Calcium is a secondary plant macronutrient and is vital for healthy plants. It is required for the formation of new cells so is needed in order for roots, stems and leaves to grow. It is also used by plants when they respond to pest and disease attacks. Calcium deficiency can negatively affect the ability of legumes to associate with nitrogen fixing bacteria so it is perhaps of greater importance for legumes than to other plants but it is also required for nitrogen uptake in all types of plants. A number of plant enzymes also require calcium.
Factors affecting calcium availability include:
- pH - calcium is less available in acidic soils and more available in alkaline soils. Additionally, if the soil is alkaline, excess calcium can bind to phosphorus preventing it from being taken up by plants.
- Cation exchange capacity - the more clay and organic matter in soil, the more cations and thus calcium it can hold.
- Other cations - excess levels of other cations can prevent plants from taking up calcium. This is particularly true of sodium.
- Moisture - a lack of water in the soil prevents the uptake of calcium by plants.
- Temperature - plants are less able to take up calcium when the soil is cold.
- A soil's parent material - soils are derived from different materials. If soil is made from limestone or another high calcium material, it will usually have more calcium. If soil is made from sandstone or shale, it will usually have lower levels of calcium.
Ideally, for healthy and productive soil you should aim for a calcium concentration of at least 5 meq/100g (milliequivalents - this is a special term used to describe the amount of some elements in soil).
Calcium deficiency is most likely to occur early in spring when the soil is cold and/or when plants are grown in acidic soil. Some soils are naturally low in calcium and plants grown in such soils are also more likely to develop symptoms of calcium deficiency. Plants are also more susceptible to calcium deficiency during periods of drought and when soil moisture is inconsistent. This may be a result of sporadic rainfall or irregular irrigation. When one or more of the above factors are present, clay soil can further increase the likelihood of calcium deficiency as it can inhibit good root development and thus decrease the amount of soil available for a plant to use to uptake water, calcium and other nutrients.
Symptoms of Calcium Deficiency
The most common symptom of calcium deficiency that is recognised by gardeners is blossom end rot - a 'disease' that causes fruit tissue to die at the flower end of the fruit. Tomatoes are most commonly affected but squashes and melons are also often affected. Lack of calcium in the soil is rarely the cause of blossom end rot however. It is more commonly a result of uneven watering - lack of water prevents calcium uptake. This is an important piece of information to note, particularly for soils that have marginal calcium levels.
Other symptoms of calcium deficiency (usually more a serious deficiency) may include chlorosis of new leaves (blotches rather than all over each leaf), necrosis along the edges of leaves and death of leaf buds. The newest leaves may also appear hooked. Root growth is often affected and so this may result in stunted plants.
Treating Calcium Deficiency
Calcium deficiency in plants is best treated by ensuring soil has a neutral or near neutral pH and that the moisture level in the soil is consistent. Calcium is most available between pH 7.5 and 9 but good uptake should still occur between pH 6.5 and 9.5 and so you should add either lime or sulfur (or another acidifying or alkalising agent) if the pH is not within this range. Supplemental watering needs to be consistent where rainfall is sporadic.
If symptoms are visible early in spring, they may be alleviated by warming the soil. Mulch can assist with the task when plants are already in the ground. If calcium deficiency in early spring is common on your property, you may be able to prevent it by placing low greenhouses/cold frames or even a sheet of clear plastic over the soil a few weeks before sowing seeds or transplanting seedlings in order to raise the soil temperature. You'll need to mulch well when transplanting seedlings or just after seeds sprout in order to maintain the heat.
If fertilisers that are high in other cations (such as magnesium and potassium) are being applied, these should be exchanged for alternative fertiliser (if magnesium is high and you need to increase the soil pH, don't use dolomite as it can upset the calcium to magnesium balance further). If irrigation water is salty (bore water is an example of water that is often salty) and you really can't get by with only natural rainfall consider using an alternative source of water (installing a rainwater tank is a good option).
If these factors do not fix the problem and a soil test reveals your soil has a calcium concentration of 5meq/100g or less, calcium fertilisers can be applied. Organic fertilisers containing a decent amount of calcium include egg shells, wood ash and bone meal. Rock dust and gypsum also contain calcium and gypsum is particularly useful on some clay soils. Inorganic sources of calcium include calcium nitrate, burnt lime, hydrated lime and even superphosphate (which is calcium phosphate).
Excess calcium is unlikely to cause toxicity in and of itself. It can reduce the uptake of other nutrients (mostly cations) however, potentially causing deficiencies in other nutrients. If such a scenario is suspected, stop any applications of calcium containing materials (including gypsum) and supplement the soil with a fertiliser containing the lacking mineral if appropriate. Always ensure the pH of the soil is appropriate for the plants you are growing. In particular, it would be beneficial to reduce the soil pH if it is a little too high.