Phosphorus Uptake by Bahiagrass from Spodosols: Modeling of Uptake from Different Horizons H. Ibrikci, N. B. Comerford,* E. A. Hanlon, and J. E. Rechcigl ABSTRACT Bahiagrass (Paspalum notation Flugge), when grown on a Myakka fine sand hi Florida (sandy, siliceous, hyperthermic Aerie Alaquod), did not respond to P fertilization even though Mehlich-l extractable P was low in the surface soil horizon. Previous research suggested that the Bh horizon was a potential source of P. The objective of this study was to evaluate the plant availability of P in the Bh horizon and to determine to what degree existing nutrient uptake theories explain the uptake of P by bahiagrass under greenhouse conditions. Sod and soil samples were collected from unfertilized bahiagrass growing on a Spo- dosol. Sod was replanted into soil samples. Plant and root samples were collected from unfertilized bahiagrass growing on a Spodosol. Sod was replanted into soil samples. Plant and root samples were collected at 18, 36, 61, and 90 d. Selected soil and plant model pa- rameters were estimated at each harvest period. Bahiagrass was able to extract more P from the Bh horizon material than from Ap or E horizon materials. The COMP8 computer model was used to simulate uptake of P by bahiagrass. Observedand predicted P uptake by bah- iagrass followed the same relative pattern (Bh > Ap > E). There was statistically good agreement between observed and predicted P uptake for 18-d-old plants, but less agreement at 90 d (within 20%) from the Bh horizon. Predicted P uptake values from the Ap and E were sta- tistically lower than observed values at both harvest dates. Selected extraction procedures indicated that there was no P release from the mineral forms in the Ap horizon. Data suggest the plant-induced min- eralization was an important P supplying mechanism in this pot study. N UTRIENT UPTAKE by soil-borne plants is dependent on the uptake characteristics of the root system as well as plant shoot demand and on the nutrient supplying capacity of the soil. Although plant species and soil conditions vary, the mechanism(s) and theory of nutrient uptake should be applicable to all soil con- ditions and plant species. During the last decade, mathematical models for prediction of nutrient uptake by plant roots growing in soil have been developed (Claassen and Barber, 1976; Nye and Tinker, 1977; Barber and Cushman, 1981). These models calculated nutrient uptake based on the size and morphology of the root system, movement of ions to roots by mass flow and diffusion, and kinetics of ion absorption by roots (Barber and Silberbush, 1984). There has been suitable agreement between ob- served and predicted uptake of K by corn (Zea mays L.) (Claassen and Barber, 1976) and forest tree spe- cies (Van Rees et al., 1990a; Gillespie, 1991), and of P uptake by corn (Schenk and Barber, 1980). How- ever, poor agreement has been observed for P uptake by other plant species, particularly under low soil P conditions. Plant species differ in their ability to ac- quire soil P (Hanway and Olson, 1981; Lorenz and H. Ibrikci, N.B. Comerford, and E.A. Hanlon, Soil and Water Science Dep., Newell Hall 106, Univ. of Florida, Gainesville, FL 32611; J.E. Rechcigl, Agricultural Research and Education Cen- ter, Univ. of Florida, Route 1, Box 62, Ona, FL 33865. A con- tribution of the Agric. Exp. Stn. Journal Series no. R-03352. z*Corresponding author. Published in Soil Sci. Soc. Am. J. 58:139-143 (1994). Vittum, 1980) due to differences in root morphology (Barley, 1970), ion uptake characteristics (Fohse et al., 1991), mycorrhizal associations (Bolan, 1991), and the effect of the plant root on soil chemistry and P solubility. Nutrient uptake model modifications meant to account for these differences have incorporated processes such as intraroot competition (Cushman, 1979a,b; Barber and Cushman, 1981; Claassen et al., 1986), uptake by root hairs (Itoh and Barber, 1983), and root exudate effects on nutrient availability (Nye, 1984). None of these models has been applied to the nutrient uptake of a perennial grass such as bahia- grass. The P fertilization recommendation for bahiagrass growing in Florida Spodosols is 20 kg P ha" 1  yr" 1 (Kidder et al., 1990). However, in a group of nine counties in central Florida, Sumner et al. (1990) showed that bahiagrass did not respond to P fertilization. Sim- ilarly, in other field studies that we conducted at the Ona Research Center (in one of the nine counties), bahiagrass either did not respond to P fertilization or showed a minimal growth response (Ibrikci et al., 1992; Rechcigl et al., 1992). Another forage fertility study showed that ryegrass (Lolium multiflorum Lam.) re- sponded to as much as 25 kg P ha" 1  in central Florida (Rechcigl, 1992). These growth responses to fertiliza- tion are interesting in the context of soil test results, which indicated very low extractable P (< 10 mg kg" 1 , Mehlich-l extractable) (Kalmbacher et al., 1992; Rechcigl et al., 1992; Rechcigl, 1992; O'Donnell et al., 1991). One explanation for these observations is that subsoil horizons may be providing significant amounts of P to bahiagrass. In central Florida, Rech- cigl et al. (1992), by placing stable Sr in the Bh ho- rizon, has shown that bahiagrass uptake of Sr from the Bh horizon was about 30% of that absorbed from the Ap horizon. Likewise, root distribution studies at the Ona Research Center have shown root densities of about 1.42 x 10 4  and 0.23 x 10 4  m m" 3  in the Ap and Bh horizons, respectively. It has not yet been shown, however, whether the P in the Bh horizon is available to plants. The objectives of this research were to: (i) deter- mine if P in the Bh horizon is plant available and (ii) apply existing nutrient uptake theories to model P up- take by bahiagrass grown in the Ap, E, and Bh ho- rizons of the above-mentioned Spodosol under greenhouse conditions. MATERIALS AND METHODS Soil, Plant, and Growth Conditions Soil samples were collected from the Ap, E, and Bh hori- zons of a Myakka fine sand from a previously unfertilized bahiagrass field at the Univ. of Florida, Ona Agricultural Re- search and Education Center, Ona, FL. Soil characteristics of a Myakka fine sand are reported in Table 1. Soil was air dried, mixed, sieved (2-mm fraction retained) and weighed (3875 g pot- 1 ) into black plastic pots. Average bulk density was 1.25 139