PHOSPHORUS AND MAGNESIUM FERTILIZATION INTERACTION WITH SOIL-PHOSPHORUS LEVEL - TALL FESCUE YIELD AND MINERAL ELEMENT CONTENT

In a previous study, P fertilization of tall fescue (Festuca arundinac ea Schreb,) pastures increased early spring leaf Mg and Ca content on low P soils, However, Mg fertilization of these low P soils increased leaf Mg content only when P was applied, The objective of this study w as to determine whether annual P and Mg fertilization of a soil with B ray-1 soil P levels varying from 8 to 97 lb/acre is required in order to increase Mg, Ca, and P contents, and yield of tall fescue forage. I n early March of each year, 25 lb P/acre and/or 15 lb Mg/acre were bro adcast onto plots near Mt. Vernon, MO, containing a Creldon silty clay loam (fine, mixed, mesic Mollic Fragiudalf) soil, with Bray-1 P tests of 8, 15, 26, 51, and 97 lb P/acre, At all soil P levels, P fertiliza tion increased leaf Mg, Ca, and P content in late March/early April, T he greatest increase in leaf content of these mineral elements occurre d when Bray-1 P was less than 16 lb/acre, Magnesium fertilization incr eased leaf Mg content only when Bray-1 P was 26 lb/acre or greater, wi th or without P fertilization. Forage yield in May was over 1000 lb/ac re greater with annual P fertilization than without when soil P was le ss than 16 lb/acre, Annual applications of 25 tb P/acre were as effect ive as building soil P above 26 lb/acre for increasing forage yield an d early spring leaf Mg and Ca content. The Mg and Ca contents of forag e harvested in May were lower than those of leaves sampled in early sp ring and this resulted from the large number of stems, which are tower in Mg and Ca content than leaf blades, Annual P fertilization of tall fescue on low P soils increased both forage production in May and min eral element content in early spring and made Mg fertilization effecti ve.