Frequently Asked Questions

Q.1 How early can I apply nitrogen to grassland in Spring?

A. Nitrogen can be applied from early January onwards or about 6 weeks before the expected turnout date. The ideal is to apply nitrogen in January as soon as ground conditions allow and there is no heavy rain forecast within 48 hours. This nitrogen will not be leached out of the soil and will be available for uptake when growth conditions improve in February and March.

Q.2 How late can nitrogen be applied to grassland in Autumn?

A. In general, nitrogen fertilizer applications The-Fertilizer-Association-of-Ireland-13should cease by mid-September on established grassland. However, nitrogen may be applied up to the end of September where soil temperatures remain high with the likelihood of a satisfactory crop response.


Q.3 Will heavy rain wash away recently applied fertilizer?

A. If there is heavy rain within 48 hours of a fertilizer application significant amounts of fertilizer can be lost in run-off water. If the rain occurs after 48 hours most of the fertilizer will be absorbed onto the soil and won’t go into solution and the risk of loss is greatly reduced.

Q.4 How long before cutting should nitrogen be applied to first cut silage?

A.Aim to have all nitrogen applied at least 8 weeks before the expected cutting date. This should ensure that there will be at least 6 growing weeks between application of fertilizer and cutting.

Q.5 How much nitrogen should I put on a first cut silage crop on fields that were reseeded two years ago?

A.Recent reseeds can take up to 110 The-Fertilizer-Association-of-Ireland-14units of nitrogen per acre for a first cut. With older pastures reduce rates to 90 – 100 units of nitrogen per acre depending on the percentage of ryegrass in the sward.

Q.6 Are there any advantages to splitting nitrogen applications on first cut silage?

A.The main advantage of splitting is it gives an opportunity to apply a small amount of nitrogen early in the season while reducing the risk of losses by not applying all the nitrogen early. For example a silage field that is not being grazed in Spring could get 40 units of nitrogen per acre in mid February to take advantage of any early growth and another 60 units in mid-March. If March arrives without the opportunity for an early split it is then advisable to apply all the nitrogen in one split.

Q.7 Does frost damage recently applied fertilizers?

A. Frost does no damage to recently applied fertilizer. Growth conditions will obviously be very poor during frosty weather and there will be little or no uptake of fertilizer but the fertilizer nutrients will retain their potency for when growth conditions improve. Fertilizer should not be applied to frozen fields due to the risk of run off losses when the thaw sets in.

Q.8 When is the best time of the year to apply phosphorous (P) and potassium (k) to grassland.

A.This depends on soil fertility and soil type. On peaty soils it is best to apply P and K during the growing season because peat has a poor capacity to retain fertilizer. Potash shouldn’t be applied to sandy soils during late Autumn/Winter as it is readily leached from sandy soils.

On non sandy mineral soils where soil fertility is poor (Index 1) it is best to apply a fairly large application of P and K in the Autumn in order to promote tillering and root development. As soil fertility improves (index 2 or better) the timing of P and K becomes less critical but there are time and labour advantages to applying P and K with nitrogen during the growing season. Risk of losses should be less with smaller more frequent applications of P and K during the growing season compared to a large single application out of season.

Q.9 When is the best time of the year to apply sulphur to grassland and how much should I apply?

A.From May onwards is the best time. 15 units of sulphur per acre is recommended for second cut silage. In very deficient areas apply 15 units of sulphur for first cut as well. 15 to 20 units per acre should be applied to grazing fields preferably in a number of applications.

Q.10 How will I know if my fields need sulphur?
A. A Sulphur deficiency is indicated by yellowing of the younger leaves. It occurs mainly on sandy free draining soils with a low organic matter content. There is no reliable soil test for sulphur and deficiency is diagnosed by means of herbage analysis. If you expect a sulphur deficiency you can confirm it by applying a sulphur containing fertilizer to the affected fields and leave a small area spread with the equivalent fertilizer without sulphur to see if there is a difference.

Q.11 Can there be problems with too much sulphur?
A. Excess sulphur can depress the uptake of selenium in herbage and reduce the absorption of copper by animals. It is important to apply sulphur only where it is needed and at the correct rates. Problems are rare but are less likely to occur if you avoid fertilizers with high levels of S (e.g. sulphate of Ammonia).

