Residual Forms of Fertilizer Nitrogen in Field Soils

Residual fertilizer N was characterized in Houston Black clay and Keene silt loam surface soils recent field microplot-N balance studies involving liberal (328–560 kg N/ha) 15N-labeled rates. Results indicate residual N is readily distributed in all the organic and inorganic soil forms, even after the first cropping season. An equivalent distribution of the residual N with the indigenous soil N, however, had not occured after three years. During this period the residual N was more than twice as susceptible to mineralization as the soil N. Most of the residual N was incorporated in amino acid N-forms.