Ensuring all soil nutrients are at the optimum level will be crucial if you are to get the most from increasingly expensive nitrogen fertiliser this spring, say experts.

While many fertilisers have jumped in price – notably P & K (Arable, 18 January) – the investment is still worthwhile and could reduce your nitrogen requirement, says Rothamsted’s Johnny Johnson. “There is a big nitrogen-potassium interaction in winter wheat, so there’s a huge opportunity to reduce the amount of nitrogen fertiliser required by ensuring you have the right amount of soil potassium.”

Jerry McHoul from Potash Ltd agrees. The company’s trials found that nitrogen use efficiency in wheat was improved from 57% where no potassium was applied to 85% where 150kg K2O/ha was used.

Potassium also improves the ability of plants to retain water, making them better-able to cope with stress conditions (eg, drought). So where biomass is increased by adding nitrogen, this should be complemented with extra K, adds Mr Johnson. Beans have a particularly high K requirement, while for oilseed rape, results from one trial suggest phosphate is more important, he says.

Mr McHoul says levels of potassium and magnesium in UK soils have been falling for several years and are now “dangerously low” in some places. He estimates about 55% of soils are at the target arable K index 2 (see box), with 25% below this, risking poor crop performance. “Many soils have been in deficit for the past 15 years, ie inputs have been below offtake. There may be a pool from previous years, but this is not inexhaustible.”

The picture for magnesium is more variable, with some places showing too high a level (eg, where magnesian lime has been applied frequently), while others are in deficit, he continues. In East Anglia over 30% of soils are index 0-1 and need extra Mg, he says. “RB209 effectively advocates using Mg fertiliser when the soil index runs down to 0-1, but I would question running any nutrient down to zero. You need to keep it around 1-2, rather than dipping lower.”

But for phosphorus, in particular, growers should remember, there is no “quick fix” Mr Johnson adds. “You can’t increase soil P quickly, you’ve got to build it up to a sufficient level and maintain it.”

He urges growers to give more attention to improving soil structure as well as nutrition. “I think many have overlooked this, but get it right and plants can make good root growth and can access more nutrients. Soil structure is going to be more and more important if farming is to achieve the yields it has to.”

Crop nutrition

• Strong N:K interaction in wheat
  
• K can improve N use efficiency
  
• Ensure soil indices are maintained
  
• Avoid running other nutrients too low – no quick fix
  
• Good soil structure crucial to maximise rooting
  
• Balance inputs with crop offtake

In the balance

Balancing fertiliser inputs with crop offtake is likely to feature more heavily in the revised RB209 fertiliser recommendations, says Rothamsted soil scientist Keith Goulding.

“There are not likely to be major revisions on indices/ analysis for P, K, Mg and lime, but there will be more emphasis on inputs balancing outtakes.”

Yara’s Mark Tucker says average P and K application rates for UK cereals are about 34kg/ha and 41kg/ha, although crop removal is nearer 73kg/ha and 100kg/ha, respectively. “Balanced nutrition is essential,” he stresses.

Including P and K with the first nitrogen dressing can help reduce passes across the field throughout the season, he says.

Target soil indices for arable rotations

• P – Index 2
  
• K – Lower Index 2 (2-)
  
• Mg – Index 2
  
• pH – 6.5 (5.8 on peat soil)
  
• S – see HGCA Topic Sheet 101 “Decision support on sulphur application to wheat”