Soil phosphate mapping
AgResearch’s Dr Alec Mackay, with Dr Kensuke Kawamura, Hiroshima University, Japan, have explored opportunities for hyperspectral imaging within New Zealand agriculture.
Increasingly better spectrometers are being used on satellites to image land surface features on earth. Hyperspectral spectrometers measure reflected light from over 1500 narrow wavebands. Combinations of bands can be used to quantify pasture biomass and pasture quality at the within-paddock scale to provide spatially variable contour maps of these parameters. One such model is the NDVI (normalised difference vegetation index), which uses two optimised wavebands to describe these characteristics.
This NDVI approach has been used to quantify the phosphorous (P) concentration in canopies of pasture grown on widely different soil Olsen P and exchangeable potassium (K) status soils. We identified the two wavebands that best describe these plant nutrient concentrations and showed very high correlations between the plant canopy concentration and the soil nutrient status for both P and K minerals.
Validation of these models is now underway which, if confirmed, could enable farmers to map their Olsen P and exchangeable K soil nutrient status continuously across the farm’s landscape based on hyper-spectral imaging of the pasture.
Such spatially variable maps could then be used to generate variable rate fertiliser application plans for the fertiliser spreading industry to use on farms. Such maps would identify areas which, if receiving additional P, would lead to excessive loss by overland flows to water ways or areas in which additional P fertiliser would lead to higher pasture growth. To date this required intensity of soil mapping has been prohibitively expensive and has prevented the widespread uptake of variable rate fertiliser technologies on grazed pastures and cropped land.
This technology is also being used in current research trials to quantify variability in pasture mass and mineral concentration and how this impacts animal grazing behaviour and, therefore, the potential to redistribute nutrients within paddocks around the farm.
Dr Kensuke Kawamura, Hiroshima University, contributes spatial modelling and hyperspectral imaging expertise to our research programme.
Mr Mike Tuohy, Massey University, provides the hyperspectral radiometer and technical skills required to operate this equipment.
The work is being progressed with FRST and the Japan Society Promotion of Science (JSPS) funding.