Calcium for plant health
- Ca helps maintain cell wall structure; a large proportion of the Ca within a plant is located in the cell walls.
- Many parasitic pathogens invade using polygalacturonase enzyme which breaks down part of plant cells. Ca significantly reduces activity of this enzyme.
- Ca concentration controls movement of sugars within plant cells.
- Too much Ca can inhibit activity of important enzymes within plants (balancing with is Mg important).
- Calcium helps maintain correct balance of cations/anions in plant cells and roots:
- If insufficient Ca present in growing medium, root extension stops
- Aids freeze/thaw protection.
- Additionally, calcium regulates various cellular functions; is vital for secretion of mucilage by root caps; and binds pectins during development of a stolon tip into a tuber, i.e. essential for tuberisation in potatoes.
Measuring calcium in soil
There are two main methods for measuring the ‘amount of calcium’ in the soil, but they give different information.
Exchangeable calcium content
- This is the standard measurement used by the labs if asked for ‘calcium’.
- Approximation of plant available Ca2+.
- Expressed as mg (Ca2+) / L (air dried soil) (can also be expressed as ppm but NOT equivalent to mg/kg).
Mineral carbonate content
- Expressed as CaCO3 % w/w of air dry soil.
- Cannot be used if soil has been limed within previous 3 months.
- Sample is sieved to < 2 mm before analysis so any large lumps of chalk are NOT accounted for.
- The definition of ‘calcareous’ depends on CaCO3 content, not Ca2+ (CaCo3 may be measured as a percentage or as ppm):
Types of calcium fertiliser
- Calcium carbonate (CaCO3) or dolomite (Ca∙Mg(CO3)2 or CaCO3∙MgCO3)
- Mostly insoluble in water, become more soluble in acidic conditions
- Availability of Ca from chalk is low
- Raise soil pH and therefore can reduce Al toxicity
- Calcium sulphate: CaSO4
- Can be used to improve stucture of soils which are ‘tight’ due to too high Mg/Ca ratio
- 100 times more soluble than CaCO3
- Does not change soil pH
- Useful for treating soils with too high sodium content (e.g. after flooding with saline water)
- Calcium ammonium nitrate
- Ammonium nitrate is enriched with lime to produce CAN
- May also contain Mg depending on source of lime
- Calcium nitrate
- Solid: 5 Ca(NO3)2∙ NH4NO3 ∙10 H2O
- Liquid: Ca(NO3)2∙ 4 H2O
- Does not acidify soils
- Very soluble
- Single superphosphate (SSP)
- Ca(H2PO4)2 [monocalcium phosphate or TSP] + 2 CaSO4 [gypsum]
- Supplies Ca, P and S readily available
- Triple superphosphate (TSP)
- Ca(H2PO4)2 ∙ H2O
- Supplies Ca, high P content (no S; contrast to SSP)
Calcium and magnesium 2018 (Roles of calcium and magnesium in soil structure and availability in different fertilisers)