With bagged ammonium nitrate fertiliser prices rising above £600/t, soil mineral nitrogen testing has become essential for one Hampshire farm manager as he seeks to optimise nutrition across all his crops.

Charlie Batten used a range of spring nitrogen rates, from 130kg/ha to 240kg/ha, on his oilseed rape last year.

This highlighted how easily nitrogen can be wasted if you don’t check how much of the valuable nutrient is already in the crop and soil.

Mr Batten is south-east regional farm manager for management group Velcourt, and he sees early-season soil testing for cereals and rapeseed as vital to check if soil nitrogen supply is higher than normal following the dry and mild winter this year.

If it is, he can tailor nitrogen fertiliser rates to the crop’s needs.

“The high price of nitrogen emphasises how important it is to get soil testing correct. It is not about cutting nitrogen, but achieving the optimum rate,” he tells Farmers Weekly.

See also: How tests for additionally available N can cut fertiliser use

Charlie Batten and William Baldwin (right) © David Jones

+Canopy management

Across the 180ha of oilseed rape he grows at Home Farm, West Tytherley, on the Hampshire-Wiltshire border, Mr Batten manages nitrogen using a canopy management approach.

The spring rate of solid ammonium nitrate in 2021 was in the 110kg/ha range, depending on soil mineral nitrogen levels and crop biomass.

Some forward crops can see savings on expensive nitrogen, while some later-drilled ones usually need a more generous treatment.

The aim is to provide all crops with the nitrogen required to produce an optimally sized canopy. Trial work shows that excessive early nitrogen can reduce yields significantly.

If the soil test shows high levels of nitrogen, or if the biomass is high, the first dose of fertiliser is adjusted according.

The second application is more often a flat rate of 90-100kg/ha.

That means, a crop assessed to need 130kg/ha will get 40kg/ha in late February and then 90kg/ha in late March, while a crop needing 240kg/ha will receive its first split of 150kg/ha and then the standard 90kg/ha later.

“It’s often the early-drilled crops that tend to get less nitrogen, while the later-sown ones need a bit extra to achieve an optimal canopy size,” he says.

Mr Batten directly manages in Hampshire, from north of Andover to Lymington on the south coast growing 544ha of oilseed rape across the 2,700ha of land.

They drill from early August into the second week of September across a range of soil types, so matching nitrogen rates to crop biomass is very important.

© MAG/David Jones

Spring nitrogen

The first assessment of spring nitrogen requirement is made in January with a green area index (GAI) measurement.

At the end of January, one block on the farm had a GAI of 1.8-1.9, and as one unit of GAI is equivalent to 50kg/ha, the crop would contain nearly 100kg/ha of nitrogen already.

The aim is to build the canopy size to GAI 3.5 by early flowering in April.

This index can be calculated by cutting all the oilseed rape crop in a 1sq m patch, measuring the fresh weight in kilograms and multiplying by 0.8.

As this is a lengthy process, there are apps available that can assess GAI using a mobile phone.

The second assessment is used to measure the amount of nitrogen available in the soil. This can be done with the CF N-Min test, devised by CF Fertilisers.

The test measures the amount of nitrogen in the soil at that time – the soil mineral nitrogen (SMN) – but also what will become available to the crop by nitrogen being mineralised in the soil – known as the additional available nitrogen (AAN).

This soil test is conducted at the end of winter before nitrogen is applied. The SMN and AAN values are added together to give an indication of how much nitrogen is available in the soil.

The GAI measurement is used to calculate how much nitrogen is needed to achieve an optimum canopy size.

Bagged nitrogen is applied to make up the shortfall between the crop’s requirement and nitrogen supplied by the soil.

On-farm testing

The soil mineral nitrogen testing on the farm is conducted by William Baldwin, operations director at soil sampling and trials group Envirofield.

He uses a buggy to cross each field to take soil samples down to a 600mm depth, which are then sent off to the laboratory for nitrogen testing.

The charge is £155/field, with at least 15 core samples taken across each field, and the results will be back with the grower in about 10 days.

“The testing allows growers to manage nitrogen fertiliser rates to the crop’s requirements,” says Mr Baldwin, adding that he is seeing an uplift in business this season driven by the high cost of nitrogen.

He suggests if growers use soil nitrogen mineral testing, they should employ a tramline trial to compare the nitrogen rates suggested by the soil test with normal farm practice to build up confidence in the testing results.

Back on the farm, Mr Batten uses the soil nitrogen testing across five fields of the 860ha farm at Norman Court, West Tytherley, so he can build up a picture of soil nitrogen supply across all the land he farms and not have to test all the fields.

Grain analysis from wheat crops and the nitrogen content of malting barley through the rotation give him added information about whether his nitrogen strategy is appropriate.

“We are convinced that it is a small price to pay to enhance our nitrogen recommendations,” he says.

Dry winter

With a mild and dry winter this season, he is hoping for a high level of soil mineral nitrogen, which may help him save some nitrogen costs this spring.

He believes testing works well and gives the group value for money, whether ammonium nitrate prices are well above £600/t or just below £300/t like they were back in June 2021.

The soil mineral nitrogen test tells him how much N is available in the soil, and he then looks at the relationship between nitrogen and grain/rapeseed prices to decide on an economic nitrogen rate.

Yield across all 588ha of oilseed rape on the Hampshire farms have averaged 3.75t/ha over the past five year for normal double-zero varieties, and slightly less at 3.45t/ha for high-erucic acid rapeseed (Hear).

One added advantage of applying the exact amount of nitrogen the crop needs is more even ripening, and the more even oilseed rape crops have been easier to harvest, often not using glyphosate as a desiccant.

“Over the past three years at Norman Court we have not needed to desiccate oilseed rape, as we are growing more even crops which makes harvest much easier,” he says.

Lodging reduction

There has also been a dramatic decline in the amount of lodging seen since the soil testing and variable nitrogen rates were introduced some six years ago.

“It’s very rare that we see any lodging, and lodging is very clearly linked to yield loss,” he says.

Managing the biomass of the crop is very much about nitrogen, while the role of plant growth regulators is to manage the crop height and increase the proportion of fertile tillers, he adds.

Velcourt has been using soil nitrogen testing for a number of years, but this season it is extending the testing across the whole 56,000ha of farmland it manages, from Devon across to Kent and up the Moray Firth in Scotland, to look at fine-tuning its use of nitrogen fertiliser.

Nick Anderson, head of crop technology, says grain nitrogen analysis after harvest indicated significant variability, with some farms not putting enough nitrogen on crops and others putting on too much.

A deviation of 0.2% grain nitrogen from the optimum could indicate lost yield potential of 0.5t/ha. Meanwhile, high grain concentrations represent wasted resources and potential environmental costs.

“Diffuse nitrate pollution and nitrous oxide emissions are things that should concern all of us. It is essential we use all tools available to us to optimise nitrogen rates,” he says.