Little is known about the effect of combined phosphorus and nitrogen (P-N) fertilization on the N requirement of sunflower (Helianthus annus L.). This study was carried out to evaluate the effects of varying levels of P and N, as well as the interaction P · N, on the N uptake, yield and N apparent utilization efficiency under field conditions. Splitplot design experiments were conducted in the mid-western Pampas in Argentina. Four levels of N (0, 46, 92 and 138 kg N ha )1 ) and three levels of P (0, 12 and 40 kg P ha )1 ) were applied to two Typic Hapludolls over two growing seasons (1997-98 and 1998-99). N uptake and soil N-NO 3 contents were determined at the V 7 , R 5 and R 9 growth stages. The sunflower yield ranged from 2.5 to 5.0 Mg ha )1 . The total N requirement was around 45 kg N Mg )1 grain, and this result suggests that it is not necessary to use different N requirements (parameter b) for fertilized crops when a yield response is expected. To achieve a 100 % yield maximum a N supply (soil plus fertilizer) of 181 kg N ha )1 at P 40 was needed. However, at P 0 , the highest yield was about 80 % of the maximum yield with a N supply (soil plus fertilizer) of 164 kg N ha )1 . P application increased the apparent use efficiency of the supplied N.

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Quantification of nitrogen mineralization at the field scale is necessary to improve current N fertilizer recommendations. The objective of this study was to identify a suitable set of explanatory variables and an appropriate modelling framework to estimate the average daily amount of N mineralized (DANM) during a corn (Zea mays L.) growing season. In six field experiments in Flat Inland Pampas, Argentina, DANM was evaluated in three different homogeneous zones (HZ), which were defined based on terrain elevation. Mixed-effect generalized linear models were developed that included growing season as random effect as well as HZ, sand, clay, silt, soil organic carbon, and normalized difference vegetation index (NDVI) as fixed effects. Results showed that DANM ranged from 0.19 to 1.25 kg N ha −1 day −1 with a mean of 0.64 kg N ha −1 day −1 , which is equivalent to 110 kg N ha −1 for the corn growing season. The best model included HZ, sand, and early NDVI measurements, and predicted DANM with an RMSE of 0.19 kg N ha −1 day −1 . Thus, the model should be improved and tested in the Inland Flat Pampas before it could be used to assess N fertilizer application rates.

There were two objectives in the study: 1) To determine exchangeable K and non‐exchangeable K in soils with different potassium depletion levels and mineralogy as plant sources 2) To establish a relationship between the mineralogy vs exchangeable K (Ke) and non‐exchangeable K (Kne) mobilization. An extraction experiment of soils was carried out in a greenhouse, with a total of 6 consecutive crops of ryegrass. Different supply rates for plant K were determined by Ke and Kne mobilization according to the soil intensity of use. The contribution of the Ke was greater and generated higher amount of K uptake during maximum availability period (from 0 to the 1st harvest) than in the later period when soil K was already depleted. For this the initial exchangeable K and the illite concentration of soils accounted for almost 100% (R2 = 0.981 P=0.01) of the K taken up by ryegrass. For the following period (from the 2nd harvest to the 6th), Kne forms became more important. Plant K supply was not only a result of initial exchangeable K and illite concentration but presumably also of primary K‐bearing minerals.

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