Agrii agronomist Tom Land has confirmed that the variety and structural make-up of fertiliser is changing.

He spoke at a recent farm workshop, hosted by Lemken, at the Ballymoney premises of John McElderry Tractors in north Co. Antrim.

According to Land, fertilisers should now be assessed in terms of their size, shape, weight and strength.

“One of the key challenges facing many arable farmers now is the need to get fertiliser out to a spreading width of 36m,” he explained.

“If this is not being achieved effectively then striping within the crop will be the end result.”

Fertiliser application

Land pointed to the need for fertiliser spreaders to be continually re-calibrated, in light of these changing market conditions.

“We are no longer getting the number of European products that once would have been the case. Fertilisers are now becoming available from places like Egypt, Algeria and North America,” he said.

“This means that the fertilisers coming on to farms now are different to the products that we have gotten used to.

“So, Agrii always encourages the use of a tray test in order to calibrate the fertiliser spreader at the beginning of the season.”

Fertiliser spreader tray test

Land told the meeting that farmers can tray test themselves or they can have an outside agent carry out the test for them.

“Take ammonium nitrate as a case in point. Product made in Europe is highly consistent. However, due to international factors, we are now seeing ammonium nitrate coming in from countries like Georgia,” he continued.

“And these products will have different flow characteristics, caused by their differing bulk densities.

“So, before farmers set-off, they should carry out an MOT test on their spreaders. This should include a full inspection of the hopper and the veins, which can become damaged over time.”

In addition, Land pointed out that power take-off (PTO) speeds should be checked and that the drop guides are intact. The inside of the tank and the likes of agitators should also be inspected.

Farmers are being specifically encouraged to check the physical characteristics of the fertiliser products that they purchase.

“A crush test can determine how hard a fertiliser granule is. This factor has major implications for farmers wanting to put the like of urea out to 36m using very high PTO speeds,” he explained.

“And so the very basic question becomes – can the fertiliser withstand these physical forces?

“We know from our own trials that a crush test of 3-5kg is within the optimal range. However, in the case of fetilisers yielding a test result of, let’s say, 1kg, then we know that such products will not take much abuse at the back of a spreader.”

Fertiliser blends

Sieve testing blends is also recommended. Most fertiliser manufacturers now supply a selection of sieves to allow farmers and contractors identify the size distribution within blend.

According to Land, the more particles in the smaller sieve size, the harder it becomes to spread the product as a whole. The veins will also give the product a harder time in the field.

“Particle distribution is really important,” Land stressed.

“Ideally, we want to see a bell shaped distribution, courtesy of the sieve test. A bulk density test of a fertiliser product will also deliver useful information for the farmer. E.g., urea has a much lower bulk density than ammonium nitrate.”

Assessing the size distribution within a fertiliser sample is important

Bulk density directly affects the ballistic properties and has a direct impact on the spread width of a fertiliser. In essence, more dense particles can be spread wider and can be spread with higher spinner disc speeds.

Carrying out a tray test is straightforward. Trays are placed 1m apart with the two central trays placed directly adjacent to one another. Fertiliser landing in the trays placed on the ground as the spreader passes are placed into individual glass tubes.

The spread pattern can then be visually assessed.

“The results of the tray test allow the farmer to set the spreader up so as to secure the most even distribution of fertiliser.”

A spreader test will give a result in terms of a coefficient of variation, or CV.

“A CV value of around 15% is pretty much optimal in the field. However, if this value rises to 20% then striping will be seen in the field,” Land continued.

“And at this stage, yield will be lost and fertiliser will be wasted.

“Understanding variation is important. Fertiliser operators should think carefully about disc heights and speeds. This is important in order to avoid overlapping or under lapping.

“And understanding the type of fertiliser being used is also important.”

The spread of fertiliser

Land made specific reference to the wearing out of veins on many fertiliser spreaders. He attributed this, in part, to the growing the use of polysulphates.

Referring back to the value of tray tests, Land confirmed that an uneven distribution of fertiliser will necessitate a re-calibration of the spreader.

