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Developing Your Fertiliser Plan

Developing Your Fertiliser Plan

Before you get to picking differing fertilisers for differing fields you need:

  1. Up to date soil analysis for every field
  2. To apply lime as required as a priority
  3. To know the phosphorus (P) and potassium (K) index of each field. The target is index 3 for both.
  4. To target slurry to fields that need it most (P/K Index 1 or 2 and/or silage fields)


KEY POINT: The efficiency that NPK applied is utilised by grass is greatly reduced where soil pH is low.

The value of slurry:

1000 gallons (7% DM) of slurry is equal to a bag of 5-530 to the acre. Growing 15T of grazed grass requires 40u/acre of K, whereas 1 cut of silage requires 100u/ acre of K. Slurry should be priorities to low index fields or to silage ground.

P and K requirements for 15T of grass.

As a cow grazes grass she removes P and K from the ground as grass. However, the cow does not retain all this P and K. She passes back about 40% of the P and 10% of the K she has removed back in her dung and urine onto the paddock as she grazes.

P and K (units per acre) required for grazed grass at high stocking rates after P Fixation & K Leaching are factored in:

Soil index P units/acre K units/acre
3 24 40
2 28 65
1 36 90


Be-wary of paddocks with bales removed.

Unlike a cow grazing when P and K are removed as silage they are not returned until slurry is spread back on that ground. Hence why, the nutrient requirement for silage is greater than that required for grazing.

KEY POINT: As a general rule of thumb, 1000 gallons of slurry is required to replace 4 round bales.

What nutrients to apply and when?

Ground needs nitrogen (N), P, K and sulphur (S) in differing ratios to meet the requirements of grass. For example, Sulphur is an important nutrient because it allows the plant to convert fertiliser nitrogen into plant protein. For every 10-12 units of N applied 1 unit of S is needed for this system to work at its optimum.

Nitrogen (for stocking rates 171-210 (2-2.5 lu/ha)):

The response to nitrogen is good in the spring, average spring nitrogen response = 10kg DM per 1 kg N applied. Nitrogen efficiency of slurry is also at its highest in the spring.

  • Spread 60-70 units of N per acre by 1st of april. Within your 1st and 2nd rounds you should be looking at applying 2500 gals of slurry to 30% of your farm per round.
  • Spread 90-100 units of N per acre by 1st May.

At higher stocking rates 100 units may be required by early April with 130-140 units needed by early May.

  • Apply c.30-40 units of N per acre per month (c.2030 units of N per acre per rotation) throughout the grazing season.
  • Apply c.40 units of N per acre in August to build grass.



Apply 50-75% of your P requirements in the spring with the remainder in the summer.

Soil index Maintenance (units/acre) Build up (units/acre) P units/acre (grazing)
3 20 0 20
2 20 24 44
1 20 40 60


*P build up levels are based on the new NAP action plan approved in Brussels on the 4th of December 2017. These rates are for stocking rates of greater than 130kgN/ha and will probably require users to complete a P building department regulated program.


If taking 1 cut of silage add 16 units P per acre, If taking 2 cuts add 28 units of P per acre to grazing requirements.

This extra P can be balanced by slurry if available. 3000 gals slurry = 15 units of P.


At our recent Dairy Day, held in Corrin Event Centre on the 12th of January, Dr. Stan Lalor of Grassland Agro discussed the competition between the minerals in the soil that lock up fertilizer P and the plant. Stan stated that “soils contain iron and aluminium that acts like a magnet for fertilizer P, locking it up. The lower the soil pH the stronger this magnet is.” In 2018, Grassland agro have brought 2 new P fertilisers, Top Phos 23 and Physlag 27, to the market designed to overcome this lockup.


As explained by Stan, Physlag 27 or P 27 for short is specifically designed for low pH soils. The P is released from the granule slowly giving the plant a greater ability to compete with the magnetic effect of the iron and aluminum.   Top Phos 23 is a new and unique chemical form of P to the market. It is water soluble and highly available in cold weather, making it ideal for early spring application. The form of P within the granule is unique, blocking the iron and aluminum binding effect.   Top Phos 23 also contains sulphur, which can also help to balance the N applied in the spring.



Apply maintenance rates during the growing season + build up rates in August/September. If taking 1 cut of silage add 100 units K per acre, If taking 2 cuts add 180 units of k per acre. Again this extra P can be balanced by slurry if available. 3000 gals slurry = 90 units of K.


Soil index Maintenance (units/acre) Build up (units/acre) P units/acre (grazing)
3 40 0 40
2 40 25 65
1 40 50 90



Apply 16-20 units per acre. Sulphur is best applied little and often, from the spring onwards. Apply 3-5 units of S per round

Spring Fertiliser Action Plan

Once ground conditions allow and soil temperatures are above 5c:

  • Spread slurry on light covers
  • Spread Urea (0.25-0.5 bags per acre)
  • Spread P as Top Phos/10-10-20/18-6-12


Example Programs:

Index 3
Spring Summer Autumn Total
N 100 80 70 250
P 14 3 3 20
K 28 6 6 40
S 8 6 6 20


Fertiliser Plan
Spring Summer Autumn Total
1.5 bags urea 4 bags greengrow pasture boost 1.5 bags Koch kan 250
1 bag greengrow pasture boost 30
1 bag top phos 23 24


*1000 gals of slurry in the spring will add 5units N, 5 units P and 30 units K

The baseline program above will maintain index 3 soils. Simple adjustments to this on specific fields could include:

  • Low P fields. Get extra P out early on low P fields. Apply extra Top Phos or use 10-10-20 instead of urea
  • Low K fields: Apply extra K in the Autumn. Slurry, Muriate of Potash, or an NK compound
  • For paddocks cut for bales, spread 1000 gals of slurry for every 4 bales.


Please contact your Area sales Manager or inside sales to discuss your fertiliser requirements for the coming season.

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