The knowledge of spatial distribution of agricultural soils physical and chemical properties is critical for profitable and sustainable crop and food production. The collection of soil data presents however obvious problems arising from sampling a dense, opaque and very heterogeneous medium. Conventional methods consisting of ground-based grid survey are laborious, expensive and lack appropriate spatial resolution to allow best farm management decision. Over the past 50 years, airborne geophysics using conventional aircraft has however become an effective and standard technology in mining and mineral exploration programs for mapping various signatures on the Earth’s surface and at depth. The last decade, Light Sport Aircraft (LSA) have also matured into operational tools providing new horizons compared to conventional aircraft, with lower capital outlay and operational cost as added values. Agricultural soil mapping can now also benefit from the powerful information and knowledge encapsulated into standard magnetics and gamma-ray spectrometric data.
Results of such airborne gamma-ray spectrometric and magnetics survey over a farm land (~ 153 ha) in South Africa are presented. The survey was flown by GyroLAG on a LSA gyrocopter at 20 m agl (above ground level), 20 m line spacing and 120 km/hr speed. A total of ~ 67 000 measurement points were collected in less than 2 hours work and providing a resolution of ~ 438 points/ha or ~ 23 points/m2. Final data and maps can usually become available within a few days or less.
The airborne magnetic data, once cleaned up from various human artifacts (i.e. pivot spraying system, underground drain and water supply pipeline), has been used to calculate the Magnetic Susceptibility of the soil and map its lateral variation.
The airborne gamma-ray data allowed identification of four main soil zones based on the visual inspection of maps of the acquired K, Th, U, Total Count and various ratios of elements such as Th/K and K/U. This substantially enhanced the soil zoning of that farm land for which only two soil zones were identified by ground investigation and soil sampling.
Airborne Th concentration and Total Count maps also mimic the Clay and Sand soil contents respectively providing a proxy for those soil parameters.
The ternary plot of K, Th and U, and also the magnetic data, all from the airborne mapping, show that the identified soil zones also display a substantial degree of lateral variability within each zone. To further characterize that aspect, a Fuzzy C-Means (FCM) cluster analysis was performed for rapid and largely automated integration of both airborne magnetic and gamma-ray data. This resulted in the generation of a high-resolution lateral variability map of the soil beyond the simpler main soil zones delineation.
Distribution and concentration of nematodes species as established from ground soil samples is also compared to the soil variability and then qualitatively discussed.
Achieving the combination of rapid, affordable and effective airborne soil zoning and variability imaging with airborne gamma-ray spectrometric and magnetics mapping provides a new powerful management tool for the agriculture industry.airborne geophysics, gamma-ray spectrometry, magnetics, agriculture, soil zoning.
