Phosphorus uptake is often enhanced by ammonium compared to nitrate nitrogen nutrition of plants. A decrease of pH at the soil root interface is generally assumed as the cause. However, an alteration of root growth and the mobilization of P by processes other than net release of protons induced by the source of nitrogen may also be considered. To study these alternatives a pot experiment was conducted with maize using a fossil Oxisol high in Fe/Al-P with low soil solution P concentration. Three levels of phosphate (0, 50, 200 mg P kg-1) in combination with either ammonium or nitrate nitrogen (100 mg N kg-1) were applied. Plants were harvested 7 and 21 days after sowing, P uptake measured and root and shoot growth determined. To assess the importance of factors involved in the P transfer from soil into plants, calculations were made using a model of Barber and Claassen. In the treatments with no and low P supply NH4-N compared to NO3-N nutrition increased the growth of plants by 25 % and their shoot P concentration by 38 % while their root growth increased by 6 % only. The rhizosphere pH decreased in the NH4-N treatments by 0.1 to 0.6 units as compared to the bulk soil while in the NO3-N treatments it increased by 0.1 to 0.5 units. These pH changes had a minor influence on P uptake only, as was demonstrated by artificially altering the soil pH to 4.7 and 6.3 respectively. At the same rhizoshere pH, however, P influx was doubled by the application of NH4- compared to NO3-N. It is concluded that in this soil the enhancement of P uptake of maize plants after ammonium application cannot be attributed to the acidification of the rhizosphere but to effects mobilizing soil phosphate or increasing P uptake efficiency of roots. Model calculations showed that these effects accounted for 53 % of the P influx per unit root length in the NO3-N and 72 % in the NH4-N supplied plants if no P was applied. With high P application the respective figures were only 18 and 19 %.