There is growing evidence that potassium deficiency in crop plants increases their susceptibility to herbivorous arthropods. The ability to remotely detect potassium deficiency in plants would be advantageous in targeting arthropod sampling and spatially optimizing potassium fertilizer to reduce yield loss due to the arthropod infestations. Four potassium fertilizer regimes were established in field plots of canola, with soil and plant nutrient concentrations tested on three occasions: 69 (seedling), 96 (stem elongation), and 113 (early flowering) days after sowing (DAS). On these dates, unmanned aerial vehicle (UAV) multi-spectral images of each plot were acquired at 15 and 120 m above ground achieving spatial (pixel) resolutions of 8.1 and 65 mm, respectively. At 69 and 96 DAS, field plants were transported to a laboratory with controlled lighting and imaged with a 240-band (390-890 nm) hyperspectral camera. At 113 DAS, all plots had become naturally infested with green peach aphids (Hemiptera: Aphididae), and intensive aphid counts were conducted. Potassium deficiency caused significant: (1) increase in concentrations of nitrogen in youngest mature leaves, (2) increase in green peach aphid density, (3) decrease in vegetation cover, (4) decrease in normalized difference vegetation indices (NDVI) and decrease in canola seed yield. UAV imagery with 65 mm spatial resolution showed higher & Dustin Severtson classification accuracy (72-100 %) than airborne imagery with 8 mm resolution (69-94 %), and bench top hyperspectral imagery acquired from field plants in laboratory conditions (78-88 %). When non-leaf pixels were removed from the UAV data, classification accuracies increased for 8 mm and 65 mm resolution images acquired 96 and 113 DAS. The study supports findings that UAV-acquired imagery has potential to identify regions containing nutrient deficiency and likely increased arthropod performance.

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Negative potassium (K) balances on farmlands globally are widespread because fertiliser K input is often less than losses (leaching) and removal of K in hay, straw and grain, which leads to a rundown of plant-available K. When soil K reserves are not large and the plant-available K pools are not well buffered, the risk of K rundown in soils is high. In the south-west of Western Australia, soil K rundown, particularly by continuous cropping or in systems where a large portion of crop biomass is removed, is increasing the prevalence of crop K deficiency even on soils where K was not previously a limiting factor for crop yields. While fertiliser K is required for adequate supply of plant-available K, maximising K use efficiency is also important for cropping profitability and sustainability in dryland agriculture. Plant K uptake and use efficiency can be affected by soil types, crop species and sequences, seasonal conditions, and K management. In water-limited environments, crop K nutrition, especially root access to subsoil K, plays a crucial role in promoting root growth, regulating plant water relations and alleviating biotic and abiotic stresses. Optimised use of both soil and fertiliser K is increasingly necessary to sustain crop yields under stressed conditions in the context of K rundown in soils.

Context Phosphorus (P) use in agriculture can lead to eutrophication. Agronomic soil tests such as Colwell P and P buffering index (PBI) define critical soil P levels for pasture production. These tests have potential for re-use as environmental risk indicators of dissolved reactive P (DRP) loss from paddocks but are constrained because a 0–10 cm sample does not necessarily align with the dominant hydrological loss pathways of runoff or leaching. Aims To identify influences on the benchmark environmental measure of DRP (CaCl2-extractable P or CaCl2-P) by agronomic-based measures such as PBI, Colwell P and depth, and Colwell P to PBI ratio (P environmental risk index; PERI). To estimate CaCl2-P at any depth from a 0–10 cm sample, and the potential for change in DRP loss risk through the adoption of evidence-based fertiliser management based on soil testing. Methods Archives of 692 0–10-cm soil samples, along with 88 sites sampled at 0–10 cm and 0–1, 1–2, 2–5, 5–10, 10–20, and 20–30 cm were analysed for Colwell P, PBI, CaCl2-P, PERI, and P fertility index (PFI). Derived relationships between CaCl2-P and Colwell P for different PBI were applied to 30 981 0–10-cm samples to estimate the potential for DRP reduction resulting from the adoption of evidence-based fertiliser management. Key results CaCl2-P, Colwell P, PERI, and PFI decreased with depth, with an associated increase in DRP loss risk from surface soil. The CaCl2-P decreased with increasing PBI. The CaCl2-P, Colwell P, PERI, and PFI could be estimated at any depth from a 0–10 cm sample, with r2 > 0.77. The CaCl2-P was estimable from PERI, and soils with low PBI or with high PFI had high DRP loss risk. The CaCl2-P was positively correlated with Colwell P, with the slope decreasing with increasing PBI and becoming invariant when PBI > 100. When applied to the current soil Colwell P and estimated current CaCl2-P and compared to CaCl2-P at the critical Colwell P for different relative yields (RYs), DRP loss risk could be reduced by 24% for a RY target of 95%, and 59% for a RY target of 80%. Conclusions Because current Colwell P levels in soils exceed critical values, DRP loss risk can be substantially reduced by adopting evidence-based fertiliser management with little or no loss of utilised pasture. Implications Fertiliser management based on evidence of P requirements determined from soil testing has a significant role in reducing DRP loss risk.