PHOSPHORUS UPTAKE OF MAIZE AS AFFECTED BY AMMONIUM AND NITRATE-NITROGEN - MEASUREMENTS AND MODEL-CALCULATIONS

Phosphorus uptake is often enhanced by ammonium compared to nitrate ni trogen nutrition of plants. A decrease of pH at the soil-root interfac e is generally assumed as the cause. However, an alteration of root gr owth and the mobilization of P by processes other than net release of protons induced by the source of nitrogen may also be considered. To s tudy these alternatives a pot experiment was conducted with maize usin g a fossil Oxisol high in Fe/Al-P with low soil solution P concentrati on. Three levels of phosphate (0, 50, 200 mg P kg-1) in combination wi th either ammonium or nitrate nitrogen (100 mg N kg-1) were applied. P lants were harvested 7 and 21 d after sowing, P uptake measured and ro ot and shoot growth determined. To assess the importance of factors in volved in the P transfer from soil into plants, calculations were made using a model of Barber and Claassen. In the treatments with no and l ow 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 chang es had a minor influence on P uptake only, as was demonstrated by arti ficially 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 N H4-compared to NO3-N. It is concluded that in this soil the enhancemen t of P uptake of maize plants after ammonium application cannot be att ributed to the acidification of the rhizosphere but to effects mobiliz ing soil phosphate or increasing P uptake efficiency of roots. Model c alculation showed that these effects accounted for 53 % of the P influ x per unit root length in the NO3-N and 72 % in the NH4-N supplied pla nts if no P was applied. With high P application the respective figure s were only 18 and 19 %.