This work was carried out to determine the influence of root exudates on ph
osphorus solubility in the presence of a strong phosphate sorbant. Root muc
ilage was collected from maize grown under field conditions. Polygalacturon
ic acid (PGA) was also used as a model of exudates. Adsorption of phosphate
by synthetic goethite was measured in the absence of either (1) root mucil
age collected from maize in the field, or (2) PGA. The ranking order of the
adsorption on goethite was phosphate at low concentration >PGA >mucilage >
phosphate at high concentration. Then, competitive adsorption between phosp
hate ions and PGA or mucilage was studied. Increasing the concentration of
PGA or mucilage added to the goethite suspension decreased the amount of ph
osphate adsorbed on goethite. It is likely that PGA, mucilage and phosphate
compete for the same adsorption sites on goethite: the effect of the coati
ng of the mineral by the mucilage is another possible explanation.