Nutrient Nurture – Flagging up the need for magnesium

 

Magnesium deficiency in wheat

As wheat crops reach flag leaf, and divert energy into developing the ear, they’ll look for the building blocks to reach their potential. CPM explores what these are.

Of all the big cations, magnesium is the most readily leachable.

By Andrew Blake

Be prepared to think beyond fungicides when approaching cereal flag leaf treatments. Application plans at that growth stage should not be solely about disease control.

That’s the advice from FMC’s technical manager for crop nutrition, Charlie Bannister.

It’s well know that magnesium (Mg) is involved in photosynthesis, so any deficiency, especially at the important crop-building period (GS39-45), is likely to restrict grain production. Less well understood, Charlie believes, is the element’s impact on root growth and other processes within the plant.

Following this season’s especially wet spring, Mg deficiencies are particularly likely, he suggests. The reason is that the element is easily leached from the land, especially from sandy soils and those low in organic matter.

“Of all the big cations, Mg is the most readily leachable. Being a small ion with a double charge means it should be held firmly by the soil colloids. But in its hydrated state where it’s surrounded by water it’s larger than the other major cations. This waters down the charge and means that actually it leaches more than calcium or potassium.

“The data in the public domain isn’t very extensive. Data that does exist show that Ca tends to be the biggest leachate followed by K and Mg. Sulphur also figures fairly highly in losses from the soil. So we set up our own work.”

This involved trials, on 13 sites in the winter of 2012-2013 and 15 sites last winter, he explains.

“We chose a range of soil types in the north of England and carried out soil analyses to assess nutrient status. We then analysed the elements coming out in the drainage water.”

The findings were in line with published work but also threw up an interesting result, says Charlie.

“Of all the leachates Ca was the greatest; but it was always the highest level in the soil analyses so that was understandable. K was also present in varying degrees.

“But if you compared the amount of Mg in the soil analyses to the amount in the leachates on a pro-rata basis, Mg was the biggest loss.

“Set that against the rainfall we’ve had since last autumn and especially this spring, and I think we could well see widespread Mg shortage problems especially on soils already prone to deficiency.”

The priority in assessing the risk of Mg deficiency and corrective treatment is to analyse one’s soil, he stresses. That’s because light land, calcareous soils and those low in organic matter are inherently low in Mg.

“We’d expect a response in those situations. We’ve also had significant rainfall over the past few months which can only exacerbate any probable deficiency due to Mg-leaching properties.

“Tissue analyses are fine but we want if possible to pre-empt a situation where we discover we’re low in Mg then react to that situation. We need to be ahead of the game and soil analysis and risk assessment allows us to do that.”

Mg deficiency sets up what Charlie calls a “cascade” effect in the leaf which finally exhibits itself in the typical symptoms sometimes seen.

Research by leading plant nutritionists, for example Ismail Cakmak of Sabanci University in Turkey, have shown that many physiological and biochemical plant processes beyond photosynthesis rely on adequate Mg supplies, notes Charlie.

That’s because Mg is thought to be essential in the energy-driven process of transporting compounds within plants, he explains.

“Low Mg means that the process which moves sucrose and amino acids out of the leaves to the roots or grains is compromised. Carbohydrates build up as a log jam in the leaves which leads to the creation of so-called reactive oxygen species (ROS) – free radicals – which in turn damage the cell structure. Photo-oxidation then occurs which shows as typical yellow blotches on the leaves.

“Low or deficient Mg can have a significant effect on root growth at the expense of shoot growth. Whilst shoot growth is affected, the real impact is on root growth and this kicks in more rapidly than the effect on shoots.

“So you can have an immediate detrimental effect on overall plant growth before it becomes obvious above ground.”

K also has an important role in moving carbohydrates and amino acids within the plant. “But according to scientific research it’s slightly less crucial than Mg.

“In 2014, there were many cases of apparent K deficiency in wheat. At first sight it looked like Mg deficiency, but tissue analysis showed it to be K.

“Bearing in mind that both are involved in carbohydrate and amino-acid transport, it’s reasonable to think that low K can lead to a similar cascade effect as seen with Mg deficiency.”

By contrast manganese (Mn) is generally required less at flag leaf than earlier in the season. “It’s still needed because it’s the pivotal element that splits water in photosynthesis, but a high percentage of the total dry matter has already been produced by GS39-45.

Both elements are important throughout the life of the crop, but the emphasis changes as the crop matures, he explains. “As the crop matures there’s a greater need for Mg for moving carbohydrates and amino acids up towards the head where the grain is filling.”

Mg is such an essential nutrient that Charlie believes there may be a case for applying the element throughout the year, especially on soils prone to deficiency.

“But the traditional timing is at flag leaf, when I’d suggest magnesium nitrate at 3-6 l/ha, the rate driven by the degree of risk, is appropriate. It’s a good mixer and doesn’t interfere with fungicides applied then.”

Mg’s role in preserving the crop canopy shouldn’t be forgotten, he adds. “It’s the central atom in chlorophyll, so there needs to be ready supply to maintain green leaf area. Any deficiency can lead to the cascade effect when green leaf can be destroyed. It’s worth remembering that leaf damage potential increases under high light intensity which you tend to get more of in high summer.”

