A plan of attack to battle potash deficiencies
A plan of attack to battle potash deficiencies
By Hannah Velten
INCREASING potash deficiency on silage making ground is limiting yields and where slurry has not been applied to land this spring, the problem will be greater.
About 50% of soils used for silage are potash index 1 or 0, according to John Hollies of the Potash Development Association.
"Fertilising policy must concentrate on raising these soils to potash index 2 and then replenishing soil reserves after each cut of silage to avoid slipping back into index 1." (see table)
To reach target indices and maintain them, producers should base fertiliser decisions on three criteria, advises Mr Hollies. "This means working with facts and accurate measures of soil index, analysis of nutrients provided by organic manures and expected silage yields. In a year like this it is important to make the best of a difficult job."
The main cause of potash deficiency is not putting on enough potash after heavy cuts of silage to replace what is taken off, says George Brew, an ADAS senior forage consultant.
"The situation is not helped when producers waste potash supplies in slurry by applying it on grazing land, which has a relatively low requirement for potash, instead of applying it all to silage grass."
Potash requirements for first cut silage – up to 140kg/ha (115 units/acre) for index 0 – are generally met by winter slurry and manure applications, says Mr Brew.
"An average dressing of diluted dairy slurry at 6% DM, applied in spring at a rate of 3000 gallons/acre will supply about 85 units/acre of potash. However, when these levels are exceeded, grass will take up more than it requires and it will be taken off with the first cut."
To replenish levels of potash after the first and subsequent cuts, ADAS Gleadthorpe-based researcher Brian Chambers suggests putting on 100kg/ha (80 units/acre) of 25:0:16 fertiliser, which is adequate for soils of above index 2+.
However, for index 2- and below, a balanced 22:0:22 compound is more suitable or applications of muriate of potash, says Mr Drew. He does not recommend applying slurry after the first cut as this can lead to silage contamination. "Anything other than dirty water tends to dry on grass, rather than reaching the soil."
But on many farms slurry stores are full because of wet conditions and cattle being housed longer than usual, says Dr Chambers. "Producers must follow sensible slurry application rates on second cut land – 30-40cu m/ha – and for those broadcasting, slurry must be put on as soon as possible after first cut and not be allowed to contaminate new growth."
Managing livestock manures booklets, MANNER software and Fertiliser Recommendations RB209 are available from ADAS (01623-844331). *
Accurate measures of soil index, analysis of nutrients provided by organic manures and expected silage yields should dictate the potash fertiliser policy on-farm, says John Hollies (inset).
POTASHDEFICIENCIES
• Target of index 2.
• Accurate fertiliser policy.
• Avoid slurry contamination.
Potash requirements
Index Total kg/ha
First cut (23t/ha)
0 140
1 110
2 60-80
3 30
4+ 0
Second cut (15t/ha)
0 120
1 100
2 60-90
3 40
4+ 0