Measurement of the effects of pH on phosphate availability
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Measurement of the effects of pH on phosphate availability N.J. Barrow
&
Abhijit Debnath & Arup Sen
Received: 31 March 2020 / Accepted: 22 July 2020 # Springer Nature Switzerland AG 2020
Abstract Aims Many soil scientists think that soil phosphate availability is highest at near-neutral pH and decreases with decreasing pH. This belief does not appear to have ever been subjected to experimental test. Methods In a pot trial, we measured response curves to phosphate at seven pH(CaCl2) values ranging from 3.99 to 7.26 using three plant species: mustard (Brassica campestris), lucerne – also known as alfalfa – (Medicago sativa) and rice (Oryza sativa). We used a form of the Mitscherlich equation that allowed us to estimate the phosphate contribution from the soil as well as the slope of the response curve and the maximum yield. Results Plants grew best near pH 5.5 and worst at nearneutral pH. We think the large decrease in growth with increasing pH was caused by decreasing rate of P uptake by plants. There was a smaller decrease in growth as pH decreased below 5.5. We think this was caused by aluminium toxicity. Conclusions The conventional belief that phosphate availability is greatest near neutral pH is wrong.
Responsible Editor: Tim S. George N. Barrow (*) Faculty of Science, School of Biological Sciences, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia e-mail: [email protected] A. Debnath : A. Sen Department of Agricultural Chemistry and Soil Science, Bidhan Chandra Krishi Viswavidyalaya, Mohanpur, West Bengal 741252, India
Keywords pH . phosphate . P response . P availability . lucerne . alfalfa . rice . mustard
Introduction Most textbooks teach that phosphate availability increases with increasing pH reaching a maximum at near neutral pH. This is based on the idea that phosphate reaction with soil is controlled by precipitation of iron phosphate at low pH, aluminium phosphate at medium pH and calcium phosphate at high pH leaving a minimum at near neutral pH (Price 2006, Penn and Camberato 2019). Thus phosphate is thought to be present as discrete particles and we refer to this as the particle hypothesis. The alternative view is that phosphate reaction involves specific adsorption of phosphate ions onto variable charges surfaces followed by diffusive penetration; the adsorption-penetration hypothesis (Barrow 1999). The two hypotheses can be distinguished by observing the effects of the background electrolyte. When measured in a dilute solution of a salt of a monovalent cation there is a large decrease in sorption with increasing pH. As the concentration of salt increases, or if a divalent cation is used, the decrease is much smaller and may be reversed at high pH (Barrow 1984, Barrow and Debnath 2015). According to the adsorption-penetration hypothesis this is the consequence of compression of the double layer as a result of which the negative potential becomes less negative and reaction is increased. The particle hypothesis has no means to explain this. We therefore arg
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