Nitrogen... too much of a good thing?

In a Victorian TOPCROP field trial last year deep soil samples for nitrogen were taken at 17 sites.

At eight of these soil nitrogen levels were very high and this increased the risk of malting barley being downgraded to feed quality — in fact, none of the crops at these eight sites achieved the higher-priced malting grade.

This result has prompted TOPCROP coordinator Margaret Evans to remind growers they should carefully choose their paddocks to be sown to malting barley.

She says that while factors such as sowing date, growing-season rainfall, crop-disease levels and varieties can all influence grain protein levels and hence the malting grade achieved, so too can high soil nitrogen levels.

She says data from deep soil nitrogen testing can be used along with other information including paddock history to assist farmers decide what paddocks to sow to malting barley and how much nitrogen fertiliser they should apply.

The CSIRO's John Angus says a combination of factors including legume residues and summer and autumn rain may lead to large but unpredictable amounts of mineral N at sowing.

He says summer weeds can reduce these levels but, again, their effect is also unpredictable.

"Deep soil tests are a way of providing a guide to the N available to crop roots and are most important where any of these factors is operating," he says.

"A sampling depth of 60 cm is sufficient before sowing, but deeper sampling may be justified if the soil is sampled after sowing and/or the wetting front has penetrated well below 60 cm."

Program 3.4.2 Contact: Dr Margaret Evans 03 5430 4414, Dr John Angus 02 6246 4911