Rain encourages soil microbes to mineralise soil organic nitrogen (N) into plant available mineral forms. Some mineral N can leach, moving down in the soil profile with subsequent rain.
Deep N soil testing can better estimate the amount of N available in the soil for the crop to make use of, compared to a 0 – 10cm soil test. A more accurate view of nitrogen available in the soil improves the capacity to match nitrogen fertiliser rates to crop demand.
Considerations for deep N soil testing
Nitrogen is more likely to be found deeper in the soil following significant rain events.
Soil sample close to or just after sowing.
Crop root depth is a good guide for how deep it’s worth sampling.
Break soil samples into increments e.g., 0-10, 10-60 or 10-90cm.
Ensure correct sampling technique to avoid any fertilizer bands if sampling after sowing.
Charlie Walker is the Head of Agronomy at Incitec Pivot.
GRDC Get the right advice on soil test results, Dig deep for nitrogen picture
Back Paddock Soil testing for deep nitrogen – use and misuse
Charlie Walker, Incitec Pivot, Chris Dowling, Back Paddock Co, Therese McBeath, CSIRO