Sample soil nitrogen for cost-effective application

19 January 2001

Sample soil nitrogen for cost-effective application

Soil mineral nitrogen

sampling and testing takes

time and money. So why

bother? Louise Impey found

out from a Cambs grower

while Andrew Blake asked

ADASs Peter Dampney what

is the best practice

ANNUAL soil sampling for nitrogen residues saves money and ensures N rates are optimised, says a Cambs-based grower who has been carefully assessing the amount of residual soil nitrogen available to crops for 20 years.

"It is especially relevant this year," says John Young of Breken Farms, Yaxley, near Peterborough. "Fertiliser prices have risen substantially, while grain prices are at their lowest."

Normally sampling saves him £2000 a year across the 650ha (1600-acre) farm, he estimates. But the potential for savings this year is even greater, as the liquid nitrogen used has gone up by over 70%.

Testing began with land earmarked for barley to ensure malting targets were hit, but now includes fields after oilseed rape and high fertility situations, such as where permanent pasture has been ploughed up. "There can be enormous variation from year to year and it is the only way to be precise. I do not want to waste money putting on too much nitrogen and I also want to avoid lodging. There are environmental considerations too."

Sampling is done in December and January, once temperatures have dropped and mineralisation is minimal. "The soil must also be quite wet, which has not been a problem this year at all."

Each field is sampled at 10 different points, normally with two samples taken at each point, one from the top 30cm of soil, the next from 30-60cm deep. The different horizons are kept separate to get the true picture. This season, on ADAS advice, a third sample will be drawn from 60-90cm.

Samples are kept cool and sent to ADAS Wolverhampton as soon as possible. Cropping history, growth stage, yield expectation and intended end use are sent for each field.

Two weeks later a nitrogen recommendation for amount and timing is returned. Even after a cost of £100 a field, the system saves him more than £2000 a season, estimates Mr Young. "It is very accurate and very helpful. We do have to put time into it, but the end result is worth it. And because we use liquid fertiliser, we can be far more precise with applications."

Dribble bars mean fertiliser can be applied accurately even in windy conditions, he adds.

Soils on the farm are very variable, which is what makes widespread soil mineral nitrogen worthwhile, says his ADAS consultant, Richard Pooley. "In general, our advice is to target it where you will get most benefit. Some growers are able to get a few tests done and then extrapolate the results for similar soil types across the farm.

"Others use it for particular situations; examples are the second year after ploughed out grassland and fields after strobilurin use, as where strobilurins are used higher nitrogen utlisation may have occurred."

With fertiliser making up over 25% of the variable costs in a typical combinable crop rotation there are good reasons to become more scientific about its use, he says.

Growers keen to assess the accuracy of nitrogen applications could check grain nitrogen level over recent seasons, suggests Mr Pooley. A feed wheats economic optimum is 2%, and a milling wheats is 2.2%, dry matter basis. "Every 0.1% variation represents an under or over supply of 30kg/ha." &#42

BREKEN FARMS N

&#8226 650ha – variable soils.

&#8226 Diverse cropping.

&#8226 N samples to ensure premiums, avoid lodging, max yield.

&#8226 Returns £2000 a year over cost.

Cambs grower John Young reckons soil mineral nitrogen tests save him £2000 a year.

Sliding ratios

Shifting prices mean historic nitrogen recommendations based on a 3:1 ratio, where 3kg of wheat was needed to pay for 1kg of nitrogen, are now too high. "With current prices, this ratio has shifted to 5:1. That represents a 12-25% reduction in fertiliser use, or 20-30kg/ha of nitrogen less," says Mr Pooley.