The Physiological Impacts of Soil Alkalising Agents on Legumes.

Rothwell, Shane Andrew (2014) The Physiological Impacts of Soil Alkalising Agents on Legumes. PhD thesis, Lancaster University.

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Abstract

To meet predicted future requirements for food production, sustainable intensive agricultural systems will continue to supply supplementary fertilisers. Furthermore, crop nutrient availability needs to be optimised to maximise output, traditionally achieved by applying lime (calcium carbonate) to target soil pH ranges (6-6.5) that optimise nutrient availability and subsequent yield. Additionally, industrial by-products that typically have been regarded as waste are being increasingly used as alternatives to traditional fertiliser sources. One of these, cement kiln dust (CKD), is a potential liming agent and potassium fertiliser. In a field trial, CKD equally increased soil pH and available potassium when compared to agricultural lime and a commercial potassium fertiliser. However, pod yield of field bean (Vicia Faba L. cv Fuego) was reduced by ~30% by liming with equivalent rates of lime or CKD. In subsequent pot trials, recommended liming rates (targeting pH 6.5) consistently reduced stomatal conductance by 26-35% and 20-59% and photosynthesis by 11-20% and 17-30% in dwarf bean (Phaseolus vulgaris L. Nassau) and tall pea (Pisum sativum L. cv. Alderman) respectively. Consequently, mean shoot dry weights were reduced by 13% and 17% in Phaseolus and Pisum respectively. Although lime significantly increased rhizospheric calcium concentrations, xylem sap calcium concentrations (a potential antitranspirant) were only increased in Phaseolus and not Pisum and thus did not cause stomatal closure. Xylem sap and tissue analysis of Pisum suggests that reduced gas exchange is caused by an increase in the plant hormone abscisic acid (ABA) in response to a lime-induced phosphorus deficiency. When synthetic ABA was supplied to detached pea leaves at the same concentrations induced by liming, stomatal conductance decreased. Furthermore, the ABA deficient mutant pea 'wilty' showed an attenuated stomatal response to liming, apparently confirming that increased ABA is mediating some legume responses to low phosphorus availability under recommended rates of liming. This research contributes to a mechanistic understanding of the physiological processes limiting gas exchange and growth when lime application limits crop yields, and raises questions about the suitability of current liming recommendations.