Submitted to: Journal of Plant Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 15, 1996
Publication Date: N/A
Interpretive Summary: Grass seed growers in western Oregon often do not monitor crop or soil N during the season. Often applied N rates greatly exceed Oregon State extension recommendations. This can be detrimental to seed yield and soil quality and may account for large yearly variations in seed yield. N uptake of ryegrass grown in two contrasting climate years was determined with respect to growing degree days and plant growth. Different fertilizer sources were also examined to determine difference in growth and N uptake. In years of contrasting climate, ryegrass N uptake was nearly the same. Differences in seed yield between years was explained by lower soil pH due to elevated ammonium levels resulting from soil and climate conditions and severe lodging in the wetter season. Information reported here is valuable to farm managers and consultants in the context of nitrogen fertilization of ryegrass grown for seed in western Oregon. It begins to establish criteria for the future development of site specific nutrient management plans and provides knowledge that will aid in improving N use efficiency through improving N fertilizer timing and N-source use.
Technical Abstract: To develop optimum N fertilization practices with the least impact on environmental quality and with the greatest economic return, it is imperative that a greater understanding of crop and soil N dynamics be developed. This paper reports on research with these objectives: (1) To determine the relationship between plant N and dry matter accumulation and soil N status as affected by N-source fertilization as a function of accumulated growing degree days (GDD); and (2) to determine if western Oregon soil conditions favor ammonium over nitrate nutrition during the period of grass seed crop growth. Wetern Oregon field plots of Italian ryegrass (Lolium multiforum Lam.) were treated with different N-sources to manipulate soil ammonium (NH4) and nitrite (NO3) ratios. Italian ryegrass accumulated the greatest portion of plant N and dry mass between tiller elongation and midheading. Reduced growth and seed yield in 1991, compared to 1992, were associated with lodging and low soil pH. Highersoil ammonium levels in 1991 was most likely responsible for a greater reduction in soil pH for that year. Declines in soil pH due to elevated ammonium levels during climate years normal to western Oregon, wet and cool, may have an additive effect to other factors limiting seed yield. Under cool wet soil conditions, ammonium was the predominate mineral N-form regardless of the N-source applied. These findings can be useful in the future development of site specific nutrient management plans and adds knowledge that will aid in improving N use efficiency through improving N fertilizer timing and N-source use.