Sunflowers drive acid dissolution of rock phosphate when banded with ammonium zeolite

Ammonium exchanged clinoptilolite (zeolite) with rock phosphate (RP) has been known to accelerate the process of RP dissolution and enhance plant growth responses; however, the mechanism underlying accelerated RP dissolution remains unclear. The present study examined whether the discharge of protons by plant roots and, to a lesser extent, by soil bacteria drives RP dissolution in this system. A total of 14 combinations consists of P-source (rock phosphate or calcium orthophosphate), placement (vertical band or dispersed), exchanged zeolite (untreated or exchanged with NH4, K or Ca), and plant (with or without plant) were installed in 1-L pots containing peat and perlite. Growth responses of sunflower (Helianthus annuus L. 'Hysun 25') were greatest after 27 days in banded and dispersed orthophosphate treatments, but RP banded with ammoniated zeolite treatments yielded 20% less total leaf length. The pH of the soil solution in RP and ammoniated zeolite bands decreased by two units concomitant with a highly significant four-fold increase in solution P concentration. Root growth proliferated in RP and ammoniated zeolite bands. Sunflower growth was very slow in all other RP treatments with little change in pH or solution P concentration. These data indicate that P dissolution from RP was mainly plant driven by proton excretion following ammonium absorption. Hence, RP banded with ammoniated zeolite could be used as an efficient plant-driven controlled-release fertiliser in pot (container) plant culture.