Calcium – Magnesium Stress
It’s an often overlooked nutrient, but calcium plays a very critical role within plants in terms of growth and nutrition. Calcium is not only a structural nutrient but also a regulatory one.
The largest utilisation of calcium in plants is as a structural component of leaves where it basically cements cellulose and pectin together to form a very strong leaf structure that supports a very high surface area to volume ratio enabling efficient solar collection for photosynthesis.
Once fixed within the plant it is not mobile and is a very important constituent of all plant cell walls. Calcium cannot remobilise from older tissue if a shortage is encountered. Calcium distribution within the plant is inextricably linked to efficient transpiration. When transpiration is reduced i.e. in dry conditions, supply can rapidly become inadequate. Calcium is taken up passively in the transpiration flow. Hence when transpiration is reduced so is calcium delivery. Physiologically calcium plays a major role in many crop maladies such as blossom end rots, potato tuber disorders, bitter pit and stone fruit cracking. These issues can be exacerbated by excess nitrogen particularly ammonium. Basically, these issues all boil down to an impaired delivery to sites of high calcium need.
Calcium helps maintain chemical balance both in the soil and in the plant. It plays a critical metabolic role in carbohydrate removal and also neutralises cell acid which can accumulate during stress periods. Many soils contain high levels of insoluble calcium in forms like calcium carbonate, but crops grown in these soils often suffer deficiencies due to the cations effect on pH as well as being site competitive with many cations (particularly Mg and K).
The analysis (TWOXO trial site) below show a Wolds calcareous soil near Louth, North Lincolnshire.
As expected we can see a low magnesium level, but also note the low calcium level.
As soils dry, plants initiate adaptive responses that regulate yield determining processes e.g. leaf growth and expansion, chlorophyll content and photosynthesis. We have measured these differences in trials this year, the correct metabolites have increased leaf size, leaf chlorophyll levels and subsequent photosynthesis rates. Understanding the regulation of these processes will improve our agronomy results.
In dry conditions calcium is the first element to precipitate due to low solubilities of many of its salts. Concomitantly other elements that form salts with calcium such as phosphate and sulphate that are biologically mediated, also precipitate out. These same processes hugely impact potassium and boron also; elements that are crucial in regulatory and reproductive processes in plants.
Reduced soil moisture leads to reduced rhizosphere biological activity which negatively impacts many nutrient acquisition processes. The order in which cations are precipitated out of the soil solution generally follows this sequence:
Ca, Mg, K, Na
For anions the least soluble are phosphates followed by sulphates.
Nitrates, sulphates, calcium, boron, molybdenum and manganese travel primarily by mass flow to the roots. Phosphorus, potassium and iron move by diffusion from areas of high to low concentration.
In the case of phosphates, in medium textured soil, it takes 1 year for PO4 to move one inch by diffusion.
The nutrients have to be in the soil solution for uptake as well as reaching the root via mass flow, diffusion or root inception. Thus, dry conditions can severely impact on calcium, the challenger is then when it rains the sudden release of calcium can cause a quick deficiency in magnesium so plan ahead to ensure minimal impact on your crop.
It’s been a mixed growing season of extremes – either too wet, too dry too cold or too hot… or combinations thereof! Nutrient availability is severely impacted by soil conditions with soil solution being the essential vehicle to supply nutrients to plants With our extensive trials programme we will hopefully understand more how to plan, prepare and afford to build a reliable profitable programme for optimal crop production.