Calcium is an important nutrient for all crops. It is also important for soil structure, water movement, nutrient balance and biological health of the soil.¹  Understanding how calcium exists in the soil and the factors influencing availability is essential for managing it effectively. 

Calcium can be found in 3 primary forms in the soil and is very mobile, so it can be lost to leaching, be retained by soil particles or precipitated as secondary minerals such as calcium phosphate. ^3..¹

There are three primary forms of calcium in the soil.  

• Insoluble mineral - bound calcium
  • Various minerals in the earth provide natural sources of calcium, releasing calcium slowly through weathering or biological decomposition. These include the common liming agents such as calcite (calcium carbonate) and dolomite³.
• Exchangeable calcium adsorbed to soil colloids
  • Calcium is adsorbed or “held” on the cation exchange sites of soil colloids, which is a measure of the soil’s ability to hold positively charged ions. Calcium can readily desorb from the exchange sites and replenish the soil solution as needed for plant uptake³. Exchangeable calcium is loosely bound and is considered the primary source of available calcium for plants.
• Soluble calcium in the soil solution
  • This is the calcium that is not bound to soil particles but is dissolved in the soil water and is readily available for uptake by plant uptake. Soluble calcium can be easily lost from the soil through leaching especially in high rainfall areas.

Calcium availability is primarily determined by soil type, soil pH and the holding sites, or the cation exchange capacity, of the soil. 

Clay soils and organic matter provide “holding sites”, called cation exchange capacity, which have a greater capacity to hold and exchange positively charged nutrients, including calcium¹.  Soils with a low cation exchange capacity are typically low in calcium. For example, sandy soils, with lower organic matter and fewer holding sites, are more prone to calcium leaching and deficiency.¹  Moderately weathered soils typically have greater amounts of available calcium when compared to highly weathered soil.³ 

Soil pH levels influence calcium availability, a soil pH between 6.0 and 7.5 is ideal for calcium absorption by plants. Both highly acidic and alkaline soils reduce calcium availability. To improve calcium availability in acid soils, applying lime is a common method to raise soil pH and increase calcium availability.

Calcium competes with other cations for uptake onto the cation exchange sites in the soil, for example  magnesium, potassium, sodium, ammonium, iron and manganese.¹  Excess magnesium or ammonium in the soil can also suppress calcium availability¹. Calcium can form strong bonds with phosphorus and iron which makes calcium unavailable for plant uptake.

Calcium is an important nutrient for all crops and is often supplied through liming materials such as agricultural lime or dolomite, which raises soil pH and improves the availability of other nutrients¹.  Not all calcium sources affect pH, for example, gypsum supplies calcium and sulphur without changing soil pH, making it valuable for applying to soils within the correct pH range, sodic or alkaline soils¹.

A steady supply of calcium throughout the season is critical for crops.¹ To maintain soil structure and support plant growth, aim for luxury levels of calcium in the soil. As a guide, aim for a base saturation for calcium of between 60-80%.

For more information about the benefits of gypsum for topping up calcium reserves in the soil, talk to your local fertiliser advisor.

Gypsum is a naturally occurring mineral. Winstone Gypsum is BioGro certified. 

References

1.Calcium Management article; Calcium in the Soil: Regenerative Calcium Management - Agresol - Regenerative Agriculture Consulting Australia

2. Calcium: Improved plant health and nutrition:401-2019.pdf www.agronomy.org/files/publications/crop-and-soils Agronomy & Soil Science Society USA

3. Nutrient Management, Base Cations- Calcium: Soil Management www.ctahr.hawaii.edu/manuisoil/c_nutrients03.aspx

4. Soil Quality, Australia: Cations and Cation Exchange Capacity | Fact Sheets | soilquality.org.au

5. The Fertiliser Institute, Calcium: tfi-clacium.pdf