Effects of Phosphorus and Potassium on Forage Nutritive Value and Quantity: Environmental Implications

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.