Effect of Nitrogen and Phosphorus Fertilizers on a Bromegrass and Alfalfa Mixture Grown Under Two Systems of Pasture Management. II. Nitrogen and Phosphorus Uptake and Concentration in Herbage1

Abstract Yield response and botanical composition changes of a bromegrass (Bromus inermis L.) and alfalfa (Medicago media Pers.) herbage mixture were related to applied N and P fertilizers in previous experiments. Herbage quality also can be affected by these elements. To determine the relationship between N and P fertilizers applied and nutrient uptake, the concentration of N and P was measured in the herbage mixtures taken from two pasture management treatments, “Put and Take” and “Fed Barley.” Nitrogen and P uptake by bromegrass and alfalfa was significantly related by regression analyses to N and P fertilizer applied. Nitrogen uptake ranged from 90 to 211 kg N/ha and P uptake from 6.3 to 18.9 kg P/ha. N uptake in herbage was greater than applied fertilizer N (45, 90, and 135 kg N/ha) with some exceptions and P uptake was less than applied P fertilizer (20 kg P/ha). Nitrogen uptake measured for 2 years after the last addition of fertilizer was curvilinearly related to residual N fertilizer. Phosphorus uptake also was curvilinearly related to residual N except for the “Fed Barley” treatment and linearly (positively) related to P fertilizer for 2 years after the last addition. Concentration of N in bromegrass herbage was highly related to applied N fertilizer and ranged from 2.1 to 3.5%. Concentration of N in alfalfa also was highly related to applied N fertilizer in the second cut of the “Put and Take” pasture management treatment and in the first cut of the “Fed Barley” pasture treatment. Percentage N in alfalfa ranged from 2.8 to 4.0%. Concentration of P in pasture herbage was increased by P fertilizer and ranged from 0.16 to 0.30% in alfalfa and 0.16 to 0.31% in bromegrass. As N uptake in herbage was greater than applied N fertilizer with the exception of the highest rates of N applied without P, symbiotic N fixation by the alfalfa plant and soil N would make up for the difference between applied N and that removed by the herbage. As P was applied at a greater rate than what was removed in the herbage, a net accumulation of P would occur in the roots and soil. As the forage was pastured, much of the nutrients in the herbage would be returned to the soil by cattle excrement which would not occur if all herbage or seed were removed under other harvesting systems.