“Agrii undertakes a large number of crop trials on an annual basis. We will commission the tray test of our spreaders at the beginning of each season and prior to us using a different product,” Land added.

“The reality is that fertiliser is now being spread up to 36m on either side of the spreader. And in some cases, this figure is now rising to 42m.

“Most suppliers tweak the specifications of their fertiliser ranges on a pretty regular basis. The variation within these products will also change with each different season.

“These are very expensive products. And it’s imperative that farmers get the best possible return from them.”

The naked eye will tell if striping has been an issue within a crop. But by this stage, the damage has been done, in terms of the uneven distribution of fertiliser.

Sap testing

However, a more immediate way of determining the spreading patterns of applied fertiliser is by sap testing the crop in situ. This is an approach recommended by Agrii agronomists.

Dry tissue testing determines what the plant has already metabolised by analysing the total nutrients in the tissue sample, whereas sap analysis measures what the plant has absorbed from the soil but not metabolised by testing just the liquids or sap in the sample.

Where headlands are concerned, Land confirmed that modern fertiliser spreaders are delivering much more accurate distribution patterns.

“Section control is making this a reality,” he further explained.

He also pointed to the use of global positioning system (GPS) operated systems as a means of bringing yet more accuracy to bear, when it comes to spreading fertiliser.


Turning to the use of polysulphates, Land recommended an application of 100kg/ha for cereals, 200kg/ha for oilseed rape and 100kg/ha for pulses.

“All applications should take place in the early spring,” he explained.

Polysulphate fertilisers are a versatile and valuable source of prolonged release sulphur (S). They contain potassium (K), calcium (Ca) and magnesium (Mg) in soluble, readily absorbed forms.

According to Land, they are also a very fast-acting source of plant nutrients. This relates back to their high levels of solubility in water.

“The gypsum carrier within the fertiliser is slower acting than the other sulphate sources within the product,” he explained.

“And, in fact, it is the gypsum that is seen lying on the soil surface for some weeks after a polysulphate has been applied.”

Assessing the bulk density of a fertiliser sample is important

Land discussed the relative merits of double superphosphate (DSP) and diammonium phosphate’s (DAP) plant phosphorous sources.

“DAP is a much faster acting product. This is because plants will readily take up the added ammonium along with a soluble source of phosphorous.

“DSP, on the other hand, works best when it is a case of building-up general soil phosphorous reserves.”

There are products now available which will act to release locked-up phosphorous within the soil.

Land believes that it should now be possible to grow top quality crops of maize, for instance, on high phosphorous soils without the need for applied DAP or DSP.

“The phosphorous required by the crop is already there. So when it comes to growing maize, there is only a requirement to put CAN down the spout with the planted seed.”

Foliar fertilisers

Significantly, Tom Land regards foliar nitrogen (N) as a greening tool, which can be applied to cereal crops, mid-season.

This assertion is based on Agrii foliar feeding trials, carried out using an amide N source.

“Delivering the building blocks of N has to come from a bag,” he stressed.

“Bag N fertiliser is 60% efficient. The remaining 40% gets ties up in the soil’s nitrogen cycle.

“Foliar N is not physically adding to the building blocks of protein in the plant. However, it can play a role in mid-season, during dry conditions, when cereal crops require a greening effect.”

But where other crops are concerned, Land identified opportunities for foliar N, when applied late in the season.

“This is certainly the case with maize crops, if grown in areas where there is a limitation on the amount of bagged N that can be applied.

“Trials have shown that a foliar N application late in the season will help maintain greening levels.

“And the same principle holds with large canopy crops, including oilseed rape and maincrop potatoes,” he said.

“By taking this approach, the active lifetime of the crop canopy can be extended.

“There is also evidence to show that the application of foliar nitrogen along with a sclerotinia spray will help improve the overall oil content of the final crop.”

According to the agronomist, trials have shown that if bagged nitrogen is applied at the flowering stage of the crop, subsequent oil production levels will be depressed.

“What we have found is that foliar nitrogen applied at this stage is helping to keep the crop green,” he stated.