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Phosphorus (P) is a primary nutrient for grassland and tillage crop production.
In plants it is akin to starter/ignition in our car or tractor engines as P has a major role in rooting, early spring growth and tiller production, and controlling plant energy cycles.
In 2019, it will be important to calculate farm P requirements in advance of fertiliser planning for the year ahead as maximum P allowances may have changed drastically since last year. Firstly, under the updated Nitrates Directive that came into force for spring 2018, the maximum area for soil sampling area has reduced from 8ha to 5ha.
Therefore, additional soil samples maybe required in 2019.
Farmers can apply for increased P fertiliser allowances where low P soils (index 1 and 2) are identified in order to build up soil fertility.
Secondly, where additional concentrate feed was imported on farms during the dry summer in 2018, it will reduce the quantity of P fertiliser allowed in 2019.
Thirdly, early indications are that P fertiliser is likely to be more expensive, compared with the last number of years, due to more limited and expensive P supply in 2019.
Given these factors, it will be important to soil sample and develop a fertiliser plan that will provide guidance on P fertiliser rates and timing to maximise grass and crop production for the season ahead.
Soil results and lime applications
Maintaining optimum soil pH =6.3 on mineral soils and pH 5.8 on peat will be the first step to increasing the availability of soil P and for achieving efficient use of applied P in organic manures and chemical fertilisers.
The continuous application of P fertiliser on low pH (acidic) soils is a false economy due to higher P locked up in the soil and low P recovery by the grass or crop.
Therefore, check soil test results and identify fields that require lime in 2019 and develop a liming plan to correct soil pH.
Update the farm fertiliser plan
Take soil samples and update the farm fertiliser plan as soon as possible based on the new soil test results and take account of additional concentrates fed in 2018.
These are vital pieces of new information required when calculating farm P allowances in 2019.
The fertiliser plan will give specific field-by-field recommendations on the rate of P required depending on the crop type (eg grazing/silage).
It will help to decide on where organic manures/slurry will be applied and the correct balance of chemical to match grass growth requirements during the season.
Maximising grass growth will be critical on many farms as depleted fodder stocks will have to be replenished over the coming months.
Impact of concentrate feed in 2018 on farm P allowances in 2019
The additional concentrates fed in 2018, which amounted to 1t/LU on many farms, will have a large impact on farm P allowances.
Each tonne of concentrate feed (ration) used on farm equates to 5kg of P imported to the farm. Feed ingredients such as pulp and distillers contain lower P levels which need to be taken into account.
Under the Nitrates Directive, 300kg concentrate feed (1.5 kg P) is deducted for every 85 kg Org. N/ha (=1 cow/ha) on the farm. This small deduction covers the P that cannot be managed or recovered within the farm due to grazing management.
This concentrate P deduction is calculated based on the previous year’s feed usage and total farm organic N loading (stocking rate equivalent).
Table 1 shows the impact of different concentrate feeding rates (t/ LU) and the reduction on farm P allowances at P index 3. For example where 1.6t/LU were fed it completely eliminates the farm P allowances at index 3 in 2019.
Where P is limited, there are a number of options to increase farm P allowances in order to maintain soil P fertility levels to sustain grass production.
Organic fertilisers
Organic fertilisers (cattle slurry) will be key to filling the P gap on many farms with limited chemical P allowances. Where cattle slurry is applied to P index 1 or 2 fields, the P in the slurry is deemed to be 50% available in the year of application. This will help increase the farm’s chemical P allowance as the 50% unavailable P can be supplied by chemical P fertiliser. For example, a farm stocked at 2LU/ha can increase the farm P chemical allowance by 4kg P/ha.
Soil P build-up allowances
Under the updated Nitrates Directive, more intensive livestock farms can apply for additional P to build up soil P levels for the next three years.
For example, at index 1, the P rate has increased by 30kg P/ha and at index 2 the rate has increased by 20kg P/ha.
This is available to farms stocked above 130 kg Org N/ha. Soil samples are required every 5ha and you must prepare a fertiliser plan.
Along with this, you must attend a short NMP training course and apply to the DAFM annually.
Where soil fertility levels are low and there is a large demand for both grazed and conserved grass, it is worth exploring this option.
The timing of P application as either slurry or fertiliser is very important and has a large impact on P use efficiency by the grass or crop.
For grassland, P is required in spring as soils begin to warm up and enter the phase of rapid growth. Aim to apply P, as close as possible to the onset of significant grass growth which is usually during March or April depending on soil type.
The grass growth profile will be different on well-drained soils compared to wetter and more poorly drained soils and hence the nutrient demand will be different over the growing season. These differences on well-drained and poorly drained soils need to be taken into account, especially in early spring in relation to the timing and rates of N-P-K-S in the first and second rounds of fertiliser.
For example, on well-drained soils cattle slurry may be used as a source of P in early February if soil temperatures (> 5.5°C) and ground conditions are favourable.
However, on poorly drained soils, with very low grass growth during early season, it may be prudent to hold off the first round of fertiliser until conditions improve in late February/early March.
Where no slurry was applied, an N-P-K compound should be applied in the second or third rounds (March/April) to help boost soil P availability before the onset of high grass growth rates. Aim to apply 50% of the P requirement in spring and supply the remaining 50% in two or three applications in May/June to meet grass P demand and maintain sufficient P in the herbage for animal health.
Authors
Mark Plunkett and David Wall, Teagasc, Johnstown Castle, Wexford and Tim Sheil, J Bolger, Agri-Merchants, Wexford.
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