Starting 2019 on the right fert

Published 23 November 18

2018 was a challenging year but as winter approaches, James Holmes, AHDB’s Senior Scientist, believes now is a good time to take stock and start planning nutrient management for spring 2019.  This is especially important for the coming season as you will likely be under increased financial pressure and it will be important to promote grass growth as early as possible.

There are two major considerations that will help you plan the use of nutrients next season. The first is checking how up-to-date your soil test results are. The second is how much of the phosphate and potash you planned to use in 2018 was actually applied or even needed?

Soil should be sampled and analysed every three to five years. Sampling 25% of fields each year is an efficient way of maintaining this schedule and now is a good time to revisit the results and start to plan nutrient management next year. If your soil test results are more than four years old, then plan to take new samples and send them away for analysis. When taking samples, try to collect them from the same point in the season and, if possible, the same point in the rotation that they were collected last time.

Your soil test results provide essential information on phosphate, potash, magnesium and crucially, pH. Lime is often overlooked, yet is arguably the most critical factor to improve grassland productivity. A list of companies that offer soil tests is available to download here.

The target pH for optimal grass growth is 6 – 6.5. It is important to correct soil acidity, otherwise grassland productivity can be affected dramatically. Lime can be applied when ground conditions allow. However, if silage is to be grown, lime should be applied two to three weeks before the first spring nitrogen application.   

Management of phosphate and potash is remarkably straightforward. Your soil test results will tell you the level (Index) of phosphate and potash in your soils. How much phosphate and potash you apply depends on these results.

The target level for phosphate is Index 2 and Index 2 minus (2-) for potash. So if at the time you sampled the soil it was at the target index then for the next four years you should only plan to apply what is ‘taken off’ the field i.e. in grass eaten or grain and straw. If a soil is below the target level then you should plan to increase it. Should a soil be above the target level then generally it is not necessary to apply phosphate and potash fertiliser to ensure optimum crop growth.

If you are satisfied with your soil test results, the next question is how to deal with applications missed this season? First and foremost, you will be considering when to apply the missing application but you should also consider how much needs to be applied.

The answers lie in how well (or not) grass grew this season. This is because phosphate and potash applications should be adjusted where yields were lower than expected. The AHDB Nutrient Management Guide (RB209) provides typical offtake values for fresh grass and silage that can be used to adjust application rates if actual figures are not available.

For example, if the first cut of silage was 5 tonnes (t) less than expected you should make allowance for this. At 25% dry matter, you would have expected 5 t fresh weight of silage to take up and remove approximately 10 kg P2O5/ha. Therefore, if you did not apply phosphate up front at the beginning of the season and were planning to apply after each cut or later in the season, then the total amount you planned to apply should be reduced, taking in to account lower yields across the whole season if necessary.

If you plan to make up missing applications of fertiliser in spring 2019, you may choose straight or compound fertilisers. However, large applications of phosphate and potash should be avoided and smaller applications spread during the season. If you use a compound, be sure to choose one that has the most suitable ratio of nitrogen, phosphate and potash.  

As always, it’s crucial to account for the phosphate and potash supplied by applications of manures when planning manufactured fertiliser applications. A single application of slurry at 30 m3/ha in the spring will supply 37 kg N/ha, 36 kg P2O5/ha and 75 kg K2O/ha with a total fertiliser value of £90/ha. Applying slurry to silage ground, or to a proportion of the first grazing round covers permitting, will allow considerable cost savings to be made.

Of course, much depends on your grassland management strategy but it is worth targeting your most productive fields for early nutrient application. For more information, download Managing Nutrients for Better Returns and the Grass and Forage Crops section of the AHDB Nutrient Management Guide (RB209).