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Implications of soil water repellence for crop growth and nutrition

Yeap, Simon Guo Hong (2020) Implications of soil water repellence for crop growth and nutrition. PhD thesis, Murdoch University.

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In water-limited environments, dryland crop and pasture production on water-repellent sandy soils is often constrained by reduced water infiltration, accentuated overland flow and soil erosion, unstable wetting patterns, and the development of preferential flow paths in the soil profile, which consequently cause considerable spatial heterogeneity in soil water content, increased prevalence of isolated dry zones, and decreased overall soil water retention. The same processes are also likely to affect soil nutrient bioavailability and plant nutrient uptake. Indeed, while problems with crop nutrition on water-repellent sandy soils have been reported by many Australian growers, the role of soil water repellence in crop nutrition has not been studied to date and the mechanisms remain unclear. While various methods exist to manage soil water repellence for improving crop and pasture production (e.g., deep soil cultivation, clay spreading, wetting agent application, stimulation of wax-degrading microorganisms, furrow/on-row sowing and water harvesting, and no-tillage and stubble retention), the outcomes for crop nutrition post-amelioration are not well understood.

Several field and glasshouse experiments were, therefore, conducted to assess the implications of soil water repellence and its management on crop growth and nutrition on several sandy soil types from the southwest region of Western Australia. Preliminary field results showed that soil water repellence, if left unmanaged, could adversely affect wheat plant density, shoot dry matter production, K nutrition, and grain yield on a Grey Bleached-Ferric Kandosol (deep grey sandy duplex soil) at Meckering with a moderate water repellence value of up to 1.6 M using the molarity of ethanol droplet (MED) test, supporting the hypothesis that soil water repellence can adversely affect crop growth, nutrition, and grain production. However, it was also revealed at another site, with a Ferric Chromosol (sandy loam yellow duplex soil) at Kojonup, that increased soil water repellence could also increase canola plant density, shoot dry matter production, Cu nutrition, and seed yield when sown with 1 L/ha of banded wetting agent, despite prolonged severe water repellence (MED of 3.4 M) throughout the growing season. Although the underlying mechanisms could not be established from this preliminary study, it was concluded that soil water repellence may have both adverse and beneficial implications, but specific effects on nutrient availability in the root zone and crop nutrition were not defined.

Additional field studies were conducted to assess the effect of soil management practices (spading, one-way plough, subsoil clay spreading, and blanket applications of wetting agent) to alleviate soil water repellence on crop growth and nutrition. While all treatments except for one-way ploughing alleviated soil water repellence, only spading significantly improved wheat emergence, shoot dry matter, K nutrition, and grain yield on a Grey Tenosol (pale deep sandy soil) at Badgingarra. By contrast, at Moora, one-way plough treatments improved canola shoot dry matter and nutrition (Ca, S, B, Cu, and Zn contents) but did not mitigate severe water-repellence on a Ferric Chromosol (sandy ironstone gravel duplex soil), and had no effect on plant density or seed yield. However, the improvements due to soil cultivation can be attributed to the alleviation of soil compaction, given that the alleviation of soil water repellence by blanket-applied wetting agent (50 L/ha) and subsoil clay spreading treatments (250 t/ha; 50 % clay; 159 mg K/kg) had negligible effect on crop growth, nutrition, and grain production. Alleviation of soil water repellence was, therefore, not important for crop production at the Badgingarra and Moora study sites, presumably due to the presence of other soil constraints.

To avoid the confounding effects from multiple limiting factors evident in the field studies, a series of controlled glasshouse experiments were conducted to examine the effects of topsoil water repellence, topsoil thickness, fertiliser placement, variable low water supply, plant density, and/or surface topography on soil water content, soil nutrient availability, and early wheat growth and nutrition in 27 L containers. All glasshouse experiments demonstrated that severely water-repellent topsoil with a wettable furrow, which ensured uniform seedling emergence, significantly increased wheat seedling development, tiller number, shoot dry matter production, and nutrition (especially N, P, and K) during the early vegetative stage in wheat (40-51 DAS), under low but regular water supply (3.4-5.4 mm every two days). The growth stimulation was attributed to in situ water harvesting caused by preferential flow in the wettable furrow which increased the soil wetting and root depth relative to the completely wettable topsoil treatments that exhibited an even but shallow wetting depth. The even but shallow wetting patterns in completely wettable treatments consequently led to an overall decrease in plant-available water and plant water use efficiency, resulting in poor wheat growth and nutrition, especially under a limited water supply. These findings underscore the high efficacy of in situ water harvesting for improving early wheat growth and nutrition on water-repellent soils relative to completely wettable soils, thus demonstrating a beneficial role of soil water repellence in crop growth and nutrition. Adopting in situ water harvesting principles (i.e., furrow sowing, banding wetting agent in the furrow, and using winged knife-points and/or press-wheels) can, therefore, be an effective strategy for managing crop growth and nutrition on water-repellent sandy soils by maximising the use efficiency of limited soil water supply during the crop establishment period.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Agricultural Sciences
United Nations SDGs: Goal 12: Responsible Consumption and Production
Supervisor(s): Bell, Richard, Harper, Richard and Scanlan, Craig
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