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Chemical Fertiliser Deadline Day approaching or is it??

The image above is known as Liebigs barrel. If one lath of the barrel is low, then the barrel can only be filled to the level of that lath. Stuart Childs, Teagasc Dairy Specialist has more information.

In this example, each lath represents a soil nutrient or other growth factors such as air, light, temperature, water  or soil pH.


For an example of what Liebigs barrel is attempting to portray, think back to earlier in the year. Many parts of the country were short of water and so grass yield dropped as the barrel could only fill as high as the lath representing water would allow. At the time, this wasn’t very high! Similarly, more recently, soil has become waterlogged in some parts of the country and this starves the soil of air and growth drops as air has become a limiting factor for growth. When we apply this principle to nutrients, the impact may not be as apparent. We get a situation where we can grow a certain level of grass but due to a lack of some soil nutrients, we don’t achieve a full return for other nutrients applied. This is bad in two ways;

  1. Loss of grass yield
  2. Less than optimal fertiliser use efficiency

The last day (September 14th) for spreading nitrogen and phosphorus fertiliser is fast approaching. However, this deadline only applies to two of the big three fertiliser nutrients. Potassium (K) or potash, as it is also known, is a significant nutrient in grassland systems but is all too often forgotten about due to the fertiliser restrictions that are associated with N and P.

Potassium is a key nutrient.

  • It is very important for root growth
  • It plays a role in activating in excess of 60 plant enzymes for plant growth
  • Helps maintain water balance and chlorophyll levels in plants
  • Increases tolerance to foliar diseases

In short, without adequate K, grass doesn’t grow well. It is actually used and required in very significant quantities especially in silage ground as when silage is harvested all the K is removed in the crop. Grazing is a little more forgiving in that animals tend to pass a lot of K out in their dung and urine and therefore recycle K onto the land. However, if it’s not there in the first place, they will struggle to recycle it!

Table 1. K requirements for grazing ground (kg/ha)

Soil K IndexK Advice (kg/ha)3
1 90 75
2 60 45
3 30 15
4 none none
  1. For stocking rates greater than 2 LU/ha (>170 kg/ha Org N), increase rates by 5kg/ha for each increase in stocking rate of 40 kg/ha.
  2. For stocking rates less than 2 LU/ha (170 kg/ha Org N), decrease rates by 5kg/ha for each decrease in stocking rate of 40 kg/ha.
  3. Chemical K application rates can be calculated by deducting the quantity of K applied in organic fertilisers.

Table 2. Total K requirements for silage in (kg/ha)

Soil K IndexFirst cut silage/hay2nd and 3rd cut2,3
1 1854 75
2 1554 75
3 1254 75
4 05 05
  1. Increase K by 25 kg/ha for each extra t/ha of dry matter (or each 5 t/ha of fresh weight).
  2. Note that the advice in this column is per cut.
  3. Where K build up has already been applied for previous grass silage crop (ie first cut) apply K based on crop offtake (ie Index 3).
  4. Typically no more than 90 kg/ha K should be applied at closing for silage and the remainder should be applied at least 3 months in advance or after silage harvest.
  5. No K is required in the year for sampling. For subsequent years use Index 3 advice pending a further soil test. 

Think back on what K you have applied on your ground this year. Is it adequate to meet these maintenance requirements (Index 3).  Index 3 grazing ground requires 24 units of K per year. Index 3 silage ground requires 100 units of K per year. If you have not applied these levels through slurry or chemical form, your K index will slip and grass growth will begin to suffer.

If you are at Index 1, then the simple way to start the process of rectifying this is to apply a bag of muriate of potach (MOP) to these areas in the next few weeks or at some stage before the end of the year when ground conditions are good to travel the land.

If you are at Index 2, then apply half a bag of MOP.

You should do this each year until your soil samples show that you have achieved your target Index 3.

If you apply maintenance (Index 3) levels of fertiliser throughout the year and build-up levels at the end of the year for a few years, you will achieve your target in a short enough space of time as K levels are easily adjusted.

Of course it is important to note also that as they can be moved up so readily, they can drop quickly too so much so that silage ground repeatedly cut without returning K to the land in the form of slurry or chemical fertiliser could drop a full index in one year.

So while the deadline for application of N and P fertiliser is fast approaching, don’t wash the fertiliser spreader and put it away just yet. Check your soil samples and see if you need to apply K to your soils. If you do, act on it at the first opportunity.   

Talk to your local advisor for further information in relation to K fertiliser requirements and recommendations.