provide sufficient calcium for the crop cycle. Calcium from limestone is typically about 70 percent Ca. Gypsum will dissociate in the soil solution into Ca²⁺, and SO_{4 -}. The rate of gypsum dissolving into the media solution will depend on particle size of the gypsum, with small particles dissolving more rapidly than large particles. The calcium (Ca²⁺) will become attached to the surfaces of media particles after incorporation and blending, and is held on the media particles by the negative electrical charge on those particle surfaces. As plant roots give off H⁺ ions to the media, calcium and other cation species are exchanged from the media particle to the soil solution, and subsequently to the root surfaces.

Unlike some of the other ionic species taken into the plant, calcium is taken up with the transpirational stream when the plant is experiencing a high transpirational rate. Plants that are not transpiring at a high rate will not be taking up large amounts of Ca. Once Ca²⁺ is in the plant, it will be translocated upward through the xylem and becomes a constituent of the plant cell wall tissue, or is deposited in the cell vacuoles.

Symptoms of low calcium levels

The plant’s calcium level depends on the growing media calcium supply. Deficiency symptoms include roots browning, short, stubby axillary roots, and shoot injury showing necrotic (brown) areas, distortion and stunting.

Plants with calcium deficiencies are smaller than normal, with dark green lower foliage, as well as flower bud abortion or reduced flower size. If a plant has damaged root tips, the deficiency symptoms can occur even if the media calcium level is adequate, as the plant’s root tips take up this element. Blossom end rot (a well-known Ca deficiency symptom) on tomato es and tip burn of lettuce are a result of low Ca and possible moisture stress.

Adjusting Ca in growing media

As peat moss is typically at a pH of 3 to 5, limestone can be incorporated to raise the pH. If the media contains coir, the starting pH of the media will be higher than one based predominantly on peat. Coir usually is at a pH of 5.5 to 6 and, after blending, would require less limestone to adjust the media pH level.

If the water source used for irrigation contains high levels of calcium, it would then be advisable to use fertilizer with low or no calcium, such as a 20-10-20 formula. Target ratios for fertilizing should be potassium (K) to Ca to magnesium (Mg) 4:2:1.

Coir typically contains more potassium and magnesium than peat, but equal or less Ca. K⁺, sodium (NA⁺), NH⁴⁺, and Mg²⁺ can be antagonists in Ca²⁺ uptake, so it is important to keep these elements at appropriate levels. Therefore, if the level of an antagonist such as K⁺ is too high in the media solution, Ca⁺ may not be taken up at the appropriate rate.

Calcium nitrate (CaNO₃) or potassium nitrate (KNO₃) combinations can be used in a fertility program to reduce micronutrient toxicity caused by using the peat-lite fertilizers (such as 15-16-17, 20-10-20), as these peat-lite formulas have higher levels of micronutrients than some others, and an abundance of nitrate (NO3-) in their formulas. These peat lite formulas can raise iron (Fe) and manganese (Mn) to toxic levels in low pH conditions, so careful monitoring of media pH is critical.

Nutritional best practices

It is advisable to check pH and EC on a regular basis, and with bedding plants, this should be done weekly. A useful tool is the pH/EC combination meter, and there are a number of options available. The pH electrode often has a limited lifespan, so growers should get a tool that has a replaceable pH electrode. If the unit is properly cared for, including using an electrode storage solution, the maximum life of the instrument will be obtained. The cost of a replacement electrode is much lower than replacing the pH meter, and the lifespan of the pH electrode should be from two to many years with proper care.

Plant tissue testing and media tests are tools that should also be regularly utilized by growers. Crops that have a short growing cycle may only need a tissue test one time, but plants that are growing on a longer cycle may need to be tested several times during the growth period. Many lab options are available to a grower, but choosing a lab is not a random decision. Select your lab based on their quality of testing and service. Stick with the same lab for tissue and media tests, as labs differ in their testing procedures. Switching between labs can only create confusion as to the results of the tests.

Managing calcium in your production situation is not only important for optimum plant growth, but to yield a greater understanding of the nutritional management of the crop as well. Many growers keep very good records and routinely test media and plant tissue for nutrient status. Good record keeping of the testing results and environmental conditions the plants were grown under will provide the best information for future crops.

Sinclair is an extension educator at Penn State University specializing in floriculture, propagation, breeding and nutrition.