Why calcium is critical for soil, plant and animal health

Calcium’s superpowers have been known by agronomists for years, yet not enough attention is paid to the crucial role this mineral plays in soil health on farm.

This is according to Mark Tripney, agronomist with LKAB Minerals, who says more emphasis should be placed on the monitoring and maintenance of soil calcium levels (see “Importance of calcium in soils”).

See also: Video: Soil health focus reduces cow variable costs by 40%

More than 66% of grassland soils in the UK are deficient in calcium, according to data from Lancrop Laboratories, and only 28% are adequate.

However, of UK soil samples sent to the company for analysis, just 26% request a test for calcium. This compares with 100% of soil test requests sent from overseas.

In addition, data gathered from 43,000 soil samples sent to NRM in the past five years show there is a huge difference in available calcium between sandy and silty clay soils, says Dr Sajjad Awan, the laboratory’s soil and crop nutrition agronomist.

Topsoil calcium content – a function of soil moisture, organic carbon and clay content – ranges from 1t/ha-7t/ha.

“Analysis is very cheap in comparison to the amount spent on fertiliser,” says Mark, adding that a good interpretation gives the potential to balance soils correctly. “Don’t just put your soil analysis in the drawer once you’ve got it, for compliance.”

Calcium’s role in plants

Mark says he is struck by the number of important roles played by the secondary nutrients in plant function: magnesium has eight roles; sulphur, seven; and calcium, six.

He is concerned there is a lack of knowledge about these, and points out that, by contrast, nitrogen, a primary nutrient, is relevant to only four plant functions.

“There are only three essential nutrients for root growth: calcium, phosphorus and boron. Why aren’t we getting that focus on calcium, on getting the plant working to start with?” he asks.

As well as root growth, calcium is vital for nodulation/nitrogen use, disease resistance (because of its role in cell structure), tolerance to abiotic stress factors (non-living factors such as heat and drought), vegetative growth and water usage.

“Whereas potassium regularly recirculates within the plant, once calcium is in the leaf, it doesn’t move around. This means a good exchangeable calcium supply is needed in the soil to make sure the plant grows,” says Mark.

Calcium’s role in retaining nutrients

There is a strong correlation between calcium and soil cation exchange capacity, says Sajjad.

An estimate of cation exchange capacity – the ability of a soil to hold positively charged cations, including important nutrients – is provided by most laboratories, but he questions how much notice is taken of it.

“Organic matter is an excellent source of nutrients and provides surfaces for cation exchange. Calcium is positively charged, while clay and organic matter particles are negatively charged, so they attract,” he explains.

“You cannot increase clay content, but you can improve soil organic matter content, so there are more negative surfaces retaining more cations in the soil and fewer losses.”

Mark adds that getting the soil chemistry right can ultimately determine the physics, or good soil structure, and this, in turn, promotes soil biology. “That’s the way to healthy plants and healthy animals,” he says.

Careful sourcing

Manures contain high amounts of calcium (average 14kg/t) and potassium (average 11kg/t), but there is huge variation in nutrients between samples from different sources, he says.

Calcium may not be sufficient from manures alone, so liming may be required. (Slurry contains much smaller quantities, at 1kg/cu m calcium and 3kg/cu m potash, on average.)

Although calcium’s ability to buffer soil pH through applications of lime is well known, it is important to source the right product for the job where correction is needed, says Mark.

A local quarry might appear to be the best option, but if it is producing magnesium limestone rather than calcium limestone, it may not be suitable.

“Just because it’s cheap, doesn’t mean it’s the right material. There’s a huge variation in quality,” he says.

“Ensure it meets the AQS [AgLime Quality Standard]. The finer the material, the higher the reactivity: 0.15mm has a massive effect on pH, whereas 2-4mm has very little effect and is not really what you should be spreading on the land. If you’re not putting reactive material on, you’re not getting any benefit.”

Efficient use of nutrients

Getting soil pH right is critical for efficient nutrient use. Mark points out that at pH6, 11% of nitrogen is being wasted, and higher-output grassland looking to produce 12-16t/ha dry matter will not operate efficiently.

In addition, he adds, at pH6, 48% of phosphorus is potentially locked up.

“It’s cheaper and much more efficient to buy good-quality lime – ground or granulated – to optimise pH early in spring to make that phosphorus more available, than put on phosphorus out of a bag.

“Get up to pH6.3-6.5 and you get better nitrogen utilisation, higher-quality silage, better dry matter, better protein… the whole works.”

Mark Tripney and Dr Sajjad Awan were speaking at the recent LKAB Calcium Conference