2 February 1996

Dont squander P&Kin fields and save cash

By Jessica Buss

TARGETING use of phosphate and potash on silage fields can lead to vast cash savings that will help to make up for dearer fertiliser prices.

That is the message from Devon-based Genus consultant Alex Lindsay. He warns that, despite the price hikes, cutting N applications below MAFF recommendations will reduce crop yields. But in many cases phosphate and potash are over-applied because soil indexes remain unknown.

"There is a vast difference between the amount that should be applied to a soil of index 0 compared to that with an index of three," he says.

Three times too much

It may be that up to three times the amount needed is being applied. For example, a P index of 0 requires 100kg/ha (80 units/acre) whereas a P index of 3 requires 30kg/ha (24 units/acre). Over-supplying P fertiliser in this way costs an extra £20/ha (£8/acre) (see table).

It is difficult to assess how much to apply without knowing the indexes. A soil index of two is often assumed, says Mr Lindsay. "When it is lower, producers may fail to get the maximum benefit from the soil, resulting in a smaller cut of silage. And when the index is above three the recommended application is nil, so any application will be a waste of money.

"To save money target fertiliser use by soil sampling each field at least every three years. Then plan nutrient applications for each field and remember to account for those applied in slurry.

"When slurry is applied to some fields in spring the nitrogen it supplies is often ignored. Test or estimate slurry nitrogen contents and reduce bag applications accordingly."

Early application of nitrogen is essential if early grazing is required, says Co Cork dairy farmer Paddy OKeeffe. "All the evidence points to getting the nitrogen on in January.

There is no evidence it will be lost and there is evidence it will help grass production", he told a joint BGS/MGA conference in Devon. (see page 40)

For those seeking an early nitrogen dressing, SACgrassland specialist Graham Swift says that urea can be applied earlier than ammonium nitrate as it takes longer to break down and is less likely to be leached.

P and K for first-cut silage


P (kg/ha)100603030

Cost (£/ha)291799

K (kg/ha)90906030

Cost (£/ha)1919137

P+K (£/ha)48372215