• Bert Quin

Definitions of Fertiliser Forms – Fluid & Suspension Fertilisers Giving Way to Wetted Prills

Suspension fertilisers

In most overseas countries, this term is used specifically to refer to a fluid made by mixing high-quality fertiliser ingredients, previously finely ground to below 100 microns (0.1mm), with 40-60% water by weight. Because most ingredients used are quite soluble in water, a saturated solution is formed, with the rest remaining in suspension as fine particles. This is commonly aided by the addition of about 10% of a clay such as bentonite, capable of keeping the fertiliser particles in a suspended state for some hours without stirring.

In New Zealand, the term is, perhaps unfortunately, used more loosely, to the extent of being used to refer to virtually any flowable mix of fine particle fertiliser and or lime in water, most of which require constant stirring to avoid settling out. Such products are applied in a number of ways, including spray booms fitted with very wide nozzles or outlets, from buckets with spinners incorporated in the case of helicopters, or from trucks fitted with specialised on-board grinding, mixing and spreading equipment.

Fluidised fertilisers

In most countries, this term can be used to refer to any mix of fertiliser and water, including those in which the fertiliser is fully dissolved in water (ie, a true solution) like urea ammonium nitrate (UAN). The term can cover a wide range of particle sizes and concentrations.

The term has not come into common usage in New Zealand. Because of this, and because of the deeply entrenched misuse (relative to overseas usage) of other terms, a prefered definition of fluidised fertilisers is ‘highly concentrated (less than 30% water by weight) liquid fertiliser products containing at least one of the major nutrients N, P, K and S, with or without lime, all with fine-form particle distributions’.

Liquid fertilisers

In most countries, this term is typically but not exclusively used to refer to any fertiliser in which all the ingredients are fully dissolved, ie, a true solution. In New Zealand, it has been most commonly used to describe both relatively dilute solutions of fertiliser in water, and such products containing partially-dissolved substances of organic origin, typically from seaweed or fish-processing waste. They are mostly applied, heavily diluted with water because of low ‘recommended’ application rates per hectare, through spray booms fitted with anti-clog nozzles.

Fine Particle Application (‘FPA’)

This term, not commonly used in many countries, has been used by some commercial entities, particularly helicopter-spreading operations, to describe mixes of finely-ground particles (typically below 100 microns) with water, and/or the application of these via spray booms. The name has been used to infer automatically increased effectiveness, regardless of whether it is present for a particular ingredient. There is typically a claim or at least inference that the total uptake of all nutrients occurs by foliar uptake, i.e. through the leaves, but the actual mechanism by which this occurs is never specified; neither is any hard scientific data provided to support performance claims. Partly because of very high costs of fine-grinding of all ingredients regardless of benefit, ‘FPA’ products tend to be recommended and sold at nutrient application rates that would be considered far to low to maintain production by agronomy and soil fertility experts.

A disadvantage of the ‘FPA’ term itself, besides the confusion over agronomic and cost-effectiveness relative to granular fertiliser for most ingredients, is its confusion with dry or deliberately dampened fine products such as RPR. These fine-particle are often sold deliberately dampened with a few percent moisture, with no other stated purposes other than to control dust and improve spreading accuracy. In fact, it has been scientifically proven that grinding RPR much below 100 microns, which FPA automatically does, confers no additional agronomic benefit because of the well-established common-ion effect between calcium from the RPR and that from the soil.


Most countries use this term almost exclusively to refer to relatively concentrated water-based organic manures, typically applied at high rates per hectare from tanker trucks fitted with special outlets. In New Zealand, the term is used by some to refer to what would be better described as a fluid inorganic fertiliser, especially ones containing a lot of large particles and/or only partially dissolved granules.

Foliar fertilisers

All such products are designed with the intention of most if not all of the nutrient content being taken up by the plant directly through the leaves (foliage). To assist in this, the nutrient ingredients are typically present in the form of highly-soluble chelated compounds, with wetting or ‘fixing’ agents added. They are supplied either in ready to use solution form or in powder form for completely dissolving in water. They are typically used on high-value crops only, because of their high cost per unit nutrient. The method by which foliar uptake is achieved or at least promoted is usually indicated to some degree.

Granular fertiliser

Granular fertilisers are made by finely grinding all the components, some of which may then be subjected to reaction with one or more mineral acids, before passing the mix through a granulation drum or over a pan granulator, to produce approximately spherical granules, generally of 4-6mm diameter, before drying and hardening.

Another technique is ‘compaction’, where the finely-ground ingredients are squeezed together under high pressure, typically into a 4-6 mm thick continuous ‘biscuit’, which is then broken into granule-sized ‘chips’. Most potash is sold in this form.

Regardless of whether granule or compaction technology is used, sizing is most commonly 4-6mm, for both ballistic (spreading evenness and width), and agronomic reasons. They are intended to pass through the foliage to the soil surface. On pasture, the rule of thumb is typically that the roots of each plant should have access to at least one granule as it dissolves in the soil. While this may be approximately true for applications of maintenance single superphosphate, it is most certainly not true for granular urea. In a typical dairy pasture, there are typically over 400  plants per m2. However, at a typical granular urea application of 30 kgN/ha (65 kg urea/ha), only 45 granules are applied to each square metre. In cropping situations, both granulated and fluid fertilisers are more commonly placed by specialised machinery close to the rows of plants or seeds, to improve proximity to roots.

Prilled (mini-granular) fertiliser

A new approach rapidly becoming popular in New Zealand is that of using ‘prilled’ or mini-granule fertilisers, typically in the 0.5-2.5mm diameter range. These provide literally 10 times more particles at the same weight applied as granules, meaning vastly more even coverage. Prilled urea supplied at 30kg N/ha typically supplies 500 prills/m2. Prills also have the further advantage if wetted that many will adhere to plant leaves, allowing some foliar uptake, but without the risk of ‘burning’ and ‘leaf scorch’ commonly induced by liquid and suspension/fluidised fertiliser N applied at more than 10 kgN/ha.