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 Plains. This study quantified wheat (Triticum aestivium L.) hay production and N uptake over seven years following six years of alfalfa (Medicago sativa L.), cicer milkvetch (Astragalus cicer L.), or grass (Old World bluestem, Bothriochloa ischaemum L.) production in western Oklahoma. Precipitation over the seven years averaged 550 mm . yr(-1). The major 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) following milkvetch, and 950 kg . ha(-1)yr(-1) following grass with N uptake attributed to the residual effect from legumes (calculated by the difference 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 alfalfa and 710 kg . ha(-1). yr(-1) following grass with N uptake attributed to the residual effect from alfalfa averaged 8 kg N ha(-1). yr(-1). Thus, the residual N effect attributed to legumes was substantial on the fine sandy loam soil and relatively small on the deep loamy sand soil.