Tests show “huge” variation in soil N content

By Tom Allen-Stevens

09 February 2012

Results from the GrowHow N-Min samples analysed so far in 2012 show a large variability in the amount of nitrogen crops can expect to come from the soil this season.

N-Min measurements to date vary from as little as 34kg N/ha to as much as 281kg N/ha, says the company’s arable agronomist Allison Grundy.

The findings all come from samples where no organic manures have been applied, so the variation cannot be attributed to the breakdown of organic material in manures, she adds.

“The difficult growing conditions last year combined with the mild weather through to the end of January are the most likely cause of the variation,” explains Allison Grundy.

“Lack of moisture during spring 2011 meant a lot of crops failed to take up all the N supplied. Add to this the impact of a mild autumn, which encouraged mineralisation, together with drier conditions than normal this winter, reducing leaching and limiting crop uptake, and the result is that some fields are full of N.

“In other parts of the country, where moisture hasn’t been a limitation the crops themselves are now full of N as they have taken advantage of the N left behind from the 2011 season. Both the N in the soil and the N in the crop must be taken into account when calculating how much spring nitrogen to apply,” she adds.

The GrowHow N-Min service measures both the amount of N in the soil (the SMN or Soil Mineral Nitrogen) and the amount of N that will mineralise from the soil over the growing season (the AAN or Additionally Available Nitrogen).