Mapping biological soil nitrogen supply using mid infrared technology

by Dr Daniel Murphy and Dr Nui Milton, Centre for Land Rehabilitation,

School of Earth and Environment, The University of Western Australia.

Nitrogen (N) is the primary nutrient limiting crop production in farming systems throughout the world. In our agricultural soils 40-80% of the crop N requirements are met through microorganisms. They breakdown residues and organic matter to release plant available N (i.e. biological soil N supply). The remaining N requirement is met through fertiliser applications. To improve N fertiliser management it is important to know the timing and location of biological soil N supply, so that fertiliser N is only applied when and where it is necessary.

Splitting applications of fertiliser N at strategic plant growth stages is becoming common. This maximises the opportunity for crop uptake at the right time and minimises the risk of nutrient leaching. Growers can also spatially adjust fertiliser rates in the field using information gained from yield mapping, through knowledge of best/worst performing areas of a paddock, and by soil type. The next extension of this approach is to utilise spatial soil maps that tell us about the soils capacity for biological N supply together with information on the chemical and physical fertility of the soil. This allows soil constraints limiting crop production to be identified. For example where biological soil N supply is high, less fertiliser N may be needed to achieve optimal yields. Alternatively where biological soil N supply is low greater reliance on fertiliser N is required for adequate crop growth. Variable rate applications of fertiliser N

How do we know what the capacity of the soil is for biological supply of N?

In the laboratory we incubated soil and measured available N as an index of biological soil N supply. This method takes one week to complete and is thus both too costly and slow for use as a decision support tool for fertiliser application rates. However, this may all change in the future. Our current work is exploring the possibility of using mid infrared technology to develop calibration curves for a range of soil biological, chemical and physical soil properties (e.g. biological soil N supply, organic matter, pH, electrical conductivity, cation exchange capacity, clay content). The advantage of the mid infrared technology, is that once calibrated soil samples can be collected from the field and scanned rapidly (2 minutes per sample) to provide predictions for a number of soil properties. This process considerably reduces analytical costs meaning that growers could afford to have more soil samples analysed enabling spatial maps to be generated or deeper soil layers to be assessed.

Mid infrared is not as accurate as measuring each soil property by standard analytical techniques but does have a place in the development of soil spatial maps for the purpose of zoning fields to allow for variable management strategies and to identify soil constraints currently restricting crop production. The application of this technology is demonstrated in this article where soil was collected under an oat crop at Dangin in 2003 using a 25m x 25m sampling grid (180 separate sampling points over 10 ha). Biological soil N supply was measured using standard laboratory methods (Figure 1) and also predicted using mid infrared technology (Figure 2). This intensive sampling grid was used for assessing the required sampling grid size for farm management application. These spatial maps show the extent of similarity between grid sample points. There was good agreement between measured (Figure 1) and mid infrared predicted (Figure 2) values of biological soil N supply. This data suggests optimal crop yields would require additional fertiliser to be applied to the red and yellow areas. In a good rainfall year, low fertiliser N application would also be of benefit in the light blue area. Fertiliser N may not be economic on the dark blue areas as soil N supply is already sufficient for crop N demand.

Figure 1. STANDARD LABORATORY DETECTION OF BIOLOGICAL SOIL N SUPPLY - Data from soil samples (0-10 cm) collected on a 25 m x 25 m sampling grid. Colours represent data categorised into 4 ranges where Red = very low biological soil N supply, Yellow= low biological soil N supply, Sky Blue= moderate biological soil N supply and Navy Blue = high biological soil N supply.

Figure 2. MID INFRARED PREDICTED BIOLOGICAL SOIL N SUPPLY - Data for the same 10 ha area as that shown in figure 1 (i.e. the pattern of colour on figure 2 would be identical to that on figure 1 if the mid infrared prediction was 100% accurate). The same colour groupings apply.

What stage is this mid infrared technology at in Western Australia?

Currently this work is in a research development stage, with mid infrared calibrations being developed for a range of soil properties in Western Australian soils. Future work requires validation of data to test the transferability of calibration curves from one region to the next and also across soil types. We hope to answer these questions over the next 2-3 years. Potential future development could include portable mid infrared machines that would enable in-field application of mid infrared to predict soil properties instead of having to collect samples and transport them to the laboratory.

This research is being funded through the GRDC Farming Systems Program and Soil Biology Initiative. We wish to thank Bill and Ritchie Walker from Dangin, for their ongoing support to our research efforts.