NITROGEN-FERTILIZATION OF GRASSLAND

We first recall that nitrogen fertilization is indispensable for grassland graminaceae and acts on the growth rate : an input of 80 U.N./ha at springtime allows to reach 2 tons of dry matter per ha in 35 days instead of 60. The influence of nitrogen fertilization is highly dependent on pedo-climatic conditions. To optimize it, one must first know the quantity of nitrogen that can be provided by the soil: nitrogen quantities exported by O witnesses constitue a good estimate of that parameter and usually lie in the range 80 to 110 kg per ha per year. It must also be kept in mind that fertilization advice is given for a determined production level, and on a probabilistic basis for example, the application of 100 units of nitrogen will result in a yield of 10 tons of dry matter per ha with a 73 % probability for phleum, and only with a 42 % probability for english rye-grass. From the point of view of environment conservation, it can be shown that the input of nitrogen fertilizers considered as optimal by the agronomists represent a rather low level of risk. More precisely, for fertilization levels up to 200 units of N per ha (fractionated in two applications), the apparent utilization coefficients calculated on the basis of yearly fertilization and outputs are close to 0.58, with a maximum of 0.73. Since grasslands have a very dense root network and permanently covers the soil, it is generally estimated that unused nitrogen is transformed and stored and has a low probability of being washed out into the aquifers.