Refining deep soil nitrogen testing to reduce environmental losses

Many grain growers conduct deep soil nitrogen (N) testing annually to understand how much N they have stored in the soil, and how much is needed to meet the demands of the growing crop. This is generally done through the collection of multiple soil cores across a paddock down to 60 – 100 cm depth, which are all bulked together into one sample, to produce one value of stored N per paddock.

A weakness with this approach is that it does not provide an understanding of how potential N storage varies across a paddock, and where the N is stored in the soil; is it all accumulated in the top 10 cm, or is there a big bulge of N at 90 cm deep, beyond the reach of most roots and at risk of leaching to groundwater?

Furthermore, the actual timing of this deep soil N (DSN) testing can vary, as sampling may occur at sowing, late autumn, or in late winter-early spring. This variability can result in large variations in the final N test results from the laboratory. Given test results are used to calculate the amount of N fertiliser to be applied to the crop, there is potential for large over-supply of fertiliser, potentially resulting in nitrate leaching to groundwater or the production of nitrous oxide (N₂O), a potent greenhouse gas.

This project aims to use two on-farm demonstration sites to illustrate the value of considering paddock variation when sampling for N, and depth-incremented sampling to understand where the N is distributed in the soil profile.

Collecting deep N samples at key stages throughout the season will also demonstrate how the test values can change according to when the sampling was done. Assessments of plant growth stage and greenness (NDVI) will be done at each stage to assist in defining the optimum time for N sampling to be done.

Results from this work was published in Research for the Riverine Plains, 2018 and is available via the following link

2018 Refining deep soil nitrogen testing to reduce environmental losses