Wednesday, 17 November 2021
Given soaring fertiliser prices and tightening regulations, our how to ‘make the most of muck’ webinar was timely. With the potential to save £451/ha in fertiliser costs for every 40 t of muck applied, interest is piqued. Michelle Nuttall, an AHDB knowledge exchange manager, summarises the main points.
- James Holmes – a senior scientist in our environment team
- Lizzie Sagoo – ADAS Principal Soil Scientist
Rules and guidance on organic materials
James outlined the rules and regulations, issuing a reminder that RB209 is guidance only. Defra’s eight rules cover the legislation, with one of primary concern. Rule one includes a requirement to demonstrate that nutrient applications do not exceed the needs of the crop/soil. This has implications for farmers – including a shift away (in some situations) from the autumn spreading of organic materials.
AHDB funded ADAS to explore rule one’s implications. The study found a switch in manure applications to spring could reduce nitrogen pollution (by as much as 60%). However, the same scenario increased phosphate pollution and ammonia emission risks. We’re working closely with Defra and the Environment Agency to evolve their rules – and our guidance – to help growers to make better use of resources, while protecting water quality.
Value of organic materials
Lizzie explored organic material advice, starting with the value of different types of manure. Due to recent fertiliser price increases, the nutrient value of ‘typical’ cattle muck has nearly doubled. For example, when applied at 40 t/ha, the value has increased from £263/ha (spring 2020) to £451/ha (autumn 2021). Given its value, it’s well worth taking steps to minimise losses and maximise crop nutrient availability. But it’s complicated, particularly when it comes to nitrogen, which can be lost through volatilisation, denitrification and nitrification. In recent decades, there’s been lots of research that has helped with the management of organic materials.
How to assess nutrient content
There are many tools – from laboratory and on-farm testing to data-based estimates – to help farmers assess the nutrient content of variable organic materials. For example, RB209 contains tables of ‘typical’ crop nutrient availability figures for nitrogen, phosphorus, potassium, magnesium and sulphur (for various manures at different timings). The MANNER-NPK software tool also provides estimates for crop available nitrogen, phosphate and potash.
RB209 figures are averages, based on many samples. However, many factors impact nutrient content in organic materials, including:
- Livestock species
- Livestock diet
- Bedding type and quantity
- Water use
- Material storage
As a result, averages need to be used cautiously. To improve the accuracy of nutrient content estimates, laboratory testing is recommended. Lizzie stressed the importance of testing representative samples, based on several mixed sub-samples. In the case of slurry, it means stirring, as the dry matter content settles during storage. It is important to remember this at application too – later loads (from the bottom of stores) often contain higher levels of dry matter and nutrients.
How to reduce leaching
To minimise nitrogen losses, it is important to consider several factors, including:
- Organic material type
- Amount of readily available nitrogen
- Soil type
- Application timing
Nitrate leaching is higher when applications are made during the autumn and the early winter period. This is because of the greater rainfall between application and the end of soil drainage in the spring, compared to later winter/early spring applications.
Leaching is greatest for slurries and manures that contain a higher proportion of readily available nitrogen. Generally, losses are more from lighter-textured sandy soils. However, it is a common misconception that clay soils are nutrient retentive. Additionally, although cracks shrink when clay soils wet-up, they don’t close. This means that readily available nitrogen may leach, especially if materials are applied shortly before a rain event.
Lizzie outlined results from three experimental trial sites that featured hydrologically-isolated plots that allowed drainage water to be captured and analysed. These confirmed that nitrate leaching was greater from autumn slurry applications, in comparison with those made in winter or spring.
The trials also analysed ammonium and phosphate sediment concentrations in drainage water. Ammonium and phosphate move differently through soils. They aren’t soluble and don’t tend to leach through the soil matrix. However, they still travel through soil water – via networks of small cracks and channels – to drains. This is why elevated levels can be observed in drains when high rainfall follows applications to wet soils. In the trial, peaks of phosphate and ammonium levels were clear to see in the water taken from the drains.
ADAS’s work contributed to guidelines on the relative risk of water pollution from slurry applications to drained clay soils, based on the soil moisture deficit (SMD) at the time of application:
- Where SMD is high (>20mm), leaching risk is low
- Where the soil is wet and the SMD is low (<10mm), leaching risk is high
How to reduce volatilisation
Volatilisation is a significant loss pathway – in fact, a third of the readily available nitrogen can be lost this way. Losses can be mitigated by incorporating material and using precision application kit to apply liquid manures and slurry. In comparison with surface broadcast, trials show a trailing hose can reduce ammonia emissions by 30%. A trailing shoe and shallow injection can reduce emissions by 40% and 70%, respectively. For solid manures, ammonia emissions occur shortly after applications, so incorporating material soon after application has the greatest reduction in emissions.
Optimising phosphate, potash and sulphur
Of course, it’s not all about nitrogen. In fact, some manures and slurries are more valuable because of their phosphate and/or potash content. It is important to use them on fields that are low in these nutrients and cut back fertiliser applications, accordingly. Growers should follow RB209 guidance and use ‘available’ (where the soil index is 0 or 1) and ‘total’ (where the soil index is 2 or above) phosphate and potash figures. Recent AHDB research has also led to revisions in RB209 sulphur availability figures, for a range of organic materials, for wheat and oilseed rape.
Nutrient management plans
Take the same care over your manure or slurry applications as you would your fertiliser applications. It is important to:
- Know the application rate
- Spread organic materials evenly
- Account for nutrients from organic sources in the farm nutrient management plan
- Reduce bagged fertiliser use, accordingly
As well as supplying crop nutrients, manures are an organic matter source. A typical 40 t/ha application of cattle manure will deliver about 5.5 t/ha of organic matter. Although it is difficult to put a financial value on the organic matter content of manures, research shows tangible increases in soil quality following long-term use of organic materials.