AbstractPhosphorus 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 d 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 ofBarberandClaassen.In the treatments with no and low P supply NH4‐N compared to NO3‐N nutrition increased the growth of the plants by 25 and their shoot P content 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 rhizosphere 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 calculation 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 figur
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