The apparent trend to drier springs could prompt growers to look again at how they apply fertiliser. Andrew Blake re-visits the solids versus liquids debate

With prills or granules still visible on the soil surface several weeks after application, might it not be better to apply crops’ needs as a more immediately available liquid?

It’s a question some growers relying on solid nitrogen fertiliser dressings could be asking themselves, especially after last season’s prolonged spring drought.

Omex agronomist Andy Eccles says the company has received plenty of feedback from farmers this year. “We had neighbours seeing adjacent fields treated with liquid respond more quickly, and regular [liquid] users commenting their crops got away while solid-treated ones hung around.”

GrowHow says the recent trend is for liquid applications to increase. “The fertiliser market is broadly split 15% liquid and 85% solid,” says the firm’s Allison Grundy.

Yara’s Mark Tucker puts liquids’ share of the nitrogen market even higher, with UAN (urea and ammonium nitrate in solution) or urea solutions accounting for 20% of last season’s N dressings.

“For the past five seasons, the shift towards liquid has continued at about 1% a year,” says Mr Tucker. However, he believes it has been driven mainly by farms expanding, and adopting wider bout widths while striving to maintain accurate applications and make best use of larger sprayers.

“The anecdotal evidence of liquid nitrogen giving a faster crop response in a dry spring has only reinforced such decision making.”

Liquids enter the soil more quickly than solids, which need moisture to dissolve, says Mr Eccles. “Although most liquid fertiliser ends up on the soil, a small amount is retained by the leaves, which gives crops an immediate ‘shot in the arm’- so in dry conditions, liquids get working more quickly.”

Mr Tucker agrees that foliar uptake by small plants is the most likely explanation for faster crop responses seen in dry springs.

There are signs that growers are thinking about how they can adapt fertiliser plans to improve their crops’ chances of withstanding prolonged dry periods, says Ms Grundy. But the main debate is over application timing rather than fertiliser type, she believes.

One option is to apply more of the crop’s N requirements earlier, as either solid or liquid. “In my view that’s risky because large amounts of early N increase lodging later, and applying N in less than ideal conditions can lead to leaching, surface run-off and damage to soil structure.

“We must remember that crops do have the capacity to compensate, and oilseed rape can take up N well into late June/early July.”

However, many growers anticipating another dry spring, are prepared to bring their first N dressings forward and increase them by up to 20% to compensate for reduced uptake efficiency, says Strutt and Parker agronomist, Jock Willmott. “And those that can already do so will try to incorporate at least one liquid pass into three-split programmes.”

Switching to liquid is quite simple, maintains Mr Eccles. “All sprayers will handle liquid – users just require a storage tank and a set of streamer jets/bars.”

Bars tend to be used where they can be left on a second sprayline or if the nozzle bodies have twin outlets, he explains. Jets are the preferred option for use with triple or quad nozzle bayonets. “Costs range from about £5-20 a nozzle/bar. Large manufacturers offer storage tanks for rent, sale or on loyalty schemes.”

Mr Tucker warns growers considering switching to liquid to ensure the sprayer capacity is large enough to cope with both fertiliser and crop protection duties.

“The number and size of tanks will depend on the acreage and geographical spread of the farm and the number of products required – for example, N with or without sulphur.”

Practically, liquid application, which tends to be a one-man operation, is a neater solution requiring less storage, says Mr Willmott. “Provided manufacturers can keep up with deliveries during the growing season, it makes for a very efficient system.”

Security of supply is certainly important, says Ms Grundy.

“In the spring of 2009, supplies were severely compromised and product couldn’t be delivered to a large proportion of liquid customers when they required it”.

“Most liquid products are imported so the commercial terms may not always be attractive,” she adds. “If supply is low when a grower wants to buy, those terms may not be favourable.”

Another potential downside is that sprayers applying fertilisers will age prematurely unless scrupulously cleaned after every application, warns Mr Willmott.

Mr Eccles admits liquid fertiliser can accelerate unprotected steel corrosion. “But as long as any scrapes and dents to paintwork are made good there are no problems. Liquid nitrogen doesn’t attack paintwork, and as long as section hinges, boom rests, stays and masts are routinely greased there should be no marked effect on the sprayer.

“Indeed, the phosphate in some solution compounds can actually mitigate corrosion by creating an iron phosphate layer on exposed steel.”

One widely acknowledged advantage of applying liquid rather than solid fertiliser is that it delivers nutrients more accurately, with both agronomic and environmental implications.

Even with a high-quality solid product spread through a well set-up machine, the co-efficient of variation (the measure of distribution uniformity) may be 5-10% as against the 0-5% when applying liquid, says Mr Tucker. The COV for a worn spreader can be more than 20%, adds Mr Eccles.

Where tramlines are wider than 24m, liquid N dressings are far more accurate than spun solids and offer peace of mind, says Mr Willmott.

Solid spinners are notably poor on headlands, where they often severely under-fertilise the outer 4-6m, cutting its yield by 5-10%, says Mr Tucker.

“Where fields are surrounded by environmental strips with management rules stipulating no fertilisation, liquid application helps meet those requirements.”

Two commonly perceived drawbacks with liquids are workload clashes – with applying pesticides – and scorch risk. The consensus is that, in practice, neither should present significant problems.

“It’s often thought the sprayer will be tied up doing one job when it’s wanted for another,” says Mr Willmott. “Growth regulator timing – end of March – is usually the pinch point, but at most other times the jobs fit round each other well.”

Liquid fertiliser can be applied in conditions that prohibit other spraying tasks, so extending the sprayer’s work window.

“Scorch is a risk,” says Mr Tucker. “But it can be minimised by correct nozzle choice and avoiding very hot days.”

Several pluses from cheap switch

Robert Alexander, who runs an 810ha all-arable unit near Diss, Norfolk, has no regrets about switching to mainly liquid fertilising of his crops five seasons ago.

The change cost very little, the only requirements being a set of nozzles for his original 24m Bateman sprayer and a second line on the 36m model that replaced it as his farmed area expanded.

Although he has retained the ability to apply solids, most phosphate and potash is supplied by plentiful applications of poultry muck throughout the rotation.

“Storage is so easy,” says Mr Alexander, who now does all fertilising operations alone. “I don’t have to worry about unloading lorries and finding secure shed space.”

Workload clashes have been non-existent. “If anything, it makes using the machine easier because it can always work a full day – fertilising early morning when it’s damp, moving on to spraying, and then back to fertilising if the wind increases.”

Applications, with GPS guiding shut-offs, are “spot on” and crops more even, he reports. “And I’ve no concern about putting N in hedges or watercourses.”

Having switched to a one-man system and with his crops’ uniformity and last season’s yields maintained, Mr Alexander says it would take a big move in the on-going “see-saw” of relative fertiliser prices to prompt him to revert to solids.

First liquid venture unconvincing

Richard Cowan is not convinced that liquid fertilising, which he tried for the first time last spring through his existing sprayer, is anything more than a step sideways on the 800ha all-arable farm he runs on the Oxfordshire/Berkshire border.

“I went over to liquid to get sulphur and nitrogen on more easily, use the sprayer and main tractor more efficiently, and to get crops growing earlier in spring.”

In practice he ran into several problems, not least sprayer reliability and low work rates. “The worst was when I needed to fertilise and spray at the same time on already stressed crops; and it wasn’t as good in the wind as I’d been led to believe.”

He needed to replace stream jets with dribble bars as the former were causing crop scorch.