Comparison of single vs composite soil electrical apparent conductivity-directed samples of irrigated soils for inorganic nitrogen estimation with electromagnetic induction sensors

Abstract

The management of soil can be improved by precision agriculture. A way to achieve this is by quantifying NH₄⁺–N and NO₃⁻–N in soil. The sampling of soil for quantifying these N species is a laborious task. Electromagnetic induction (EMI) sensors could ease the task, providing a large quantity of data. The correlation of inorganic N with soil electrical apparent conductivity (EC_a) in either single or composite samples is essential to accomplish this. Using an EC_a-directed sampling approach, suitable sampling points were identified at four centre pivots. Single and composite samples were collected in 300 mm increments to 600 mm at Hofmeyr and 1500 mm at Douglas, Luckhoff and Empangeni. The samples were analysed for NH₄⁺–N and NO₃⁻–N from which total inorganic N (TIN) was calculated. All three inorganic N fractions were correlated insignificantly to in-field readings of EC_a, implicating that EMI sensors are not suitable for inorganic N estimations in these irrigated soils. A comparison between single and composite samples, however, showed a good agreement of NH₄⁺–N concentrations which was not true for NO₃⁻–N or TIN concentrations. Composite samples are recommended for inorganic N measurements with an EMI sensor. Single samples could be used when only NH₄⁺–N is analysed.

Keywords:

• ammonium
• EC_a
• fertilisation
• irrigation
• nitrate
• site-specific

Acknowledgements

We are thankful for the financial support of the Water Research Commission (Project K5/2499) to conduct this research. The statistical advice of Prof. Robert Schall for data processing is gratefully acknowledged. Our appreciation also extends to all staff and farmers who kindly assisted us with data collection.