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How to reduce chemical fertiliser N in 2022

How to reduce chemical fertiliser N in 2022

Getting more from every kilo of nitrogen (N) in 2022 is the new challenge as fertiliser N prices reach record highs and legislative N limits tighten in a quest to improve water quality and reduce the impacts of agricultural emissions on climate change. Mark Plunkett, Teagasc Specialist has advice

On farm we mainly associate N supply with bag fertilisers such as such as CAN 27% N or Urea 46% N. Now more than ever we need to use bagged fertilisers as efficiently as possible (The 4 R’s):

  1. The right product,
  2. At the right rate,
  3. At the right time and
  4. In the right place

We need to adjust fertiliser N rates to factor in soil N and slurry N supply to reduce our reliance on the fertiliser spreader.

Soil Fertility & N Efficiency

The starting point to improving N use efficiency on an intensive livestock farm starts with maintaining good soil fertility. For example on mineral grassland soils pH 6.3 to 6.5, optimum soil P and K (Index 3). Soil test results will help provide a solid basis to correcting nutrient deficiencies such as soil pH and making decisions on the most suitable fertiliser types and formulation for 2022.
Recent research from Johnstown Castle shows that optimising soil fertility will improve N efficiency from 35 to 65%. This means that for every 100 kg N/ha applied, we can utilise 65kg N to grow grass rather than 35kg N.

Lime & Unlock Soil N & P

Maintaining the correct soil pH will firstly improve the availability of N, P & K and secondly increase the utilisation of applied nutrients such as organic manures or chemical fertilisers. For example, liming acidic soils will result in the release of up to 70 kg N / ha / yr from the soil organic N reserves. This will help reduce overall farm N fertiliser requirements while reducing costs in the region of €155/ha. Research from Johnstown Castle shows that correcting soil pH will increase the availability of soil P and improves the efficiency of applied P. In effect, this will reduce P requirements, as soils will supply more available P during the growing season where soil pH is maintained in the optimum range.

Recover More Cattle Slurry N

Cattle slurry is a valuable on farm source of available plant nutrients (N, P & K) and can effectively replace bag fertiliser during the growing season. Utilising the N in slurry will be one way of reducing overall farm N requirements during the growing season. The first step is to know the fertiliser value of each 1,000 gallons of cattle slurry similar to when we purchase a bag of fertiliser. Typical cattle slurry is 6% DM and nutrient values are shown in Table 1 below, plan to get slurry tested to determine fertiliser value.

To increase the available N from cattle slurry it is important to apply with LESS technology in springtime on cool, calm and moist days to reduce N loss as ammonia. It is important to remember that only 50% of the N in cattle slurry is available at time of application. While the remaining 50% (organic N) is broken down during the growing season.

Timing of Early N Applications

In the early part of the growing season, there is a lower N demand as grass growth rates are low. Only apply early N when soil and weather conditions are favourable for grass growth for example; soil temperatures >5˚C, good soil trafficability and a good weather forecast ahead. Apply early N to fields that are dry, good levels of rye grasses, grass covers >400kg DM/ha and fields with optimum soil fertility. The safest form of N is an ammonia based N such as cattle slurry or Protected urea. Apply 25kgN/ha (20 units/ac). For example, 25mᶾ/ha (2,200gals/ac) of cattle slurry can supply 25kg N/ha.

Protected urea cheaper than CAN

With increasing fertiliser prices, there is an increasing differential between the cost CAN and Urea per unit of N. At present, a unit of N in the form of urea is ~ 25% cheaper than CAN. Choose protected urea as it is a more efficient form of N and will give the same performance of a CAN based fertiliser at lower cost. Research over the last 7 to 8 years here in Ireland shows that protected urea is a very reliable source of N for grassland soils plus reducing GHG emissions by 80%.

Mark Plunkett is a Soil and Plant Nutrition Specialist with Teagasc at Johnstown Castle Research Centre, Co. Wexford

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