Written by Tony Leggett

Single superphosphate (SSP) is a proven, low-emission choice of fertiliser that could help get more farms closer to New Zealand’s carbon neutral target by 2050.

SSP’s relatively low carbon footprint was identified in 2011 and confirmed in late 2019 by AgResearch’s world-class Life Cycle Assessment (LCA) team. It reviewed the earlier estimates of greenhouse gas (GHG) emissions for a range of imported fertilisers and SSP, based on local production and importation data for 2018-19.

The study evaluated SSP and a range of other fertilisers including triple superphosphate (TSP), diammonium phosphate (DAP), ammonium sulphate (AS), calcium ammonium nitrate (CAN), and muriate of potash (KCl).

Prices might have changed considerably since the 2019 study was completed, but the relative differences in emissions produced by each fertiliser were similar for both studies, says one of its co-authors, AgResearch’s principal scientist Dr Stewart Ledgard.

He’s an internationally recognised expert in LCA, which analyses resource use and environmental emissions associated with a product or system.

In the more recent study, LCA methodology was used to account for all the sources of GHG emissions to calculate the carbon footprint of each fertiliser from the raw material source to the New Zealand port for the imported fertilisers, or source to New Zealand manufacturing-plant door for SSP. Extraction of the raw material, transportation to a New Zealand port, production of the inputs, and manufacturing emissions are included for both cases.

The study found the fertiliser manufacturing process was the primary contributor to total emissions for most fertilisers except SSP, but values for each fertiliser were lower than the earlier study, due to improved efficiency of production and transportation, and changes in the sources of raw materials.

The total carbon footprint of SSP in the 2018-19 study was lower than the earlier study (0.156kg CO₂ equivalents/kg of SSP vs 0.216kg CO₂ eq/kg in 2011), mainly due to the impact of differences in phosphate rock source and shipping efficiencies.

Emissions from shipping of the phosphate rock (PR) and sulphur (S) to New Zealand were the largest contributor to the carbon footprint of SSP, at 62% of the total. Mining of PR rock at the site accounted for a further 25% of the total, internal transport 2%, energy for production of SSP 2%, and CO₂ release from carbonate in the phosphate rock, 9%.

Another benefit from applying SSP is its sulphur content, which is about 10 times higher per kg P applied for SSP than for TSP, indicating greater GHG efficiency for the New Zealand average SSP.

The study shows the highest GHG emissions per kg of product were from the nitrogen (N)-based fertilisers – not surprising given the high energy requirements for ammonia production. TSP produced the lowest emissions of the imported fertilisers at 1.85kg CO₂ eq/kg P compared with 1.72kg CO₂ eq/kg P for SSP.

Stewart and a colleague, modeler Shelly Falconer, also used the LCA data to calculate the carbon footprint of milk produced from the milking platform of the average dairy farm, using DairyNZ data from the 2016-17 year. The average rate of N, P and K applied in fertilisers was 140, 27 and 28kg/ha/year respectively.

This showed N fertiliser use contributed 7.4% of the total carbon footprint of milk from the milking platform compared with just 0.6% for the non-N fertilisers (P, K and S).

The estimates for N emissions included nitrous oxide emissions, both direct and indirect, from fertiliser N after its application, CO₂ released from urea after application, and its manufacture. Not included were emissions from fertilisers used on the area for grazing replacements, wintering cows off and where crops were grown for production of brought-in feeds.

For the average North Island hill country sheep and beef farm (based on Class 4 land for the 2015-16 year), the contribution from N fertiliser to total farm GHG emissions was 2.8% compared with 1% for non-N fertilisers.

Stewart says he was not surprised the carbon footprint relativities between the fertilisers were similar between the two studies, and values for each fertiliser option were lower in the more recent one.

“The basic production methods [for each fertiliser option] are unchanged between the reports. Electricity is not a large part of the production process of fertilisers here in New Zealand, but TSP for instance, a lot of that is made in United States where they use less renewable energy compared to New Zealand.

“The average shipping distance for phosphate rock was less in 2019 than the earlier study, and there have been gains in the efficiency of shipping since the earlier study,” Stewart says.

The LCA approach is helping to guide new thinking in environmental management because it accounts for all the emissions from all inputs through the life-cycle of the product.

“It’s why we use the LCA approach when we want to look at future farm systems because it provides a more complete picture rather than just focusing on the system itself in isolation," Stewart says.

“It includes all the emissions for each of the inputs. So, a mitigation might work well at farm level, but its total picture may not be so appealing.”

He says the LCA approach also recognises there are additional costs to produce inputs that are not included in a one-dimensional system analysis.

“It is starting to become more valuable as we look to use LCA to evaluate new mitigations or new future farm systems,” he says.

A next step could be to include the financials for a farm system evaluation so its profitability could be compared alongside environmental emissions.

Work is now underway with many other countries to agree a set of global LCA weightings for outputs, so claims can be compared internationally and prevent companies using LCA to manipulate weightings of outputs to produce a more desired carbon footprint result.

Evidence delivered by team to back product claims

AgResearch's world-class Life Cycle Assessment (LCA) team is providing an evidence base to help maintain New Zealand's export market edge.

AgResearch’s principal scientist and LCA expert Dr Stewart Ledgard says LCA usually covers the full life-cycle of a product, including processing, transport, retail, consumer and waste stages.

Results are expressed per kilogram of a product and so it is often used to help make decisions in choices of food, goods or systems to minimise our impacts on the environment.

In 2020, the team measured the carbon footprint of products such as Simply Milk and a range of Anchor Milk brands, as well as the carbon neutral beef product marketed by Silver Fern Farms.

This LCA work enabled the companies to take actions to offset product emissions, such as purchasing carbon credits from third parties, so they could be certified as carbon zero or carbon neutral.

In January 2021, DairyNZ released research it commissioned from the AgResearch LCA team that showed New Zealand to be a world leader in the carbon footprint for milk production – with the most efficient production among comparable countries using a measure of kilogram of carbon dioxide equivalent (CO₂e) per kilogram of fat and protein corrected milk.

DairyNZ chief executive Dr Tim Mackle described the dairy research as playing a key part in understanding how New Zealand dairy farms stack up and informing how Kiwi farmers can be even more efficient.

The AgResearch team has also worked with Beef + Lamb New Zealand on similar research measuring the carbon footprint of the country’s sheep and beef sectors. Previous research has also demonstrated that New Zealand has a lower carbon footprint per kilogram of carbon dioxide equivalent (CO₂e) for sheep meat than other comparable producing nations.

Another advantage of LCA research is providing accurate measurements to debunk myths or challenge assumptions about exported products, including ‘food miles’. After accounting for freight to overseas markets, New Zealand products often stack up favourably for environmental impact, given the way they are produced.

The team expects demand for LCA research to only grow with public concern about climate change, water scarcity and other environmental issues. The research is also likely to expand into areas such as social and cultural impacts.

“In future, it will go beyond just getting results for current systems and products to using LCA in designing new future systems and products with greater resource-use efficiency and lower environmental impacts,” Stewart says.

“Additionally, it will go beyond a focus on only climate change to multiple impacts, including human health, ecosystem quality and waste reduction.”