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Herbage Yield and Chemical Composition of Switchgrass as Affected by N, S, and K Fertilization¹

Little information is available regarding the nutrient requirements of switchgrass (Panicum virgatum L.). The objective of this greenhouse study was to examine the effect of N, S, and K fertilization on the herbage yield, N/S ratio, and herbage concentrations of N and S in leaf blade, stem (culms and leaf sheaths), and total herbage fractions of switchgrass grown to anthesis. Three rates of N (0, 224, and 448 kg/ha of N as NH₄NO₃ in split application), three rates of K (0, 448, and 896 kg/ha of K as KHCO₃), and two rates of S (0 and 56 kg/ha of S as CaSO₄) were applied in all possible combinations (18 treatments) to pots containing soil cores of switchgrass.

Dry matter yield of all herbage fractions increased significantly with each increment of N. However, for total herbage, stems, and inflorescences, yield increase with the second increment of N was dependent upon the application of S. K fertilization decreased inflorescence yield significantly at the highest rate, but did not affect dry matter yield of any other herbage fraction.

At the highest rate of N, fertilization with S increased N concentration in the leaves, decreased N concentration in the stems, decreased the proportion of the total N legalized in the stems, and increased the total herbage uptake of N. When S was applied, switchgrass responded to increased N fertilization by increasing S uptake. In contrast, when S was not applied, fertilization with N caused a dilution of tissue S and also decreased the proportion of the total herbage S localized in the stems. Concentration and content of N and S decreased with the first increment of K fertilization, but there was no change with the second increment of K. Critical N/S ratios for switchgrass appear to be greater than 10:1 for herbage, greater than 12:1 for leaf blades, and greater than 8:1 for stems.

Key Words: Panicum virgatum • Herbage fractions • Total N • N?S ratios

² Graduate research assistant and professors of agronomy, respectively, Univ. of Wisconsin, Madison, WI 53706.

Received for publication March 18, 1976.