Fertiliser considerations to maximise Nutrient Use Efficiency

Author: Philip Kennedy – Area Manager (New Ross) 

To maximise the efficient use of our land, organic and chemical fertilisers is something that farmers must strive to achieve. In doing so, there are a number of factors that we must take into account. When growing any crop, it is critical to ensure that the soil pH is at the correct level. The optimum soil pH is 6.3 for grassland, preferably with 60-70% calcium and 10-20% magnesium.

Agri Lime  – the forgotten fertiliser

Lime is often a forgotten fertiliser that can greatly impact soil fertility. There is not a one size fits all when it comes to lime, therefore we must choose the correct lime to suit our soil type. In general, calcium lime is the only type required in Wexford due to the high levels of magnesium in soils. Remember; if you are at Index 4 for magnesium, that is as far as the scale goes. You could be index 6 or 8 if the scale were to continue, therefore, do not use dolomitic lime. Having the correct type of lime and pH for your soil will make your Phosphorus (P) and Potassium (K) fertiliser work far more efficiently and can release up to 80kg/Ha organic N.

Know your off-takes (P – Phosphorous, K – Potassium & S – Sulphur) 

When cutting grass silage, we must remember that each tonne of grass DM per ha will remove 4kg/HA of P (3.2 units/ac) and 25kg/ha of K (20units/ac). Within a grazing situation, these nutrients are recycled and replaced from the animal’s through animal manure.  Sulphur requirements will depend on your soil type. Light soils will leach sulphur out more than heavy soils. As a rule of thumb, match your P requirements to your S requirements. Avoid spreading large quantities of S during the breeding season in order not to lock up selenium and iodine. Smaller quantities can be spread, however if you need ASN (16N 14S) Kieserite (15Mg 20S) or Calcium Sulphate (33 CA 22S), it is recommended to wait until after the breeding season as these products contain high levels of sulphur.

Slurry Nutrient Values 

Slurry is an important source of organic fertilisation; therefore, it is important that we do not underestimate the slurry values. The typical nutrient value of a 7% DM meal feed cattle slurry per 1,000 gallons per acre, is 6N, 5P and 30K. However, when using a 3.5% DM slurry (this is a more accurate figure as we must take dairy washings and added water into account when using Low Emission Slurry Spreading), these nutrient values drop to 5N, 3P and 15K. Nitrogen efficiency in slurry is variable and depends on the method of application and the prevailing weather conditions. Warm dry weather combined with a splash plate application will lead to most of the N being lost into the atmosphere. When we talk about organic fertilisers, we must bear in mind that cutting grass silage with a low index for P and K will equate to lower slurry P and K values.


Total NPKS requirements will depend on your stocking rate and how much grass is required. To give an example, a stocking rate of 210kg organic N per ha will require circa, 100kg N, 9kg P and 10kg K per Ha more than a stocking rate in the 130 to 170 kg organic N per ha bracket. The below figures are given for an average stocking rate.


Phosphorus and potassium table


As the above table shows, continued soil analysis is vital in order to use fertilisers in the most cost-efficient way possible. P requirements are high in the spring, therefore, half your P requirements should be applied in spring with a little and often approach being taken throughout the remainder of the grazing season.


Due to the fact that we get luxury uptake of K in the spring, and in order to prevent cow health issues such as grass tetany, it is best to avoid large K applications in early season. Instead, it is best to build soil K levels later in the grazing season. Applying larger K applications can be useful at certain times of the year to reduce drought stress in crops during the summer months. For example, using the likes of an 18-6-12+S product on in early to mid-May will help maximise grass quality and reduce stem production due to drought. For low index K fields, spreading a compound such as 24-2.5-10+s during the summer months can be a useful way of applying the required amount.


A reasonable heavy crop of first cut silage will have 5 tonne of DM per ha, which roughly equates to 10 tonne of fresh grass per acre. On an index 3 P 3K soil, this will remove 20kg per ha p (16 units per acre) and 125kg per ha K (100 units per acre). For index 2 P, we must add 10kg P per ha (8units/ac) and for index 1, we must add 20kg P (16units/ac) per cut. For index 2 K, we must add 30kg/Ha (24 units/ac) and for index 1 we must add 60kg/Ha (48 units/ac) K.


Farmers are looking for better quality forage now and graze silage in early spring and still cut their crop in late May. For this system, 100kg N/Ha (80 units/ac) 20kg p (16 units/ac) 100kg K/Ha (80 units/ac) and 20kg/Ha S (16 units/ac) should be applied between slurry and chemical fertiliser. In the event of low index soils, the P can be addressed for the crop, however if large amounts of K are required, it is best to apply some of this in the autumn of the previous year. This will eliminate your silage being too high in K and causing hypocalcaemia in freshly calved cows. High K silage does not affect beef cattle or lactating cows. Muriate of potash 0-0-50 can be applied even in the closed period and is a good way of addressing low index k soils. Second and third cuts will also require 25kg N 4kg P 25kg K and 4kg S per ton of DM removed. When cutting out heavy paddocks, do not forget to replace the P and K removed. Applying a product like 15-3-20+S at 2-2.5 bags per acre should maintain your index in this situation. Alternatively; 2000 gallons/acre of slurry can replace the nutrients removed.


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