14 March 1996

More measured approach

Maize fields have traditionally been dumping grounds for manure and slurry, but to grow the best crop requires a more accurate approach. John Burns reports

STANDARD fertiliser recommendations for maize are out of date and growers should adopt a more precise approach to supplying the crops nutrients.

Nigel Jones of Glos-based Huntseeds says that each tonne (fresh weight) of silage maize taken at 30% dry matter removes 3.5kg of nitrogen, 1.5kg phosphate and 4.0kg potash.

Therefore, a crop yielding 50t/ha (20t/acre) fresh weight removes 175kg/ha (140 units/acre) nitrogen, 75kg/ha (60 units/acre) phosphate and 200kg/ha (160 units/acre) potash.

"All three nutrients could be supplied from the soils reserves and in manure and slurry, without using artificial fertiliser," he says.

Avoid excesses

But, to avoid excesses which could reduce crop yield or quality and cause pollution of ground water, careful analysis of manures and soil nutrient reserves must be matched with application rates, and the timing of application.

"To be reliable, available soil nitrogen has to be measured when soil has warmed up. That is usually after the maize has been sown and so any fertiliser nitrogen needed to top up soil reserves has to be applied to the growing crop," says Mr Jones.

Mr Jones advocates using a German soil test which measures nitrate in mg/kg of dry soil. Samples are taken 30cm (12in) deep in heavy soils and 60cm (24in) deep in light soils, then tested for nutrient quantities.

Ian Richards, of the Suffolk-based Levington Research Group, suggests many maize crops do not need any fertiliser nitrogen at all.

"We could use nitrate levels in deep soil samples taken at the four-leaf stage, four to six weeks post-drilling, to fine-tune nitrogen application," he says.

"Then we could move away from routine seed-bed nitrogen and in many cases no nitrogen would be needed."

The groups research for MAFF showed a good correlation between soil nitrate level six weeks after drilling and the maize crops response to nitrogen.

Nitrate levels can vary within a field, but Mr Richards says that if enough samples are taken – a minimum of 15 cores in a 20ha (8 acre) field – results should be reliable enough to devise a fertiliser programme.

Average recommendation

Maize Growers Association agronomist Simon Draper says that until research, which will be conducted this year, is completed, the MGA can only give an average recommendation on nitrogen. This is 120kg nitrogen a hectare (96 units/acre) including the contribution from manures or slurry.

"As well as nitrogen, maize needs plenty of potash. It improves stalk strength and, therefore, standing ability, and has a big influence on cob formation.

He believes some of the lodging and poor cob formation seen in many UK maize crops are the result of potash shortage.

"We usually recommend 180-200kg of muriate of potash if the soil K index is two or less, 120kg for an index of three."n

Before spreading slurry or farmyard manure assess soil reserves and tailor applications to meet the crops needs.


&#8226 Excesses reduce yield and increase pollution risks.

&#8226 Assess what the crop needs.

&#8226 Apply at the four-leaf stage between rows to avoid scorch.


&#8226 Deficiency can cause lodging and poor cob formation.

&#8226 For soil index of 2 or less, apply 180-200kg/ha muriate of potash.

&#8226 For a soil of index 3 apply 120kg/ha.