A strategy to extract information from airborne gamma ray spectrometry for applications in plant-based industries

Radiometrics is a successful natural resource tool; however technology captive projects of the past need to progress to on-ground decisions. This paper has focussed on developing a strategy to bridge this gap. The data to decision strategy involves a four stage process. (1) Be aware of the strengths and limitations of the technology; (2) match the data to the application, (3) target research with an application focus; and (4) evaluate the value of the geophysical information in decision making. This approach was used to bridge the gap between the gamma radiometric technology and ‘on-ground’ decisions in plant based industries

Mapping rice yield and its fertilizer response at provincial-scale in Takeo, Cambodia

Our objective was to identify responsive areas for nitrogen (N), phosphorus (P) and potassium (K) fertiliser use on rice (Oryza sativa L.) within Takeo province from trial results obtained at 2336 sites. Regression tree analysis identified in order of decreasing importance the following factors which explained the variation in yield from on-farm experiments: season, location, fertiliser, soil type. Semi variograms of the same data set indicated that a maximum spread of 12 km in datum points was required to map yield across the province. Separating the results into N response classes decreased the maximum spread of datum for mapping to only 8 km. The maps generated indicated areas in which response to fertiliser is more or less likely. Whereas P responses were predicted to be relatively uniform across the province, N and K responses were more varied. Results suggest a very strong positive response to N, particularly on the central-west of the province. They also suggest negative effects of high N rates on the most fertile soils (Kbal Po, Krakor) in the east of Takeo, and in the Prateah Lang and Koktrap soils in the flooded areas of the south east of Takeo. At the provincial scale, the maps identified areas that can be used to target extension effort to where it is likely to be most effective, and areas where further research is needed to clarify reasons for poor responses. This should enhance the strategic planning capability for delivery of extension services and fertiliser inputs

Using weight of evidence modelling to map rice yield in southeast Cambodia

Agricultural services often need to represent variations in expected benefits from input use within a designated area. Our aim was to produce maps that integrated data from research trials with researchers’ knowledge using weight of evidence modeling. A validation data set was used to generate maps indicating areas in which response to fertilizer was more or less likely. Maps of predicted yield were also generated for Takeo province, Cambodia using EXPECTOR software to combine, using Baysian statistics, weighted evidence from soil, a flood risk and N fertilizer response maps. The data set comprised 2336 on-farm trial sites. The two sets of maps were in broad agreement, suggesting that provincial-scale maps of yield and yield response could be generated using the weight of evidence approach when there is a low density of field trial data. Discrepancies between the two data sets were only significant where the spread of map data points exceeded the maximum likelihood distance and where increased N fertilizer rates appeared to exacerbate insect damage to crops. At the provincial scale, the maps identified areas that can be used to target advice on appropriate fertilizer rates. This could enhance the strategic planning capability for delivery of fertilizer inputs

Using maps to show how rice yield and yield response to fertilizer varies at provincial-scale

While agronomy tends to offer advice that is generally true, what fanners need is site-specific advice. Our objective was to represent variations in expected benefits from fertiliser use within an area using maps that integrate data from research trials on farmers' fields with researchers' knowledge and experience about the area to which it is applie

Relationships between soil properties and high-resolution radiometrics, central eastern Wheatbelt, Western Australia

A quantitative investigation was undertaken to ascertain the relationships between radiometrics and surface soil properties within a 77 km2 survey area, on the Elashgin Creek catchment, Wyalkatchem, Western Australia. A high-resolution airborne gamma-ray survey, with a measurement 'footprint' width of approximately 40 m and length of 80 m, provided measurements of radionuclide distributions across the study area. Ground-based gamma-spectrometry measurements, with a much smaller 'footprint' of one metre diameter, were also made at the majority of sampling sites. Field texture and gravel content from 132 sampling sites were recorded, with particle-size and mineralogical analysis conducted on a selection of sites. Mineralogical analysis provided an understanding of the sources of radionuclides. Using discriminant function analysis (DFA), areas of shallow soil over rock and rock outcrop were successfully discriminated using airborne radiometric and DEM data. The presence or absence of gravel grades was also successfully discriminated with radiometric data using DFA. In addition, a statistically significant correlation was identified between airborne thorium/potassium ratio and ironstone gravel content in the top 0 to 10 cm of soil (r2 = 0.56, P-value < 0.001). Stronger linear relationships were identified between airborne data and surface texture (e.g., 0?10 cm % clay and total count, r2 = 0.71, P-value < 0.001). Signal attenuation prevented the accurate determination of deeper subsurface soil texture (e.g., 20?30 cm % clay and total count, r2 = 0.24, P-value < 0.05). Of the airborne radiometric data, total count exhibited the strongest correlation to soil texture and potassium the least. It is concluded that high-resolution radiometric data can aid the mapping of important soil properties such as shallow granitic bedrock, presence or absence of gravel and the percentage clay in the top 0?10 cm of soil

Determination of spatial distribution patterns of clay and plant available potassium contents in surface soils at the farm scale using high resolution gamma ray spectrometry

Variation in dryland crop yield is often related to underlying soil properties such as water availability and soil fertility. There are often significant difficulties in adequately defining the spatial distribution of such properties at the farm scale. Gamma ray spectrometry (radiometrics) is a relatively new soil sensing technique that can potentially address this by improving the mapping of soil texture and plant available potassium (bic-K). Three sites North Nolba, South Nolba and Summerset were investigated using exploratory linear correlation analysis. Mapping analysis was focused on the Summerset site. In contrast to the two Nolba sites, the soils from Summerset had sufficient soil texture range and parent material conditions that allowed for calibrations to be developed. Soil properties were mapped at Summerset using multivariate linear regression and tree-based models with radiometric, topographic and location data as the inputs. A multivariate linear regression analysis using radiometric data was associated with greater than 70% of the variance in bic-K and soil texture at Summerset. Field checked maps indicated that up to 66% and 60% of the variation in clay and bic-K contents respectively, could be predicted. The overall lowest map errors in root mean square error (RMSE) were 2.4 dag/kg clay and 103 mg/kg bic-K contents. This study concludes that for a site with weathered soils of sufficient soil texture range, radiometrics can reliably predict clay and plant available potassium contents. Radiometrics has practical farm scale applications at a precision that is useful for understanding potential yield variation across a farm