| Summary: | Core Ideas Recommended P and K rates are largely unknown for native grasslands. Switchgrass requires low levels of P and K in a two‐cut forage system. There are limited forage quality gains from P and K fertilizers. Nitrogen is a more limiting nutrient in switchgrass forage systems on marginal soils. Finite nutrients, including P and K, are required in large quantities for many warm‐season forages. However, native perennial grasses such as switchgrass (Panicum virgatum L.) have less of an affinity for fertilizers, although recommended P and K rates are largely unknown. Objectives were to test switchgrass forage yield response to P and K rates representative of warm‐season forages and identify compositional impacts for developing switchgrass forage fertilizer recommendations. Switchgrass cv. Alamo was subjected to two harvests per year after applications of 0, 28, 56, 112, and 224 kg P ha−1, and split applications of 0, 134, 269, 404, and 538 kg K ha−1 yr−1, from 2014 to 2016. Two sets of controls were used, a “nutrient control” (0 P and K with supplemental rates of N and S) and a “control” (no fertilizer). Harvest timing (first or second cuts) did not influence (P ≥ 0.05) P or K removal or forage yields. For all study years, the control (no supplemental fertilizer) resulted in substantially lower (P ≤ 0.05) yields than the nutrient control, meaning yield did benefit (44 and 49% increases) from supplemental nutrient applications. Therefore, minimal P and K inputs are required for a yield response, as there were very few detectable yield increases beyond the lowest tested application rates (134 kg K ha−1 and 28 kg P ha−1). Results indicate P and K are required in low amounts and that N is a more limiting nutrient in switchgrass forage systems on marginal soils of the southeast.
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