Calcium is an important nutrient for all crops. Soil calcium concentration and availability to crops is influenced by pH, CEC, soil type, and the concentration of other minerals in the soil. The solubility of the calcium source is important to consider when making a calcium application. The source, rate, timing, placement, and environmental conditions must be considered to implement the 4Rs for improved plant performance, yield, and crop quality.
In the 1958 first edition of Soil Fertility and Animal Health, the author called calcium “the premier of the soil’s nutrient elements,” noting the importance of calcium for production of proteins, protection against microbial pathogens, cellular growth, and plant reproduction. Specifically, calcium provides stability and rigidity of plant cell walls and is essential for root development and function (St. John et al., 2013). Dicotyledonous plants (like cotton and soybeans) have a greater calcium requirement (0.5 to 2.0% of dry matter; St. John et al., 2013) than monocotyledonous (corn and grasses) plants (0.3 to 1.25%) and are therefore more likely to exhibit calcium deficiency and disorders. Generally, calcium has low mobility in plants since it becomes a component of cell walls and membranes. Visual deficiency symptoms occur at the new growing points for both roots and shoots. Additionally, poor leaf canopy, bud development, or the shedding of blossoms result without available calcium.
Calcium concentration in the soil varies depending on soil type and mineralogy. Crops grown on sandy soils with low cation exchange capacity (CEC) are more likely to exhibit calcium deficiency although calcium deficiency can occur in all soils. The availability of soil calcium is related to the calcium concentration from the soil CEC and the soil pH, which can influence calcium availability from mineral sources like calcium carbonate. When creating a calcium management program, it is important to know not just the soil’s calcium content, but also the solubility of the calcium entering the soil solution for root uptake. Insoluble calcium sources may not meet the needs of the plant if the soil pH is not acidic.
Sources of calcium
Calcium is present in some fertilizers and soil amendments. However, calcium sources have a wide water solubility range, and as a result, differ in plant availability. Common sources of calcium like lime (calcium carbonate) and gypsum (calcium sulfate) have low solubility, and the calcium becomes available to the plant over a long period of time, months to years, depending on particle size of the material applied. Other sources like calcium nitrate or calcium chloride are highly soluble and available for crop uptake immediately (Easterwood, 2002). In high-value tree and vegetable crops, there are specific growth stages that require calcium uptake; therefore, soluble calcium sources are applied. Generally, calcium nitrate is soil-applied as a dry fertilizer or fertigated as a fluid fertilizer. Calcium chloride is mainly a foliar-applied source.
Crop health and calcium
Cell wall structure depends on available calcium. Without adequate calcium, cell membrane structure, function, and integrity can be lost and influence nutrient and water uptake by the roots (Palta, 1996). Increasing the calcium concentration in onions, apples, pears, and potatoes has been shown to increase crop quality, reduce plant disease and the impact of environmental stress, and limit development of diseased tissue during storage (Palta, 1996; Raese and Staiff, 1990). In onions, it was found that increased plant calcium concentration increased freeze tolerance and decreased leaking of potassium from cells (Palta, 1996). However, as with any nutrient application, the response of the onion crop depends on soil type and calcium availability in the soil prior to application.
Calcium can be supplemented to fruit trees as a foliar application or applied to the soil. Soil-applied calcium is assimilated by tree roots, shoots, and leaves, but depending on application rate and timing, more calcium can be used for the fruit. The calcium uptake for apple fruit occurs in a narrow window where 90% of uptake occurs four to six weeks after bloom. Given this short period of time, calcium for fruit uptake is often supplemented by foliar application of calcium sources, such as calcium chloride.
Does soil-applied calcium perform favorably for the tree and fruit? Results from a five-year study were published in the Good Fruit Grower Magazine (Raese, 1995) where ammonium nitrate fertilization of apple trees was compared with calcium nitrate. The calcium nitrate trees produced 11.4 boxes of fruit per tree, and the ammonium nitrate produced 10.3 boxes per tree. Agronomically and economically, adding calcium with a nitrate nitrogen source was favorable with the same rate, timing, and placement. Earlier research on apples and pears reported that tree vigor was higher with the soil application of ammonium nitrate and the incidence of fruit disorders was decreased when calcium nitrate was applied as the nitrogen source (Raese and Staiff, 1990).
Sally Flis, Ph.D. and CCA, Director of Agronomy, The Fertilizer Institute, Washington, DC