An increasing number of growers are supplementing traditional granular or liquid nitrogen applications with foliar sprays later in the season, looking to improve nitrogen use efficiency.

Foliar feeding offers potentially not just a more efficient and quicker route to get nutrients into the crop, but also minimises some of the risks associated with granular or liquid fertilisers, such as nitrogen leaching or volatilisation.

Consistency using this approach can be variable, however, says Joel Williams of Integrated Soils, especially if there is a lack of attention to detail when applying and mixing the treatment.

See also: 3 steps for farmers to reduce reliance on artificial fertiliser

Mr Williams has spent the past year putting together a series of lectures to help growers improve results.

Here are the independent plant and soil health scientist’s six top tips to avoiding a “spray and pray” approach.

1. Use clean water for melting and spraying

Just as water quality can affect some pesticides’ efficacy, the same is true for nutrient sprays.

Rainwater is ideal, as it is naturally soft and lacks the minerals some other sources contain that can lock up nutrients.

Hard water reaching 250 parts per million or above of calcium carbonate is likely to affect performance, according to research results, Mr Williams says.

A process called reverse osmosis can be used to demineralise water by pushing it under pressure through a semi-permeable membrane to remove mineral salts and other contaminants.

Kits are expensive, although Australian and Canadian farmers have found they pay for themselves in savings on herbicides within a couple of years.

Water conditioners such as citric acid or ammonium sulphate can be added to help reduce nutrient lock-up, Mr Williams says.

Both will also help acidify the water and slow any possible ammonia volatilisation.

2. Water circulation and temperature are key

Some growers will dissolve granular urea on farm to make their own foliar nitrogen source.

Remember this is an endothermic reaction, Mr Williams says, which means the water cools as the urea dissolves.

“As the water temperature drops it becomes harder to solubilise the rest of the urea, so anything you can do to raise the temperature of the water will help.”

That could include trying to raise the temperature by storing the water in a black tank in sunshine for a few days before melting urea, he suggests.

The other key tip when melting urea is to keep the water circulating constantly, including at the bottom of the tank.

A dissolving ratio of about one-third urea to two-thirds water should be possible, Mr Williams says.

© Tim Scrivener

3. Add carbon source to final spray solution

Typical final urea rates for cereals are about 10-20kg N/ha, but that’s not the only thing that should be in the final spray solution, Mr Williams says.

Carbon sources, such as humic or fulvic acid, act as a sponge for positively charged nutrients to bind to.

Neutralising positively charged nutrient minerals in the tank helps uptake into the leaf.

“The plant leaf is negatively charged, so if you spray a positively charged cation it can get stuck on the leaf surface and locked up,” he explains.

For foliar applications Mr Williams advises fulvic acid with its smaller molecule size, which helps it to pass into the leaf more quickly – a key requisite for a foliar spray.

Other carbon sources, such as molasses and fish hydrolysates, can also be added, providing other functions such as a source of slow-release fertiliser and amino acids needed for plant growth.

4. Add other required nutrition

Foliar sprays have always been useful for applying other nutrition, especially trace elements.

Other minerals, including the macronutrients sulphur, potassium and magnesium, are crucial for the enzymes that break down and convert urea and nitrate into ammonium and, ultimately, into proteins.

If there is a deficiency in any of these key nutrients it can cause a backlog of ammonium or nitrate to build up in the plant, with negative consequences for plant health, Mr Williams says.

Using plant sap or tissue analysis prior to application can help determine whether any other nutrients are needed.

5. Check for compatibility

With multiple potential additives to the tank, including wetters, spreaders or stickers that can help change the behaviour, activity or availability of the nutrients, it’s worth jar-testing the mix beforehand to test for compatibility.

Be especially careful if adding a soluble form of calcium with sulphur or phosphorus, as this could easily precipitate out to form gypsum or calcium phosphate, Mr Williams cautions.

6. Choose your spray day carefully

Application conditions are important with foliar nutrition sprays, Mr Williams stresses.

As well as the correct combination of nozzles, pressure and forward speed, water volume is important to maximise coverage without too much run-off.

Coating both sides of the leaf with spray is key as nutrient uptake takes place via the stomata on the undersides of leaves, through the cuticle and via leaf hairs.

But the most important environmental factor to pay attention to is relative humidity, Mr Williams says.

“You want relative humidity to be high to keep the cuticle layer dynamic and permeable, and the stomata open.”

Research shows greater uptake at 70% relative humidity or above, he says, with optimum spray windows likely to be either early in the morning or later in the afternoon, when relative humidity tends to be highest.

One of the other main concerns with a foliar nitrogen spray is scorching of leaf tips caused by build-up of ammonium or urea in the leaf.

Avoiding conditions that slow down the plant’s metabolic rate will help minimise the risk of scorch, such as stress caused by high temperatures, frost or drought.

Adding in nickel can also help mitigate scorch risk as it optimises urease activity – the enzyme that breaks down urea in the plant.

Case study; Henrik Simonsen, Førslev Estate, Fuglebjerg, Denmark

Henrik Simonsen, who manages 900ha of arable crops on the Førslev Estate, about 55 miles south-east of Copenhagen, has invested in the Flex fertiliser system to make his own liquid fertiliser, including foliars, on farm.

Used for the first time in 2022 on the farm, the desired custom-made fertilisers are coded into the system and then made in a 2t tank from raw materials fed in via four augers from separate tanks.

A batch can be made in 30 minutes, according to Mr Simonsen.

Raw materials for the various fertilisers include 46% urea, phosphoric and sulphuric acids, 50% potassium granules, magnesium and manganese sulphate, a pre-mixed micronutrient blend containing zinc, boron, molybdenum and copper, molasses to help stick foliar sprays to the plant, and a Flex Fertilisers-supplied microbial complex.

© Erol/Adobe Stock

This season, Mr Simonsen is planning to use three foliar nutrient sprays in spring malting barley, supplying about 50kg N/ha in total in three monthly splits.

An initial application of 60kg N/ha of a liquid 24:3 N:S, also made within the Flex system, is applied at drilling, along with some di-ammonium phosphate.

In wheat, four to five foliar nitrogen applications will be made, supplying about 80kg N/ha, again following liquid fertiliser or manure applications.

The farm used satellite imagery to create variable-rate application maps last season, which produced a much more even grain protein level across fields, Mr Simonsen says.

Fertiliser inputs were reduced by 10-15% in 2022, with yields and quality remaining good.

“We’ve definitely seen better effects from the applied foliar nitrogen in the dry conditions,” he says.

Further big-plot trials are planned for this season to assess how much further nitrogen inputs can be reduced, and how much can be applied as a foliar spray.