Spotlight on calcium
1 June 2021 - Articles
In this edition of our series on nutrients for horticultural crops, we focus on calcium (Ca). This is the fourth article in this series, after nitrogen (N), phosphorus (P) and potassium (K).
Calcium is an essential nutrient for all crops. But it is a difficult element, both in terms of uptake and distribution in the plant. Calcium uptake is influenced by transpiration, the presence of other positively charged nutrients in the root environment (i.e. influenced by the EC) and the root temperature.
Function of calcium in the plant
Calcium has a strengthening effect on the cell membranes, just like the trace elements copper (Cu) and boron (B). The plant absorbs calcium as Ca2+; it is the only element that is taken up exclusively by the root tips. Calcium uptake is a passive process, which means that it enters the roots with the water flow. It is then transported up through the xylem (dead cells) to the parts of the plant that are actively transpiring. This water flow only moves upwards.
Once it reaches its destination, calcium is stored in the vacuoles (spaces in the plant cell that absorb moisture, as calcium oxalate) or in the cell wall (as calcium pectinate).
In addition to the xylem, plants also have a transport system via the living cells, the phloem. This goes to all parts of the plant and carries photosynthesis products (sugars) from the leaves to the other parts of the plant, as roots, growing points, flowers and fruits all require sugars to grow. This transport (loading and unloading of the phloem) takes place due to load differences in plant cells and costs energy (compare it to putting something on a conveyor belt and taking it off again). This transport slows down at low temperatures. Calcium influences the permeability and load distribution in the cell wall and therefore the transport of photosynthesis products.
Calcium in the soil
The calcium content in the soil varies from less than 0.5% to more than 25% (calcareous clay soils) by weight. A calcium content of more than 3% means that free calcium carbonate (CaCO3) is present in the soil; these are soil types with a high pH. Some soil types contain calcium in the form of gypsum (CaSO4), however. These soils do not naturally have a high pH.
Calcium in the soil not only provides nutrition for the plants but also impacts on the soil structure. If clay minerals contain sufficient calcium, this will benefit the soil structure. In the Greenhouse Soil Stock we measure this as clay humus (CEC).
Follow up feeding with calcium
Calcium is particularly important for young, fast-growing parts of plants. Due to the poor distribution in the plant, symptoms of calcium deficiency first appear in young plant parts that transpire the least. With calcium deficiency, prevention is better than cure. Take water or soil samples in good time to ascertain whether follow up feeding is required. A plant sap sample will tell you how much nutrients have been absorbed.
Note the ratio between the various elements in the root environment. Structural monitoring will provide a picture of the changes and whether you need to intervene. A high concentration of other elements (potassium, ammonium, magnesium, phosphorus, iron, boron, zinc and manganese) can inhibit calcium uptake. Conversely, a high concentration of calcium can inhibit the uptake of these other elements.
Eurofins Agro can carry out various calcium analyses:
We analyse calcium in all types of substrate. In coconut-based organic substrates, we measure how much calcium reserve there is in the substrate (as well as potassium, magnesium and sodium) in a 1:1.5 volume extract with barium chloride, before crop cultivation. In the 1:1.5 volume extract with water we measure how much is released (from the basic fertilisation at the potting soil supplier) and is therefore immediately available to the plant.
The calcium content can also be analysed in (coated) mineral fertilisers, both solid and liquid ones. For this purpose, we need at least 200 g of the fertiliser. The analysis result is given in percentages by weight or mmol. In lime fertilisers we measure the neutralizing value of the fertiliser (calculated from the measured percentage of CaO and MgO). On this basis, you can work out how much of this fertiliser you will need to raise a low pH to the target level.
When calcium in crop samples is analysed, it is expressed in mg per kg, or in mmol/kg of dry matter if measured by the dry matter analysis method. It is expressed in mg per litre, or mmol per litre when analysed by the plant sap method.