A novel approach to map the depth to a soil pH constraint – a useful tool for understanding yield variability

Patrick Filippi*1, Edward J. Jones1, Bradley J. Ginns1, Brett M. Whelan1, Guy W. Roth1, Thomas F.A. Bishop1

1 The University of Sydney, Sydney Institute of Agriculture, Sydney, New South Wales, Australia, patrick.filippi@sydney.edu.au 


Subsoil alkalinity is a common issue in the alluvial cotton-growing valleys of northern NSW, Australia. This causes nutrient deficiencies, toxicity, and inhibits root growth, which can have a damaging impact on crops. The depth at which a soil constraint is reached is important information for farmers, however, this is hard to measure spatially. This study predicted the depth in which a pH constraint (pH > 9) was reached to a 1 cm vertical resolution for a 1 m soil profile on a dryland cropping farm in northern NSW, Australia. Equal-area quadratic smoothing splines were used to resample vertical soil profile data, and a random forest model was used to produce the depth-to-pH-constraint map. The model to spatially predict soil pH across the farm was accurate, with an LCCC of 0.63, and an RMSE of 0.47 when testing with leave-one-site-out-cross-validation. About 77% of the area was constrained by a pH greater than 9 within the top 1 m of soil. The relationship between the predicted depth-to-pH-constraint map and cotton and grain (wheat, canola, and chickpea) yield monitor data was analysed for individual fields. The deeper in the soil profile a pH constraint was reached, the greater the crop yield. A strong relationship was found for wheat, canola, and chickpea (Spearman’s correlation (rs) of 0.75, 0.66, and 0.58, respectively), and a moderate relationship for cotton (rs = 0.37). The modelling approach presented could be used to identify the depth to other soil constraints, such as soil sodicity. The outputs are a promising opportunity to understand crop yield variability, which could lead to improvements in management practices.


The Australian Society of Agronomy is the professional body for agronomists in Australia. It has approximately 500 active members drawn from government, universities, research organisations and the private sector.

Photo Credits

David Marland Photography david_marland@hotmail.com Graham Centre for Agricultural Innovation, Charles Sturt University

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