Potash for Cereals
Nutrient removal and balance
Potash
Potash removal 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 removal in straw
This is of increasing importance as more straw is baled because of economic pressures. Around 90-95% 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 65% of the wheat crop is now baled; this has risen 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 removal is 9.5 kg K2O/t fresh straw. Winter barley, being harvested earlier, often retains more K in the straw than wheat. Typical removal in spring barley and wheat straw is around 12.5 kg K2O/t fresh straw. 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 certain agrochemical treatments which preserve green-ness later in crop life will also have an effect upon potash retention and thus removal.
Effect of straw yield
Potash removal from the field 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 50% of grain yield, i.e. for every tonne of wheat or barley grain harvested it is assumed that half a tonne of straw is available for baling.
Potash balance
It is very useful and quite easy to keep a record of the potash balance for an arable field. Input of total K2O in fertilisers and organic manures and offtake in grain and straw are recorded and the balance is the difference (PDA Leaflet 'Phosphate and Potash removal by crops'). A negative balance over several years will tend to reduce soil Index and could indicate an impending problem unless the Index is high and being run down intentionally.
Light soils
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 barley crop removes an extra 35-45 kg/ha of potash over and above that in the grain.