The objective of this part of the study was to determine the long-term agronomic impacts of 10 and 20 Mg PG ha-1 applied all at one time in 1993 (FIPR Publ. No. 05-038- 141) to Malabar and to Myakka soils cropped to bahiagrass. Averaged over 1993-94, the 20 Mg PG ha-1 reduced regrowth forage yield in Myakka but not thereafter. Averaged over 1995-97, PG increased S and Ca, reduced Mg, and had no effect on P, K, and N in regrowth forage. In soil down to 15 cm depth, PG tended to increase pH and Mehlich 2 extractable Ca and reduced Mg significantly. The PG also tended to increase extractable K down to 15 cm in Malabar and retention capacity of soil for P down to the clay in Malabar and in the spodic layer in Myakka. At 10 Mg ha-1, PG tended to reduce extractable Al in the clay and in the spodic layer of Malabar and Myakka, respectively. In groundwater, PG reduced pH and increased conductivity at the 80-90 cm sampling depth. The PG kept Ca high at the 35-45 cm sampling depth and Mg low in runoff in Malabar. The effect of PG on Ca and Mg in groundwater in Myakka was no longer noted. No effects for P, K, and Al were noted. The PG showed no effect on micronutrients and on Hg, Se, Cr, or Pb concentrations in soil, groundwater or forage. Except for Mg loss, the fast rate at which the undesirable effects of the high rates of PG were lost indicates that PG is a safe material for long-term use as a S and/or Ca fertilizer for soils deficient in these nutrients.
J.E. Rechcigl et al., University of Florida. March 2001.