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Phosphorus Management

Effects of Phosphorus and Potassium on Forage Nutritive Value and Quantity

Environmental Implications

^a Univ. of Florida, IFAS, RCREC, 3401 Experiment Station, Ona, FL 33865
^b Univ. of Florida, IFAS, GCREC, 5007 60th Street East, Bradenton, FL 34203

^* Corresponding author (hari{at}mail.ifas.ufl.edu)

Received for publication May 3, 2003. Management practices minimizing P application in agricultural catchments ultimately reduce P export to waters. To determine stargrass (Cynodon nlemfuensis Vanderyst var. nlemfuensis) response to P and K, eight rates of P and K were applied on experimental units located on Pomona fine sand (sandy, siliceous, hyperthermia Ultic Alaquods) Spodosols, and arranged in a randomized complete block design with four replicates. The forage yield was less from 39:0 (P/K; kg ha^–1 yr^–1) treatment than the experimental units supplied with 93 kg K ha^–1 yr^–1 and low P (10 and 20 kg ha^–1 yr^–1) in all years with exception in 1998 (i.e., the year of grass establishment), indicating efficient P utilization due to K applications. No significant differences were obtained in in-vitro organic matter digestibility (IVOMD) from the applications of 10 kg P ha^–1 yr^–1 and 93 kg K ha^–1 yr^–1. The applications of 10 and 93 kg ha^–1 yr^–1 of P and K, respectively, provided efficient P utilization. Phosphorus mass balance showed that stargrass receiving 10 and 93 kg ha^–1 yr^–1 of P and K, respectively, removed maximum P (161% of the applied P) by uptake from soils. This may indicate the capability of stargrass to mine P from subsoils if sufficient K is supplied, and also suggests that stargrass may be useful for crop phytoremediation on P-impacted sites. In general, this study indicates that applications of 10 kg P ha^–1 yr^–1 in combination with 93 kg K ha^–1 yr^–1 will maintain forage nutritive value and quantity, and maximize P removals by stargrass. Moreover, the supply of sufficient K appears to be crucial for efficient P utilization by forages, reducing potential adverse effects of P over-fertilization on water quality.

Abbreviations: CP, crude protein • DM, dry matter • IVOMD, in-vitro organic matter digestibility • NPK, nitrogen, phosphorus, and potassium fertilizers