Potassium and Phosphorus Fertilization of Grass Pastures

Pasture fertilization plays a vital role in the success of modern forage-based livestock production systems. Despite the University of Florida recommendations for K and P fertilization of highly productive hayfields and pastures, many forage producers do not supply adequate amounts of K and P to replace the exported nutrients in harvested forage. The relatively large K removal associated with the sandy-textured soils with low K-buffering capacity can lead to severe K deficiency symptoms, particularly in mechanically harvested forage production systems. The objective of this study was to evaluate the response of Jiggs bermudagrass (Cynondon dactylon (L.) Pers.) and limpograss (Hemarthria altissima Stapf. and Hubbard) to K and P fertilization. The study was conducted at the University of Florida Range Cattle Research and Education Center, Ona, Florida and treatments consisted of minimum fertilization regimens that could maintain optimum forage yield, nutritive value, and stand persistence. Because limpograss is often conserved as a stockpile grass, this study also investigated the effect of harvest frequency (4- vs. 2-times per year) on limpograss responses to K and P fertilization. Potassium and P were applied in 2012 and 2013 at rates of 0, 40, and 80 K₂O/A and 0, 20, and 40 lb P₂O₅/A per year.

Cumulative dry matter yields (DMY) of Jiggs bermudgrass increased linearly (~121 to 170%) as K fertilization increased. Similarly, K fertilization increased limpograss DMY by as much as 38% relative to the zero K control treatments. Despite the positive effect of K fertilization, the 2013 DMY of both Jiggs bermudagrass and limpograss were significantly lower than those obtained in 2012. Although the amounts of K exported via above-ground biomass harvest were similar or less than those applied as fertilizer, K fertilization at the levels tested in this study was not sufficient to sustain the same level of production during the 2-yr study. There was considerable stand losses and concomitant weed infestation by the end of the study. No significant benefit from P fertilization on DMY was observed. Although K and P fertilization showed no effect on soil chemical properties in the limpograss study, soil pH and extractable P and K concentrations were affected by fertility treatments in the Jiggs study. Our data indicated that continuous above-ground removal without proper K fertilization results in poor forage performance, stand loss, and weed infestation. Adequate K supply is essential to sustain Jiggs bermudagrass and limpograss productivity and long-term persistence.