NITROGEN-FERTILIZATION OF WHEAT NO-TILL PLANTED IN ALFALFA STUBBLE

Conservation tillage practices significantly reduce soil erosion, impr ove water distribution, and decrease environmental concerns in furrow- irrigated fields. Developing optimum N fertilization practices after l egumes in these systems will help their adoption. We conducted three f ield studies on a silt loam soil to determine if N fertilizer is requi red for furrow-irrigated wheat (Triticum aestivum L.) planted in herbi cide killed alfalfa (Medicago sativa L.) stubble. Fall regrowth of alf alfa was sprayed with a mixture of 1 qt glyphosphate and 2 qt 2,4-D/ac re. 'Stephens', a soft white winter wheat was planted with a double di sk opener drill in two experiments and 'Bronze Chief', a hard red spri ng wheat in a third experiment. Nitrogen fertilizer (ammonium nitrate) was spring broadcast at four rates from 0 to 200 or 240 lb N/acre. Th e buried plastic bag technique estimated available N (EAN); above grou nd whole plant samples estimated root zone nitrate-N; and grain plot y ields estimated with a combine. Nitrogen fertilization increased grain yield in all experiments because sufficient N had not mineralized fro m soil and legume residues before uptake needs of the wheat. The wheat plant at the soft dough growth stage contained about 109 lb N/acre fr om N mineralized from soil and legume residue sources. The apparent N fertilizer recovery calculated by a combined regression relationship b etween N uptake and fertilizer rates was 76%. The average plant recove ry of mineralized N at maximum grain yields was calculated at 78%. Cro ps planted in herbicide killed alfalfa should be selected so sufficien t nitrate-N can accumulate from mineralization before maximum crop upt ake and for N uptake ability in late summer. Nitrogen fertilizer appli cations should be based on a spring soil test for nitrate-N in this no -till system. Both practices will reduce the potential for nitrate-N l eaching losses.