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 arundinacea 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 was to determine whether annual P and Mg fertilization of a soil with Bray-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. In early March of each year, 25 lb P/acre and/or 15 lb Mg/acre were broadcast 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 fertilization increased leaf Mg, Ca, and P content in late March/early April. The greatest increase in leaf content of these mineral elements occurred when Bray-1 P was less than 16 lb/acre. Magnesium fertilization increased leaf Mg content only when Bray-1 P was 26 lb/acre or greater, with or without P fertilization. Forage yield in May was over 1000 lb/acre greater with annual P fertilization than without when soil P was less than 16 lb/acre. Annual applications of 25 lb P/acre were as effective as building soil P above 26 lb/acre for increasing forage yield and early spring leaf Mg and Ca content. The Mg and Ca contents of forage harvested in May were lower than those of leaves sampled in early spring and this resulted from the large number of stems, which are lower 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 mineral element content in early spring and made Mg fertilization effective. Research QuestionPhosphorus is an essential macronutrient that often limits pasture production on soils of the eastern, southeastern, and lower midwestern USA. Tall fescue grown on these low P soils is often low in Mg, Ca, and P. Forage low in one or more of these three nutrients may cause low blood serum levels of Mg and Ca in cattle leading to grass tetany or wheat pasture poisoning. Attempting to correct a forage Mg deficiency by applying Mg to the soil has often been unsuccessful. The objective of this study was to determine whether annual P and Mg fertilization of a soil with Bray-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. Literature SummaryThe optimum Bray-1 soil P level for a cool season grass forage is 40 lb/acre. However, 60% of samples analyzed by the University of Missouri Soil Testing Lab in 1994 had soil P levels that were below 40 lb/acre. Fertilization with 25 lb P/acre in early March on soils testing “low” for P (8 lb P/acre) increased leaf Mg, Ca, and P contents in the early spring. Fertilization with over 100 lb Mg/acre in an attempt to increase leaf Mg content has often been unsuccessful. However, when P fertilizer was applied to soils testing low for P, fertilization with 15 lb Mg/acre increased tall fescue leaf Mg content. Study DescriptionAn established stand oftall fescue on a Creldon silty clay loam soil near Mt. Vernon, MO, was fertilized with various rates of P to form duplicate 10 ft by 20 ft plots that tested 8, 15, 26, 51, or 97 lb Bray-1 P/acre. In early March of 1994 and 1995, plots at each soil P level were divided into two 10 ft by 10 ft subplots. One subplot received 25 lb P/acre as triple superphosphate and the other plot did not receive P fertilizer. One of the duplicate soil Bray-1 P plots was also treated with 15 lb Mg/acre as MgCl₂, Duplicate soil P levels were randomized within blocks with five replications and P fertilization treatments were randomized within each soil P level. At two weekly harvests in late Marchlearly April of each year, at least 25 of the youngest fully collared leaves from each plot were sampled for Mg, Ca, K, and P. In addition, forage was harvested for yield and Mg, Ca, K, and P content determinations in May and October of each year. Applied QuestionsAt what soil P level is annual P fertilization not required to increase Mg, Ca, and P contents of tall fescue leaves in the early spring? Annual P fertiliiation increased leaf Mg, Ca, and P contents in the early spring at each soil P level. The greatest increases in leaf Mg, Ca, and P contents with P fertilization were for soil P levels of 8 and 15 lb/acre. Does application of 15 lb Mg/acre fertilizer increase leaf Mg content at all soil P levels? No, only when soil P levels were 26 lb/acre or greater (Fig. 1). This interaction of Mg fertilization and soil P level may explain why fertilization of low P soils with over 100 lb Mg/acre failed to increase leaf Mg levels in many studies, even when soil Mg levels were high. Does P and Mg fertilization in early March affect forage yields in May? Yes. When soil P was low, 8 or 15 lb/acre, P fertilization increased forage yield by over 1000 lb/acre in May (Fig. 2). Magnesium fertilization had no effect on forage yield. When soil levels were above 26 lb P/acre, forage production in May was not affected by P fertilization. Does P and Mg fertilization increase Mg, Ca, and P contents of tall fescue forage harvested in May and October? No. There was little effect of P or Mg fertilization on forage Mg, Ca, and P contents at either harvest. In May, forage Mg content was 0.15% and Ca content was 0.44%, with or without P fertilization at all soil P levels. Tall fescue stem tissue is very low in Mg and Ca contents compared with leaf blades and any increase in forage Mg and Ca contents at the May harvest were probably masked by the large number of stems that were present. By October, much of the applied P was unavailable and could not influence uptake and translocation of Mg and Ca. Fig. 1Early spring Mg content of tall fescue leaves in response to the interaction of annual Mg fertilization and soil P level. The Mg application consisted of 15 lb Mg/acre as MgCl₂ and soil P levels ranged from 8 to 97 lb/acre. No Mg fertilizer was applied to the −Mg plots. Data are combined over 2 yr —1994 and 1995 — and two harvests each year, and are presented on a percentage dry weight basis. *Means are significantly different within soil P level at the 0.05 level. Fig. 2Forage yield of tall fescue at the May harvest in response to the interaction of annual P fertilization and soil P level. The P application consisted of 25 lb P/acre as triple super phosphate and soil P levels ranged from 8 to 97 lb P/acre. No P fertilizer was added to the −P plots. Data are combined over two years, 1994 and 1995. *Means are significantly different within soil P level at the 0.05 level.