Discriminating stratigraphic layers of cultivated organic soils using proximal sensors

Cultivated organic soils (Histosols) cover thousands of hectares in Canada. Drained peatlands are cultivated for high-value crops but are subject to physical and chemical degradation. To facilitate the creation of agricultural management zones, field-scale mapping of peat layer stratigraphy is essential since soil degradation is correlated with peat thickness. However, manual probing can be time-consuming for large-scale mapping of peat layers and sometimes fails to detect limnic layers. This research aimed to provide more insights into the properties of peat layers, with an emphasis on limnic layers. The hypothesis was that soil properties measured using proximal sensing techniques could discriminate contrasting peat soil layers (peaty, limnic, and mineral), circumventing the need for manual probing and laboratory tests. Forty sites from four cultivated organic soils with contrasting stratigraphy were studied. At each site, soil penetration resistance (SPR) was measured to a depth of 100 cm. For each 10-cm soil layer, soil cores were extracted. Multiple laboratory tests were performed: loss-on-ignition (organic matter content), soil volumetric (θ) and gravimetric water contents, bulk density, and electrical conductivity (EC). Linear mixed-effects models followed by Tukey's test revealed that all six soil properties differed significantly at p<0.001 between the peaty, limnic, and mineral layers. Peaty layers had low θ and SPR, and the lowest EC. Limnic layers showed low SPR, intermediate EC, and high θ. Mineral layers had low θ, the highest EC, and the high BD. The results proved that proximal sensors such as a time-domain reflectometer (which measures θ and EC), electromagnetic induction and conductivity sensors, and a soil penetrometer could potentially discriminate the three soil layers to map the stratigraphy of cultivated organic soils.