Measuring N pays dividends

Measuring the amount of nitrogen available to a crop throughout the growing season offers significant fertiliser savings over the traditional field assessment method according to Mechteld Blake-Kalff of Hill Court Farm Research.

“Research shows that sampling soil mineral nitrogen is worthwhile, particularly where soil nitrogen supply is estimated to be in excess of 75kg/ha,” she said at the AICC conference. Where soil N was high it allowed growers to cut nitrogen applications, reducing costs, she explained.

But it was equally important to determine whether soil nitrogen levels were low to ensure enough fertiliser was applied, she added.

Research showed that SMN sampling and the Growhow N-min test gave a more accurate prediction of soil nitrogen supply than the field assessment method, said Dr Blake-Kalff.

“Estimates from measuring SMN are much closer to the real SNS than the field assessment method,” she said. “We can get the SNS within 50kg for 70% of the samples and 50% within 30kg.”

Results were particularly conclusive on deep clay soils. “They often have a lot less SMN than RB209 suggests,” she said. “Therefore, if you do use the field assessment method you need to consider that they may not have as much nitrogen as you think.”It was advisable to sample soil mineral nitrogen on a number of barometer fields to calibrate the system, she said. “Then you will have a base line level for nitrogen in your fields.”

Traditionally, soil mineral nitrogen sampling was carried out in the spring but current work indicated that autumn sampling could be as accurate on medium and heavy soils, she said.

“If this works out to be consistent, growers will be able to start sampling nitrogen earlier in late November and early December.”

Test accuracy was improving and it was likely that all samples would be accurate to 30kg/ha in the next few years, she concluded.

Soil nitrogen sampling terms 

 Soil mineral nitrogen (SMN) – The nitrogen available for plant uptake at the time of sampling to a depth of 90cm.

Growhow N-Min – SMN down to 60cm and additionally available nitrogen. The surplus nitrogen is the nitrogen which is mineralised and becomes available to the plant between the time of sampling and harvest. It also estimates the nitrogen uptake from soil below 60cm.

SNS – Soil nitrogen supply is a combination of the soil mineral nitrogen, nitrogen already in the crop and the nitrogen that may mineralise during the growing season.

Look after soil samples 

Minimising transport and storage time for soil samples would ensure accurate assessment of soil mineral nitrogen, said Dr Mechteld.

“Research shows that samples show a significant increase in SMN during storage, even when stored at low temperatures,” she said.

Soils with high levels of organic matter were particularly susceptible, she added.

Growers using the Growhow N-min service didn’t have to be quite as quick, she said. “The fraction that can be mineralised is also measured so there is no impact on the final answer. You just get a shift between the different pools.”


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