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TECHNICAL REPORTS

Plant and Environment Interactions

Phosphorus in Hawaiian Kikuyugrass Pastures and Potential Phosphorus Release to Water

^a College of Agriculture, Forestry, and Natural Resource Management, University of Hawai'i at Hilo, 200 West Kawili Street, Hilo, HI 96720-4091
^b Department of Human Nutrition, Food, and Animal Sciences, College of Tropical Agriculture and Human Resources, University of Hawai'i at Manoa, 1955 East-West Road, Honolulu, HI 96822
^c Agronomy Department, University of Florida, Gainesville, FL 32611-0500
^d Biochemistry Department, Iowa State University, Ames, IA 50011

^* Corresponding author (bmathews{at}hawaii.edu)

Received for publication October 24, 2004. Pasture systems in Hawaii are based primarily on kikuyugrass (Pennisetum clandestinum Hochst. ex Chiov.). Relationships among kikuyugrass P concentration, animal P requirements, and various soil P determinations are needed to help identify source areas for implementing pasture management strategies to limit P loss via overland flow. A total of 51 rotationally stocked kikuyugrass pastures (>20 yr old) with contrasting soil chemical properties were sampled. A satisfactory predictive relationship between modified-Truog (MT)-extractable phosphorus (P_MT) and dissolved (<0.45-µm pore diameter), molybdate-reactive phosphorus (DRP) desorbed from soil in a water extract (DRP_WE) was found when 0- to 4-cm-depth data for the soil orders with medium to high DRP_WE (two Mollisols and an Inceptisol) were pooled separately from those with low DRP_WE (five Andisols, three Ultisols, and an Oxisol). The oxalate phosphorus saturation index (PSI_ox) procedure was the best predictor of DRP_WE across soil orders when oxalate-extractable molybdate-reactive phosphorus (RP_ox) was used to calculate PSI_ox (PSI_oxRP) rather than when total oxalate-extractable phosphorus (TP_ox) was used (PSI_oxTP). There was little DRP_WE until PSI_oxRP exceeded 6% or PSI_oxTP exceeded 8%. A more empirical dilute-acid phosphorus saturation index (PSI_MT) was also calculated using P_MT and MT-extractable iron (Fe_MT) and aluminum (Al_MT). The PSI_MT procedure showed some utility in predicting DRP_WE, was positively related to the PSI_ox procedures, and can be more readily performed in agronomic soil testing laboratories than PSI_ox. The present research suggests that while Hawaiian kikuyugrass pastures tend to be sufficient to high in forage P, potential soil P release to water only appeared to be a possible environmental concern for the Mollisol and Inceptisol sites.

Abbreviations: DC, dithionite–citrate • DRP, dissolved (<0.45-µm pore diameter), molybdate-reactive phosphorus • DRP_WE, dissolved, molybdate-reactive phosphorus desorbed from soil in a water extract • Fe_DC, Al_DC, RP_DC, dithionite–citrate-extractable iron, aluminum, and molybdate-reactive phosphorus, respectively • Fe_MT, Al_MT, P_MT, modified-Truog-extractable iron, aluminum, and phosphorus, respectively • Fe_ox, Al_ox, RP_ox, and TP_ox, oxalate-extractable iron, aluminum, molybdate-reactive phosphorus, and total phosphorus, respectively • ICPES, inductively coupled plasma emission spectroscopy • MT, modified Truog extractant • PSI_MT, a soil phosphorus saturation index estimated from modified-Truog-extractable iron, aluminum, and phosphorus • PSI_MT[P/Al], a soil phosphorus saturation index estimated from modified-Truog-extractable aluminum and phosphorus only • PSI_ox, soil phosphorus saturation index based on oxalate extraction • PSI_oxOP, extent of soil saturation with oxalate-extractable organic phosphorus • PSI_oxRP, soil phosphorus saturation index calculated using oxalate-extractable iron, aluminum, and molybdate-reactive phosphorus • PSI_oxTP, soil phosphorus saturation index calculated using oxalate-extractable iron, aluminum, and total phosphorus

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