A research project investigating the role of phosphorus (P) in the UK food system has identified huge inefficiencies in its use, with a total of 138,000 tonnes of P going unused, wasted or lost each year in 2018.
One of the three key areas associated with this loss was over-application of phosphorus, which results in an excess of 90,000 t of P applied to soils each year. This equates to 8 kg/ha unnecessary.
Phosphorus is an essential element of the food system. However, phosphate rock, from which fertilizers and animal feed are derived, is a limited resource, and only found in a handful of countries.
In addition, the leaching of P into waterways may raise environmental concerns.
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The project known as ‘RePhoKUs’ aims to refocus the use of phosphorus in the UK food system to safeguard the future of food and water security.
Researcher Shane Rothwell of Lancaster University notes: “British agriculture is entirely dependent on imports from countries like Russia and Morocco.
“This means that the management of these reserves is of crucial importance, particularly when it comes to avoiding losses which can lead to nutrient pollution and eutrophication.”
The project involved mapping the use of phosphorus in the UK food system via a substance flow analysis. This assessed P imports, fluxes, stocks, losses and exports at the national, regional and watershed scales.
Working on the theoretical basis of mass balance, what goes into a defined system must go out, because phosphorus can neither be created nor destroyed.
The study identified three main areas of inefficiency across the UK:
- Over-application of P (90,000 t)
- Loss in water (26,000 t)
- Discharge loss (22,000 t)
“In total, 138,000 tonnes of phosphorus are wasted each year. This equates to 75% of the total P from the food system imported into the UK, so there is considerable room for improvement,” says Dr Rothwell.
As uncertainty grows across the world, a future shortage or rising import costs could leave the UK food sector vulnerable.
Improving phosphorus efficiency would therefore increase the UK’s resilience by reducing import dependency and optimizing agricultural intensification while minimizing the impact on water quality, he explains. -he.
Manure and regional P balance
One way to improve this is to improve the management of cattle manure, which is the route of greatest phosphorus flow into the UK food system (175,000t).
Cattle manures are also the largest contributor to excess soil P.
In the west of the country where livestock farming dominates, the P surplus is greater, while in the east where agriculture dominates, the surplus is lower and in some cases declining.
The northwest has the largest surplus at 13.5 kg/ha per year, which is mainly a slurry management issue, while the east has a soil deficit of -3.2 kg/ha per year .
“Where there is a P-deficiency, it cannot continue sustainably over the long term and will eventually result in a yield penalty.
“The main factor behind the high surplus associated with manure is a result of high phosphorus levels in animal feeds such as soybeans and concentrate feeds,” says Dr. Rothwell.
He says adapting P inputs to where they are needed in the UK would significantly improve efficiency and help improve this balance.
However, this comes with significant logistical hurdles and financial issues associated with transporting manure across the country.
Recovering fertilizer-grade phosphorus from manures to overcome transport barriers would help solve the problem. How will this be achieved? Who will pay it?
Tillage Grass V
Significant differences are noted with respect to soil P-use efficiency and crop type.
Cultivated arable land is very efficient, using 85% of the phosphorus applied. This means that it represents 15% of the national P surplus, or the equivalent of 2.8 kg/ha.
In contrast, grasslands receive most phosphorus in the form of manure and are only 53% efficient. This contributes 85% of the national P surplus, which is equivalent to 11kg/ha.
Inherited soil phosphate
Shane Rothwell (above) thinks finding ways to access the huge stores of excess phosphorus found in soils, known as legacy phosphorus, could prove key to reducing reliance on phosphorus applications and optimize crop nutrient requirements.
“Due to the continuous supply of phosphorus, a huge reserve has accumulated over time, strongly bound to the complexes of calcium, iron and aluminum in the soil.
“So the total amount of phosphorus available does not reflect what growers can access. Olsen P is used as an indicator of soil fertility in the UK and in reality it is only a small fraction of the total P reserves,” he says.
Some methods such as zero and min-till can increase soil P availability, but the exact science behind this is unknown.
It is believed that as soil biology becomes more active, their biological processes make phosphorus more available.
Intercropping and legume cropping are also thought to be helpful, but more research is needed.
Drawdown test of P
An accelerated P uptake test was set up in a laboratory to determine if inherited P could be utilized by crops, when no other source of P was applied.
A fast-growing species of ryegrass was grown in pots containing soil from three catchments: The Wye (arable farm, Herefordshire), Upper Welland (arable farm, Leicestershire) and Upper Bann (dairy farm, Northern Ireland). North).
During 535 days of growth and 20 harvests, investigators monitored biomass production, grass tissue P, soil Olsen P, and pore water P.
Crop deficiency has generally been found to occur at an Olsen P index of 1.
The amount of inherited P was calculated and with each grass cut, additional P was removed, emphasizing that in times of Olsen P deficiency, inherited P can be utilized.
The relationship between cumulative P uptake and Olsen drawdown was scaled from “pot to field” to estimate inherited P for three different farms and suggested that inherited P could keep crops in sufficient supply for between two and 20 years, depending on the state of soil P.
Everything you need to know about phosphorus
Phosphate leaches into water through surface runoff, dispersive soil mobilization, and accidental losses such as heavy rain after application.
However, soil chemical properties can have a significant impact on a soil’s ability to absorb and retain phosphorus.
As researcher Shane Rothwell explains, if you think of soils as a sponge, they are affected by three main factors:
- sponge size
- How full is the sponge
- Ability of P to stay in the sponge
Soils with “leaky” sponges may be more at risk of pollution if they contain too much excess phosphorus. Therefore, legislation focused on methods to reduce surface runoff and accidental losses alone may not address potential long-term P losses and individual catchment areas and soil type need to be considered.
“What we need is good quality agriculture to bring the excess phosphorus reserves back to optimal levels in those areas, and then once that has been achieved we can sustain the levels to achieve a sustainable use of P,” he says.
Nutrient planning at the regional and watershed level is also needed to better manage local phosphorus surpluses from manure production.
The RePhoKUs project is funded under the Government’s Food Security Research Program and is a collaboration between Lancaster University, University of Leeds, Institute of Agribusiness and Biosciences, University of Technology of Sydney, the UK Center for Ecology and Hydrology and the N8 AgriFood programme.