MODELING THE UPTAKE OF SULFUR BY CROPS ON 3 ALLUVIAL SOILS OF LOUISIANA - WHEAT

The Cushman and Barber mechanistic model was used to help elucidate th e mechanisms that govern the uptake of sulfur (S) by plants. Sulfur up take predicted by the model was compared to measured S uptake by wheat (Triticum aestivum [L]) grown on Gallion very fine sandy loam (Typic Hapludalf), Mhoon silty clay loam (Typic Fluvaquent), and a Norwood si lt loam (Typic Udifluvent) under glasshouse conditions. Predicted S up take was significantly correlated with observed uptake r(2)=0.85. Howe ver, the model over predicted S uptake by a factor of 10.4. The assump tion that the maximum ion Influx rate (I-max) for roots growing in soi ls is the same as the I-max measured in solution cultures experiments may not be appropriate. Better agreement between observed and predicte d S uptake values was achieved by using an estimated I-max Obtained fr om soil studies (I-s). Using I-s, predicted vs observed S uptake had a slope of 1.5 and r(2)=0.93. The model predicted that when S concentra tion in soil solution (C-lo) at the root surface (r(o)) was about 2mM or higher, this high C-lo will trigger a higher ion Influx rate of abo ut 2.6 nmol/m(2)sec. Plants grown on soils with C-lo less than 1 mM at r(o) will show a lower ion Influx rate of about 0.8 nmol/m(2)sec, sug gesting that S uptake by wheat plants is biphasic and depends on C-lo at r(o).