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

Conservation tillage practices significantly reduce soil erosion, improve water distribution, and decrease environmental concerns in furrow-irrigated fields. Developing optimum N fertilization practices after legumes in these systems will help their adoption. We conducted three field studies on a silt loam soil to determine if N fertilizer is required for furrow-irrigated wheat (Triticum aestivum L.) planted in herbicide killed alfalfa (Medicago sativa L.) stubble. Fall regrowth of alfalfa was sprayed with a mixture of 1 qt glyphosphate and 2 qt 2,4-D/acre. 'Stephens', a soft white winter wheat was planted with a double disk opener drill in two experiments and 'Bronze Chief', a hard red spring wheat in a third experiment. Nitrogen fertilizer (ammonium nitrate) was spring broadcast at four rates from 0 to 200 or 240 lb N/acre. The buried plastic bag technique estimated available N (EAN); above ground whole plant samples estimated root zone nitrate-N; and grain plot yields estimated with a combine. Nitrogen fertilization increased grain yield in all experiments because sufficient N had not mineralized from 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 from N mineralized from soil and legume residue sources. The apparent N fertilizer recovery calculated by a combined regression relationship between N uptake and fertilizer rates was 76%. The average plant recovery of mineralized N at maximum grain yields was calculated at 78%. Crops planted in herbicide killed alfalfa should be selected so sufficient nitrate-N can accumulate from mineralization before maximum crop uptake and for N uptake ability in late summer. Nitrogen fertilizer applications 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 leaching losses.