Flag-leaf greening from magnesium often boosts yield

Magnesium is a key ‘building block’ in the chlorophyll needed for plant photosynthesis, says BASIS and FACTS-qualified Samantha Johnson. She has seven years’ experience working as partner with her father, Richard, in the Yorks-based family independent agronomy business he set up 30 years ago.

“In wheat in particular we see a greening effect from applying Mg at flag leaf and at flowering, and we often get a yield response indicating the increased need the plant has for Mg at this stage.

“I look after farms in East Yorks and the Vale of York, across some very varied soil types. The East Yorks Wold soils are calcareous and we grow the usual combinable crops as well as vining peas and potatoes. The Vale of York has lighter sandy soils which allow mixed cropping including potatoes and sugar beet.

“Micronutrients play a very important role in the farms I cover. Manganese deficiency is a big factor on the lighter sandy soils, and my clients on this type of land are very aware of the visible symptoms of Mn deficiency in their crops.”

Mg is deficient in the calcareous soils of the East Yorks Wolds where it’s impacted by their high calcium levels, and the element’s availability is a major concern for potato growers due to the crop’s high potash need locking it up, she notes.

“Mg in the soil can become less available to crops for several reasons, such as in dry times, in situations with high K or Ca levels, or in soils with low pH. As with any deficiency it can result in plants more prone to disease and/or a reduction in yield.

“We advise our clients to undertake soil analysis every 3-4 years, testing for nitrogen, phosphorus, potash, magnesium and pH.”

The samples are taken to Beverley Analytical Laboratory, a local independent lab, and her recommendations are based on the test results.

Tissue testing is mainly carried out on potato crops at full ground cover, Mg levels being analysed by Lancrop Laboratories in Pocklington. “In dry years we may do mid-season tests.”

On calcareous soils Samantha advises applying foliar magnesium throughout the growing season in tank mixes. “We recommend bittersaltz or, in more complicated tank mixes, liquid Mg as Headland Magnesium, because it’s more compatible.”

To satisfy potato’s high Mg demand she recommends kieserite in the seedbed, and bittersaltz goes in with the blight sprays several times during the season. “Even if soil indices are high for Mg it isn’t always available for uptake,” she explains.

Magnesium at OSR flowering deemed important

BASIS-qualified Alex Borthwick is agronomist and spray operator on the 860ha run by R M Cottingham based at Hoe Hill Farm, Swinthorpe, Lincs. He’s worked with the firm for 15 years, having studied part-time at Lincoln University and Riseholme College.

Blackgrass aside, the main limitation to crop yield on the company’s three farms is nutrient availability, he says. “We’ve a wide range of soils.

“I do the agronomy myself, and generally tend to make all the decisions on crop nutrients, seeking advice from other sources should I be unsure or feel I need further advice.”

Some of the land on the high Wolds has pH levels of over 8, he notes. “Although soil analysis shows good levels of magnesium, there’s a lot of lock up, so we need to apply foliar Mg to maintain good plant health and green leaf area.”

Readily available Mg is especially important for good oilseed rape flowering, he believes. “And in potatoes I’m sure that using more Mg over the past couple of seasons has led to better plant health and helped improve the skin finish which has a positive impact on marketing the crop.

“We try to soil-sample each field at least every five years, generally using Lancrop at Pocklington for analysis. We usually check for pH, phosphorus, potassium and magnesium, but we also do full spectrum nutrient analysis in a few fields each year to keep an eye on what’s is going on – in particular for boron for OSR and sulphur generally, as well as looking at cation exchange.”

Tissue testing, via Lancrop, is sometimes employed. “It’s generally in fields which don’t look as good as we’d like or where we feel we might have a problem or have had one previously.

“We tend to sample leaf four in cereals just before expansive growth, so we can take action at the T1 fungicide timing if needs be. In OSR we usually test at stem extension.

“Historically we’ve applied Mg as sulphate in the form of bittersaltz. More recently I’ve moved more to using magnesium nitrate as I believe it’s taken up faster and moves into the plant more rapidly.

“The product is Magnesium Super 80, and in a few wheat fields which came out of the vegetative stage last spring not looking too great I saw a very quick response to an application.”

Nutrient nurture

An appreciation of the importance of crop nutrition and the need to understand its multifaceted nature are essential to optimise crop performance. In this sponsored series, CPM has teamed up with FMC to explore the individual elements needed for crop growth and the factors which apply individually to each element.

From functions within the plant to behaviour in the soil, antagonism, synergies, environmental impacts and limiting factors, these articles seek to unlock the answers and facilitate sustainable increases in crop yield and quality.

FMC supplies an extensive range of foliar nutrition products, encompassing those containing single elements to more complex multi-element products formulated to meets differing crop demands. FMC is also the market leader in the manufacture and supply of foliar nutrition products using humic technology, nitrate salts and advanced surfactant/suspension systems delivering rapid uptake and utilisation of elements by the plant.