Keywords: nitrogen leaching, optimum nitrogen fertilisation, N-Sensor, precision agriculture.
Nitrogen (N) fertilisation does not usually lead to increased N leaching as long as there is a significant yield response. The highest fertilisation rate at which the response of the crop to N uptake is enough to limit N leaching, is in the same range as the economical optimum. Above this optimum 10-50% of the excess fertiliser N is likely to leach with drainage. The optimum N rate is dependent on both potential yield and level of soil N supply. Both these factors vary between sites and years. Predicting the optimum N rate for a specific field and year is therefore a great challenge. There is also a large variation within fields, which increases the challenge even more.
General fertiliser recommendations that consider manure and cropping history, soil organic matter content and expected yield are very good for predicting averages. However, they fail to capture all variation. An accurate average is not good enough, since high levels of leaching in areas where fertilisation is above the optimum cannot be offset by less leaching in areas where the fertilisation is below the optimum level. This just results in lower yields.
In Sweden, one approach to complementing general recommendations is to publish crop N uptake in N fertilisation experiments during the current season. This facilitates discussion about how the weather is affecting crops and soil N regionally in the current year. However this still does not capture the site-specific conditions for every individual field. For this, field observations are necessary. Some fine tuning can be made from local soil analyses. However, the best indicator of crop response is the crop itself.
Some sensors are available that can provide estimates of crop N status. One example is the tractor mounted Yara N-SensorTM. It measures variations within fields, enabling the application of more N to areas with a high yield response, and avoiding or reducing applications in parts of the field which are not N limited. This still needs field calibration. There are several methods available to do this. One is to leave a plot in the field unfertilised, to demonstrate the soil N supply to the crop. By measuring this unfertilised crop at flag leaf emergence with a hand held version of the Yara N-Sensor, good predictions of soil N supply can be made. From this, more accurate optimum N rates can be calculated. The improvement and implementation of such methods could decrease leaching.
Sofia Delina, Ingemar Gruvaeus b, Johanna Wetterlinda, Maria Stenberga, Gunilla Frostgårdb, Katarina Bõrlingc, Carl-Magnus Olssonb and Anna-Karin Krijgerd.
a Swedish University of Agricultural Sciences, Skara, Sweden.
b Yara AB, Landskrona, Sweden.
c Swedish Board of Agriculture, Dragarbrunnsgatan 35, Uppsala, Sweden.
d Rural Economy and Agricultural Society of Skaraborg, Skara, Sweden.
24 pages, 13 figures, 3 tables, 26 references