Bahiagrass phosphorus fertilization updates

February, 2011
Maria L. Silveira, Soil and Water Science Program, UF/IFAS Range Cattle Research & Education Center

Currently, Florida has approximately 3 million hectares of pasture and rangeland with about 2 million being planted pastures. Bahiagrass occupies about 1 million hectares in Florida and is the predominant planted forage species. Most forage-based beef cattle systems in Florida rely on bahiagrass pastures as the major source of energy and protein for most of the growing season. Due to the extensive area of bahiagrass pastures and potential environmental impacts associated with nutrient transport, fertilizer management of bahiagrass pastures has received great attention by state regulatory agencies, livestock farmers, and researchers in recent years.

Bahiagrass like most warm-season grasses responds very well to N fertilization. Nitrogen rates of 50 to 60 lb N/A represents a typical application rate used in the majority of cow-calf operations in Florida. Phosphorus fertilization of bahiagrass pastures has been controversial and a topic of agronomic and environmental importance. Conflicting reports in the literature suggest that, despite significant research efforts on pasture fertilization, bahiagrass response to P fertilization has not been fully understood. While some research studies showed that bahiagrass can respond to P application, reports also suggested that bahiagrass yields are not affected by the addition of P. The lack of response is often attributed to the ability of bahiagrass roots to obtain P from the spodic horizon, which typically contains significant amounts of available P.

The discrepancies in the literature regarding bahiagrass response to P fertilization are, in part, due to the inability of soil testing alone to accurately predict forage P requirements. Although most Florida sands exhibit very low P concentrations in the surface horizons, adequate concentrations are often found at deeper soil depths. However, because soil tests typically examine the top 15 cm of the soil profile, it may not reflect the total available soil-P pool present at deeper soil depths. Soil test alone has been shown to poorly predict bahiagrass P requirements. A challenge for agronomists and soil scientists has been to develop additional tools to better predict P requirements of established bahiagrass pastures. In this context, plant analysis in combination with soil test has proven to be a useful diagnostic tool to manage soil fertility. Although using the concept of plant nutrient analysis has been long used in many agricultural systems, incorporating this concept into forage nutrient management programs in Florida has been a major change. In 2007, plant tissue and soil testing were incorporated in the UF/IFAS P fertilizer recommendations for established bahiagrass pastures. According to the revised fertilizer recommendations for established bahiagrass, tissue testing is recommended when soil test P is low/very low (< 10 and 10-15 ppm, respectively). Phosphorus fertilizer is not recommended when Mehlich-1 soil test P is medium (16-30 ppm) or high (31-60 ppm). When soil test P is low/very low and tissue P ≥ 0.15%, no P application is required. However, if soil test P is low/very low and tissue P < 0.15%, 25 lb P2O5/A is recommended for the low and medium N options.

A field study was conducted at Ona to evaluate the effects of revised P fertilizer recommendations on forage dry matter yield and nutritive value and the potential impacts on soil-test P concentrations and water quality in bahiagrass pastures growing on a Spodosol. Treatments were a combination of three annual N application rates (0, 50 and 100 lb N/A) and four P rates (0, 12, 25, and 50 lb P2O5/A). The 50 and 100 lb N/A rates correspond to the recommended UF/IFAS low and medium bahiagrass N options, respectively. Phosphorus application rates correspond to 0, 0.5-, 1-, and 2-times the UF-IFAS P fertilizer application rate of 25 lb P2O5/A. Treatments spanning a range of low levels of P fertilization were targeted in this study because of the critical need to minimize P loss to surface water while maintaining vigorous bahiagrass stands. Five suction cup lysimeters were installed in the center of each plot at 6-, 12-, 24-, 36-, and 60-inches depths to monitor the impacts of fertilization on water quality. The 6- and 12-inches lysimeters were located above the spodic horizon, whereas lysimeters at 24-, 36-, and 60-inches were below the spodic horizon. Leachate samples were collected after rainfall events > 0.4 inches.

Although P application showed no effects on bahiagrass yield in 2007, bahiagrass increased linearly to increasing P application in 2008. Across the range of N rates most often used on bahiagrass swards in Florida, increasing P application from 0 to 25 lb P2O5/A had no effect on DMY in one year (average bahiagrass yield of 6.7 tons of dry matter/A) and increased DMY by ~ 13% in the second year. Our results showed that bahiagrass response to P application can be variable depending on year and environmental conditions. This trend may also explain the apparent discrepancies in the literature relative to the effects of P fertilization on bahiagrass pastures in South Florida. There was no effect of P application on any soil P response. Leachate P was generally not affected by P application. The only exception occurred in 2008 where leachate P concentration at 6-inches depth was greater for the 25 lb P2O5/A compared to the other treatments. However, leachate P at the 30-cm depth was either lower or similar for the 25 lb P2O5/A compared to the control in 2007 and 2008, which suggested that limited P leaching occurred at depths > 12 inches. The high variability associated with leachate P in both years reflects the seasonal fluctuation in P losses by leaching. It is likely that high water table conditions experienced during the summer months increased P release from the deeper soil layers and promoted subsequent transport of P to the surface horizons. In both years, the period of large spikes in soil solution P concentration in the 6- and 12-inches lysimeters coincided with periods of high rainfall and high water table conditions at the experimental site.

Additional work with low rates of P is warranted because this study covered a relatively short time scale and because these data are from harvested bahiagrass swards instead of grazed pastures. Results also suggest that further studies are needed to investigate the impact of fluctuating water table on P availability and loss potential.

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