When it comes to nitrogen fertilisers, PCU (polymer coated urea) is without doubt one of the most nutrient efficient alternatives to traditional urea that we currently have. It’s a game-changer because the semi-permeable coating controls the way that the nutrients are released into the soil so that far less is lost, saving time and money.
However, not all PCUs are manufactured in the same way, and some are more successful than others in regard to their performance in the field. It all depends on the quality of the polymer coating.
Polymer coating isn’t new
Polymer coated granules of fertiliser have been around for some time. Many will be familiar with ‘Osmocote’, a controlled release polymer-coated fertiliser used since 1988 in the nursery trade to provide nutrition to potted plants and other high value crops for extended periods of up to six months.
Until relatively recently, it was too expensive to manufacture controlled release fertilisers like ‘Osmocote’ for wider horticulture and agriculture application. But new technology has enabled the development of coatings that are now cost effective.
Some polymer coatings are made from inorganic materials such as sulphur and polystyrene while others are made from organic substances like starch or cellulose. At present, one of the best performing coatings is an ultra-thin polymer film made of vegetable oil.
This polymer coating not only allows the nutrient release to be controlled for up to 180 days, but also enables the rate of release to be matched to the growth requirements of specific crops such as bananas, coffee, palm oil, rice, sugarcane, potatoes, and of course, pasture.
Research for PCU
There is a lot of published research across the globe supporting the use of polymer coated fertilisers and their nutrient efficiency. In NZ too, we have published, peer-reviewed research conducted on PCU by independent researchers Edmeades and McBride (2017). They carried out five field trials across various sites in the North Island with a PCU ‘Smartfert’, currently marketed by Fertco as ‘N Smart’.
They measured pasture production and pasture nitrogen concentration, concluding that ‘Smartfert’ could be used to “reduce N fertiliser application costs … reducing pasture N concentrations and hence N leaching”.
It is important to note that the PCU tested had the ultra-thin polymer coating and that the research took place in the field rather than in a laboratory.
Coatings must be robust
To achieve predicted nutrient release patterns, coatings must be permeable enough to gradually release the nutrients in the fertiliser granule. However, the coating must be robust enough to withstand handling when blended with other fertilisers, and avoid being damaged by machinery when spread. The coating must also stand up to fluctuating soil pressure and soil moisture.
Should the coating become cracked, the nutrients can leak out of the granule. For PCU, this can result in higher than optimal nitrate levels in the soil and in plants, and wastage to the environment through leaching.
Coatings must be biodegradable
While the coating must be robust, it must also biodegrade. It’s all well and good controlling the amount of nutrient in the soil so that leaching is reduced, but it’s not so great if the soil becomes littered with empty shells because the polymer coating does not break down after the nutrients have diffused out.
In Europe, the EU has restricted the use of any fertiliser products like PCU from 2026 onwards unless they can demonstrate biodegradability. Fortunately, we already have access to such a product since ‘N Smart’ has the ultra-thin coatings made from vegetable oil which have been shown to achieve 99.1% biodegradability by 152 days after application. However, it’s important to be aware that other PCUs with different coatings may not biodegrade at all.
Choose top quality
So if you decide to give PCU a go, make sure you choose a top-quality product with a coating that is robust enough to deliver the nutrient efficiency claimed, and biodegradable so you don’t end up with polymer shells littering your soil.
This article was published in the Coast & Country News.
