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FAQ: N, P & K

Nitrogen (N)

What is the difference between CAN (27% N) & Urea (46%N)?

CAN is a straight 27% N source which contains calcium. 50% of the N is in the nitrate form and 50% in the ammonium N form. Urea contains 46% N and all the N is in the ammonium form.

How long should one leave between spreading fertiliser N and slurry?

Slurry applied on fertiliser N creates ideal conditions for denitrification, i.e., anaerobic conditions and high carbon compounds. It is recommended to leave 4 to 7 days before or after slurry spreading for application.

How long should one leave between spreading lime and urea?

Lime increases soil pH which increases rate of volatilisation of ammonia. Do not spread urea for 3-6 months after lime application.

How long should one leave between spreading urea/ slurry and lime?

Where urea is spread in advance of lime it will eliminate the risk of N loss through volatilization. It is recommended to leave 10 days between applying urea and lime.

How long should one leave between spreading slurry and fertiliser N?

Slurry is a carbon source and where fertiliser is spread directly after slurry application it will result in a loss in N. It is recommended to leave 4 to 7 days between the application of fertiliser N and slurry

Q. When should I start applying my 1st Nitrogen to my dairy grazing paddocks?

A. When soil temperatures reach 4 to 5 ̊C and weather is forecast to be mild then it would be suitable to apply 20 to 25 units N/ac (½ urea). Soil temperatures will vary depending on location so check soil temperatures with Met Eireann. Consider spreading slurry on bare paddock and apply fertilizer N in the next couple of weeks.


Is the soil test for phosphorus a test of plant available phosphorus or total phosphorus?

The Morgans soil P test measures plant available P.

Where a soil test shows a P index 4 for either grassland or tillage what is the advice in this situation?

Omit P applications for 2 to 3 years and re test the soil at that stage to monitor soil P levels.

What effect has soil sampling depth on soil P levels?

Soil P levels decrease with soil sampling depth as P is immobile it tends to remain in the top few centimetres of soil.


Potassium (K)

What is the % of K in Muriate of Potash (MOP)?

50% K

What is the % of K in Sulphate of Potash (SOP) and what % sulphur is in it?

42% K & 18% S

What is the maximum application of potassium on a grazing sward in the spring and why?

90kg/ha maximum application.  Reason why is to prevent grass tetany in grazing livestock

Where a soil test shows soil K levels are high (above 151 mg/L) what is the advice for grassland in this situation?

Omit K for one year and revert back to advice for K at soil Index 3 until the soil is tested again.

On light sandy soils should one aim to build soil K levels?

 Sandy soils have little ability to hold soil K.  Therefore apply maintenance applications of K annually.

Where a soil test is showing very low (Index 1) soil K levels. What is the best way to build soil K levels?

Apply organic manures such as cattle slurry / mushroom compost to build soil K levels.  Alternatively apply 50% K (MOP) depending on crop requirements.  It is good practice to apply fertiliser compounds containing K during the growing season for example 18-6-12 / 24-2.5-10 to replenish soil K levels over time.

Can lime and potassium be applied at the same time for example in the Autumn?



Q. What is the recommended rate of sulphur for grazing ground?

A. 20 kg S/ha/year

Q. What is the recommended rate of sulphur for grass silage ground?

A. 20 kg S/ha/cut

Q. When is the best time to apply S to grazing swards?

A. Apply in peak grass growing season. For example apply 4 to 5 units S /ac with each round of N applications from the 1st April.

Q. Will sulphur affect trace elements?

A. Excess sulphur on soils not deficient may reduce the uptake of copper and selenium in herbage.

Q. Which soils are most prone to S deficiency?

A. Light soils with low soil organic matter are generally more prone to S deficiency.

Q. Will slurry / FYM supply S?

A. Yes an application of 11m³/ha (1,000gal/ac) will supply approximately 4.5kg available S. An application of 10ton/ha FYM will supply approximately 9kg of available S.

Q. What is the best way to apply S?

A. Generally in fertilizer form. For example a compound (18-6-12+S) / straight N product (CAN +S) that contains between 3 to 5% S.

Q. Is it best to apply sulphur in a single application or in a little and often approach?

A. It can be applied as a single application for example ASN (26% N & 14%S). But where possible apply little and often.