FIRST-cut silage aftermaths should need no more nutrients because of leaching in recent wet weather. But where manure and slurry has been spread on maize fields instead in the past five years, phosphate and potash levels may be depleting.
Thats the message of Mike Dewhurst, Fieldfare Associates.
It is important to take soil analyses to make sure potash supply is adequate on grass fields, he says. "After a first cut of silage, some potash must be replaced."
When a field is to be taken for second cut and has a soil index 2, it needs 80kg/ha (64 units/acre) of potash, but when the field is to be grazed aim to apply 50kg/ha (40 units/acre) at a later date or as animal manure, adds Mr Dewhurst.
Blended or compound K and N fertilisers are usually good value. N should be applied at 2.5kg/ha (2 units/acre) for each days growth until the next cutting or grazing.
Phosphate applications are not so critical. Providing soil indexes are above two, fields only need a maintenance dressing of 30kg/ha (24 units/acre) at some time during the season to prevent a long-term rundown, he advises.
ADAS soil scientist Brian Chambers suggests now is a valuable chance to reduce quantities of slurry and dirty water in store by spreading it on first-cut aftermaths.
Dr Chambers suggests surface spreading slurryrather than broadcasting, to halve ammonia losses.
He calculates it is possible to save £10/ha (£5/acre) in N fertiliser costs by surface spreading compared with bag applications. This benefit falls to £5/ha (£2.5/acre) when slurry is broadcast instead, assuming an application rate of 50cu m/ha (4000gal/acre).
At the same application rate slurry can supply up to 100kg/ha (80 units/acre)potash – worth £20/ha (£10/acre) in potential fertiliser savings.