THE FLORIDA CATTLEMAN AND LIVESTOCK JOURNAL
Agronomic and Environmental Impacts of Phosphorus Fertilization to Low Input Bahiagrass Systems in Florida
Maria L. Silveira
Phosphorus management in low input bahiagrass systems in Florida represents a major challenge of agronomic and environmental importance. Concerns relative to potential impacts of pasture P fertilization on water quality continues to be an important issue for the cow-calf industry. This is particularly true in Florida, where environmental regulations are typically more stringent than other states in the U.S.
Despite the extensive body of knowledge on the effects of N fertilization on bahiagrass yield, limited research is available about the agronomic and environmental impacts of bahiagrass P fertilization. In addition, the emphasis of most previous studies on bahiagrass P fertilization has been on agronomic responses of bahiagrass to relatively high P inputs not commonly used in a typical cow-calf operation. Very few (if any) studies have attempted to investigate the potential impacts of P fertilization on water quality using fertilizer rates that truly represent a typical situation in most ranches in Florida. Science-based information on P management strategies that balances productivity with environmental implication is important not only for ranchers but also for the success of water quality programs in Florida.
To address this important issue, we conducted a 2-yr study on established bahiagrass pastures at the Range Cattle REC in Ona to (i) investigate bahiagrass response to reduced P fertilization rates, and (ii) evaluate the potential effects of bahiagrass P fertilization on soil test P concentrations and water quality. Treatments consisted of a combination of three N application rates (0, 50 and 100 lb N/A) and four P rates (0, 12.5, 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 corresponded to 0, 0.5-, 1-, and 2-times the UF-IFAS P fertilizer application rates. Each plot also received a basal annual application of 50 lb K2O/A. Plots received lime prior to the initiation of the study to raise the soil pH to 5.5. The experiment was conducted for 2 years (2007 and 2008) and treatments applied in May of each year. Bahiagrass forage was harvested at 28-day intervals from June to October each year. Prior to treatment application and at the end of each growing season, soil samples were taken from the Ap (0-6”), E (6-12”) and Bh (12-24”) horizons and analyzed for Mehlich-1 P and water-extractable P concentrations. To monitor water quality, five suction cup lysimeters (referred to as lysimeters herein) were installed in each plot at 6-, 12-, 24-, 48-, and 69-inches depth. Water samples were collected after rainfall events > 4 inches and analyzed for phosphorus and nitrogen (nitrate + ammonium) concentrations.
Our results showed that P application resulted in no significant effects on forage yield in 2007; however, bahiagrass responded linearly to P application in 2008. Compared to control treatments (no P applied), bahiagrass yield increased ~ 4, 10, and 19% in 2008 when P was applied at rates of 12.5, 25, and 50 lb P2O5 /A, respectively. Differences in rainfall pattern between 2007 and 2008 likely affected bahiagrass response to N and P fertilization. These results confirmed that bahiagrass response to P application can be variable depending on the year and environmental conditions. This may also explain the apparent discrepancies in the literature relative to the effects of P fertilization on bahiagrass pastures in south Florida. Phosphorus application to bahiagrass pastures showed no effect on Mehlich-1 soil P and water-extractable P concentrations in the Ap, E, and Bh horizons. This indicates that P application at agronomic rates will likely have no negative impacts on soil test P concentrations. Similarly, our results showed that P application had no effect on water P concentration, which suggests that P fertilization at agronomic rates to low input bahiagrass systems has no environmental impacts on water quality. In 2007 and 2008, the period of greater spikes in P concentration in the shallow lysimeters (6 and 12” depth) coincides with periods of high rainfall and high water table conditions in the experimental site. The high water table conditions might have contributed to P movement from deeper horizons into the surface layers, thus increasing P concentration in the lysimeters above the spodic layer. The fluctuating water table conditions experienced in Florida may affect P availability to bahiagrass.