Nitrogen is key in making the most of muck

The main reason is the rocketing price of manufactured fertiliser, explains Brian Chambers, ADAS head of soils and nutrients. “In the past 10 years the prices of manufactured N, P and K fertilisers have all just about tripled. Today nitrogen is about 90p/kg – only a few years ago it was only about 30p/kg. Clearly by using organic sources growers can reduce production costs.”

Many farms have access to manures that cost comparatively small sums and attract little more than storage and application charges, Prof Chambers points out.

Another key reason for growers’ greater interest is improved technology allowing them to assess the nutrient content of manures more quickly and apply them more precisely, he adds.

There are plenty of fertiliser substitutes.

“Farmyard manure and slurries are still the main sources, but the new kid on the block is digestate. We’re now seeing significant volumes coming through, and annual output could rise to 5m tonnes.”

At the recent relaunch of ADAS’s updated MANNER-NPK software, the value (at current fertiliser prices) of pig farmyard manure, cattle slurry and broiler litter were put at £10/t, £3.40/cu m, and £37/t, respectively.

Solid fertiliser substitutes, especially composts and sewage sludge cakes, have extra long-term value in that they help raise soil organic matter levels, adds Prof Chambers. “Most of the UK’s arable land needs more organic matter.”

As manufactured fertiliser prices have increased so has the value of organic materials, and suppliers have raised their prices in line, notes Hutchinsons’ Rob Jewers. “Even so, organic materials usually represent good value for money when imported on to farms,” he says.

As well as contributing nutrients, they improve the soil’s structure and water retention and benefit the organisms living within it, he adds. Ideally, to minimise nutrient losses, they should be applied in early spring.

“But this doesn’t suit all rotations, and growers with large areas of winter cropping may have to spread in summer. This can clash with the busy harvest and autumn cultivations period.”

The bottom line
•    Manufactured fertiliser prices have tripled in past decade
•    Fertiliser costs for the average wheat grower have risen annually by 21% over the past seven years
•    Current value of pig farmyard manure, cattle slurry and broiler litter are £10/t, £3.40/cu m, and £37/t, respectively
So to what extent may organic materials replace manufactured fertiliser? The Fertiliser Manual (RB209) provides detailed analyses of a wide range of them.

Where available in sufficient amounts, manures are really good for correcting low soil P and K indices, says Ecopt’s Ian Richards. “Treat manures as fertilisers – not wastes.”

The key driver is nitrogen content, he explains. “It’s best not to try to use manure to meet a crop’s full N requirements – the supply and rate of release is too uncertain.”

RB209 advises that no more than 50-60% of a crop’s N needs should come from manure, notes Prof Chambers. “It’s the nitrogen content of the manure that should dictate how far you can go with substitution. Get it wrong and you can easily end up with flat crops.”

That is where science, in the form of equipment such as the Agros and Quantofix meters, which measure the readily available N content of slurries/digestates, backed by the MANNER-NPK software, comes into the picture to help, he explains.

“Nobody uses manufactured fertiliser without knowing its analysis – it should be the same with manures.”

Manure application is mainly dictated by the nitrate vulnerable zone (NVZ) regulations, warns Mr Jewers. “The rules are complex and a FACTS-qualified adviser should be used to assist with planning manure use and writing a nutrient management plan. Regular soil testing should be carried out on one-third to one-quarter of the farm each year so that crop requirements can be accurately predicted.”

Mr Richards urges farmers outside NVZs to follow the advice in DEFRA’s publication Protecting our Water, Soil and Air.

To read further please click this link to FWI.

Author:
Andrew Blake