Q.12 Can I spread urea in the same year as I spread lime ?
A. The response to urea can be very The-Fertilizer-Association-of-Ireland-15poor if it’s applied within eight weeks of an application of lime. A reaction occurs between the urea and the freshly applied lime particle’s which results in much of the urea being lost to the atmosphere as a gas (volatilisation).

As a general rule don’t use urea if lime has been applied that Spring. If lime was applied in the previous back end it should be safe to use urea in the following Spring provided there was sufficient rainfall to wash the lime into the soil. Lime can be applied a week after an application of urea with no increased risk of losses as the urea will have been fixed in the soil and no reaction occurs between the urea and lime particles.

Q.13 How late into the growing season can urea be used?
A. May is the cut off point for use of urea. Research has shown that even in cool wet summers urea did not give as reliable results as CAN as higher soil temperatures increase losses. A useful rule of thumb is if you kneel down in a field of grass and your knees are dry when you stand up it is too dry for urea. If there are very good drying conditions in April conditions may not be suitable for urea particularly on bare ground.

Q.14 I normally apply one bag of urea per acre to my silage fields for early grazing. By how much should I reduce the main application of nitrogen for my first cut silage crop to allow for some carry over of nitrogen?
A. Assume that one third of the bag of urea will be available to the silage crop and you would then need to reduce your main application of nitrogen by 15 units per acre.

Q.15 I spread about 3,000 gallons per acre of cattle slurry to my silage fields in February/early March. How much nitrogen will this slurry contribute to my first cut of silage?
A. It is not possible to predict accurately the nitrogen contribution from slurry as slurry quality can vary. In general reduce fertilizer nitrogen by 20 to 25 units per acre where 3,000 gallons of slurry has been applied. Autumn applied slurry will contribute little or no nitrogen to the nitrogen requirements of first cut silage.

Q.16 What is meant by luxury uptake of potassium (K) and how can it be avoided?
A. Luxury uptake of potassium occurs in rapidly growing Spring grass which has the ability to take up a lot more potassium than it needs for normal plant functioning. High levels of potassium in the plant interfere with the uptake of magnesium and low magnesium in the animal’s diet is one of the factors causing grass tetany. The risk of luxury uptake can be reduced by avoiding large applications of potassium to grazing fields in Spring. Applications of grazing fertilizers containing low levels of potassium e.g. 27-2.5-5 will not cause problems. Precautions against grass tetany should be taken regardless of what fertilizer programme is followed.

Q.17 Is there a risk of luxury uptake when large amounts of phosphorous (P) are applied?
A. Luxury uptake only occurs with potassium (K) where there can be a rapid uptake of Spring applied potassium. There is never a rapid uptake of phosphorous and luxury uptake problems do not occur.

Q.18 When is the best time to apply lime to grassland?
A. Lime can be safely applied to grassland at any time of the year. It is important to ensure that lime is well washed off grass before stock graze it, as it can cause scouring. Lime should not be applied to silage fields in the Spring prior to cutting or between cuts as any traces of lime in the pit would have a negative impact on preservation.

Q.19 How often should I take soil samples and when is the best time of the year to take them?
A. It is now recommended on intensive farms to sample every three years and every 5 years on extensive farms. It is also recommended that a soil test shouldn’t be taken until 4 – 6 months after the last application of P and K. This may not always be practical in which case the emphasis should be placed on sampling in the same month every year samples are taken.

Q.20 What steps can I take to ensure losses of fertilizer to the environment are minimised?

A. Apply fertilizer on the basis of soil analysis:

  1. Never apply fertilizer on waterlogged or frozen soils.
  2. Don’t apply fertilizer if heavy rain is forecast within the next 48 hours.
  3. Never spread fertilizer directly into streams or waterways.
  4. Maintain your spreader in good condition so that it will spread evenly and accurately.

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P & K Calculator

The P & K calculator is an online calculator for estimating P and K fertilizer requirements for grassland and crops. It is available online, or via the iOS and Android Smart Phone Apps.P & K Calculator