Spatial Variation of Soil Mineral Nitrogen as a Guide to Sampling for Site-Specific Management

Keywords: Bulking; geostatistics; variogram; kriging; mapping; sampling.

Precise management of crop nutrients in agriculture depends upon adequate information about the spatial variation in soil and crop attributes within fields. Sampling the soil to measure the concentrations of available crop nutrients is costly and time-consuming, but needs to be adequate to obtain digital spatial information for mapping or site-specific management (SSM). Many ‘contour’ maps created by interpolation are often based on too few data that are not spatially dependent. If this is so it might be more appropriate to classify the data to create management zones. A geostatistical approach based on the variogram enables spatial dependence to be determined and modelled. The variogram from this approach can be used to determine how many sub-samples are required to make up a bulked sample for analysis to remove local variation over a given area, and also to guide sampling intensity on a grid for kriging and eventual mapping or SSM. In this paper we describe a case study largely based on total mineral nitrogen to determine how many samples are required for bulking. In addition, we describe three grid surveys of total mineral nitrogen with some with additional variables to compare their patterns of variation and to illustrate how the variogram range can be used to suggest future sampling intervals for soil and crop surveys. Overall, four or nine samples were optimal for bulking. The recommended grid intervals for two of the fields were similar, but for the other it was twice as large. These intervals reflected the considerable differences in the scales of spatial variation among the fields. Sampling and analysis to measure soil mineral nitrogen is costly and the need for this is likely to be superseded by the use of nitrogen sensors that can provide intensive information relatively cheaply and at several crucial growth stages during the growing season. Nevertheless, the sensors need to be calibrated, which will still require some soil and crop sampling for which the above recommendations are important.

M A Oliver¹ and C J Dawson²

¹ Soil Research Centre, Department of Geography and Environmental Science, The University of Reading, Whiteknights, Reading RG6 6DW, UK.

² Chris Dawson and Associates, Westover, Ox Carr Lane, Strensall, York YO32 5TD, UK.

47 pages, 23 figures, 12 tables, 29 references.