Potash for Grassland

Over-high application rates

When more K is provided than is necessary, grass may continue to take up K with no change in yield, so that % K content in the plant increases. This should be avoided on cost grounds, but may also reduce magnesium availability and increase the risk of hypomagnesaemia (staggers) although the more frequent reason for such mineral disorders is that total dietary magnesium supply is inadequate.

Animal stress such as calving/lambing increases the risk of mineral imbalance in the animal. Adverse weather conditions provide a double risk in that stress on animals is increased and growth rate and nutritional content of herbage is reduced. Magnesium problems are more likely on light soils and in other situations where soil Mg is poor. Maintenance of soil Mg at index 2 by the use of magnesium lime or fertilisers is advised where there is a history of problems.

Potash requirements for grazing are low as detailed above but it is obviously prudent to avoid any potash applications, including FYM or slurry, at times of greatest risk of mineral imbalance such as during the spring flush of growth and in the autumn when grass supply and value is decreasing. There are no risks of upsetting magnesium balance in silage if proper calculation of potash needs are undertaken based on soil analysis and crop removal. Again it is essential to include the contribution of manures in the calculation. If magnesium intake from forage is low, animal requirements should be achieved by supplementary provision.

For cut swards there is a much greater risk that potash supply is insufficient for full yield than of over-high application.

This topic is dealt with in more detail in PDA leaflet 6 - Potash, Magnesium and Sodium Fertilisers for Grass.

Phosphate & potash removal

Phosphate and potash use should be based on the amounts removed.

These can be estimated using typical rates of phosphate and potash removal in each tonne of different grass crops, as follows:-

		REMOVAL PER TONNE
		kg/t P2O5	kg/t K2O
	Fresh grass 15-20% DM	1.4	4.8
	Silage 25% DM	1.7	6.0
	Silage 30% DM	2.1	7.2
	Hay 85% DM	6	18

Typical whole season removal per hectare for different systems, assuming average yields:

		Typical Yield	REMOVAL PER HECTARE
			kg/ha P2O5	kg/ha K2O
	1 silage cut and grazing	23t/ha	40	140
	2 silage cuts and grazing	38t/ha	65	230
	3 silage cuts and grazing	47t/ha	80	280
	4 silage cuts and grazing	54t/ha	90	320
	1 hay cut and grazing	5t/ha	30	90

In some situations, particularly with very high herbage yields, the annual quantity of potash removed may reach 500 kg K₂O/ha.

PK balance

Silage and hay remove 3 to 3.5 times as much potash as phosphate

This wide ratio frequently results in over-use of phosphate and under-use of potash leading in time to excess soil P, which is undesirable for environmental reasons, and to low soil K which will reduce yields. Such an imbalance of P and K is doubly costly because of yield loss from K shortage and because phosphate is considerably more expensive than potash.

Upland and Organic farming systems

The same principles of potash manuring apply to all systems of production and are even more important for systems which rely on clover to generate nitrogen by biological fixation as this is seriously affected by any shortage of potash. The intensity of stocking and conservation yields clearly affect the amount of potash removed and the 'standard' recommendations below should be adjusted accordingly.

Yields from grass-white clover swards without N fertiliser are normally some 80% of those from grass with high N application (as in example below). Lucerne and red clover in organic systems will, however, commonly give as high yields as grass, leading to very high levels of removal of potash.

It is important to ensure that potash removed in any system is replaced if soil fertility is to be maintained and a sustainable system operated.

Manures on farm simply re-cycle nutrients that were in the soil to start with, plus some nutrients from bought-in feeds. Even where manures are stored and used efficiently there will be some natural losses from the system as well as an 'export' of potash in whatever products are sold off the farm. Approved potash fertiliser or imported manures will be needed to replace these quantities and maintain soil fertility.