RESIDUAL NITROGEN EFFECTS ON WHEAT FOLLOWING LEGUMES IN THE SOUTHERN PLAINS

Increasing nitrogen (N) fertilizer prices give rise to the question of N benefits from legumes in cropping systems in the Southern Great Pla ins. This study quantified wheat (Triticum aestivium L.) hay productio n and N uptake over seven years following six years of alfalfa (Medica go sativa L.), cicer milkvetch (Astragalus cicer L.), or grass (Old Wo rld bluestem, Bothriochloa ischaemum L.) production in western Oklahom a. Precipitation over the seven years averaged 550 mm . yr(-1). The ma jor residual N effects were measured within the first five years. On a fine sandy loam soil, wheat hay yields averaged 3,070 kg . ha(-1). yr (-1) over five years following alfalfa, 2,580 kg . ha(-1). yr(-1) foll owing milkvetch, and 950 kg . ha(-1)yr(-1) following grass with N upta ke attributed to the residual effect from legumes (calculated by the d ifference method) averaged 34 kg N ha(-1). yr(-1) from alfalfa and 25 kg . ha . yr(-1) from milkvetch. On a deep loamy sand soil, wheat hay yields averaged 1,290 kg . ha(-1). yr(-1) over five years following al falfa and 710 kg . ha(-1). yr(-1) following grass with N uptake attrib uted to the residual effect from alfalfa averaged 8 kg N ha(-1). yr(-1 ). Thus, the residual N effect attributed to legumes was substantial o n the fine sandy loam soil and relatively small on the deep loamy sand soil.