Potash for Cereals

Nutrient removal

Potash

Potash removal is the other main factor which needs to be linked to soil analysis to determine correct fertiliser policy.

Potash in grain

Potassium content of grain is typically 0.5-0.6% K in dry matter for all cereals. This equates to around 5.6 kg of potash (K₂O) per tonne of grain at 85% dry matter.

Potash in straw

This is of increasing importance as more straw is baled because of the economic pressures caused by lower grain prices. Around 80 - 95% of the barley crop is baled and the reduction in numbers of mixed farms means that a far greater proportion is now sold off farms and not recycled through animals and manure. About half of the wheat crop is also baled and this could rise with developing opportunities for sale into industrial uses.

Potash content in straw is much more variable than in grain and is different for winter and spring wheat and barley, and for oats, and is affected by growing conditions.

• Effect of cereal species
  Winter cereal straw retains less potassium as there is more time for redistribution of potash out of the upper parts of the plant - typical content is 0.8-1.0% K. Winter barley, being harvested earlier, often retains more K in the straw than wheat. Spring barley and wheat straw typically contain around 1.0-1.25% K. Both spring and winter oats tend to retain much higher levels of K in straw than other cereals - often with values of over 2% K in dry matter.
• Effect of growing conditions
  With very dry pre-harvest conditions, the rapid desiccation of the crop can result in less movement of potash out of the straw. For a given yield of straw, potash removal with a hot, dry harvest can be double that of a normal season. Greater quantities of potash are also retained in the plant when the straw stays green right up to harvest and this is believed to contribute to the higher potash values found in oat straw. It is possible that treatments such as strobilurin use which retain green-ness later in crop life might also have an effect upon potash retention and thus removal.
• Effect of straw yield
  Potash removal will obviously increase with straw yield but for practical purposes because straw yields are not normally recorded, typical values for potash removal where both grain and straw are removed are quoted per tonne of grain yield. Straw yield is assumed as a fixed 65% of grain yield.

Phosphate removal

Phosphate content of straw is very much lower than potash. Removal of straw thus has little effect upon
phosphate offtake.

Typical removal standards

		METRIC kg/t of grain
		P2O5	K2O
Cereals	- Grain only	7.8	5.6
	- Grain & Straw Winter wheat/barley	8.6	11.8
	Spring wheat/barley	8.8	13.7
	Oats	8.8	17.3
Imperial conversion: kg/t x 2 = units/t

On light soils, where it is more difficult to maintain potash at a satisfactory level, it is important for cereal growers to note that baling the straw from a typical 7 t/ha grain crop removes an extra 40-80 kg/ha of potash over and above that in the grain. This is worth around £8-16/ha.