SOLUBLE NITROGEN AND CARBON IN THE SUBSOIL IN RELATION TO VEGETABLE PRODUCTION INTENSITY

We determined soluble nitrogen (N_org, NH₄-N, NO₃-N), carbon and sulphate in the subsoil down to 10 m under fields with different vegetable production intensity. There was one sandy soil planted with asparagus and four loamy sands. On two of these fields only vegetables were grown and on two fields vegetables and agricultural crops.

The amount of inorganic nitrogen (NO₃-N + NH₄-N) was between 500 and 2300 kg ha^-1 N, mostly as nitrate. Based on the amounts of dissolvable organic carbon (DOC) we estimated the denitrification potentials from 500 to 1900 kg ha^-1 NO₃-N. There was no correlation between denitrification potential and amount of nitrate. Ammonium nitrogen was found mainly in the water saturated zone; the amounts of inorganic N were negligible.

The amount of sulphate in the subsoil was between 600 and 7000 kg ha^-1 SO₄-S. One subsoil contained up to 7000 kg ha^-1 SO₄-S due to the application of large amounts of ammonium sulphate in addition to potassium sulphate. In the other fields there was only a small correlation between S-input and S-amount in the subsoil.