Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

Michael C. Amacher


The Barber and Cushman (1981) mechanistic model of element uptake by plants was used to help elucidate the mechanisms that govern the movement of S in soils and S uptake by plants. Sulfur uptake predicted by the model was compared to measured S uptake by 'Stoneville 825' cotton (Gossypium hirsutum L.), 'Centenial' soybean (Glycine max L. Merr.), and 'Coker 916' wheat (Triticum aestivum L.) grown on Gallion very fine sandy loam (Typic Hapludalf), Mhoon silty clay loam (Typic Fluvaquent), and Norwood silt loam (Typic Udifluvent) under glasshouse conditions. Predicted S uptake by the model was significantly correlated with observed uptake. However, the model overpredicted S uptake by all crops on all soils by at least 3 times and up to 21 times. Better agreement between observed and predicted S uptake could be achieved by decreasing the Influx maximum (I$\sb{\rm max}$) values obtained from solution culture experiments. The I$\sb{\rm max}$ measured in solution cultures may not be valid for soil studies or the assumption made by the model that I$\sb{\rm max}$ is the same irregardless of plant age may not be appropriate for roots in soil. If the current model is otherwise valid, I$\sb{\rm max}$ appropriate to soils could be fit directly to the uptake data. An alternate explanation of S uptake overprediction is that uptake is controlled by a feedback mechanism that can turn off S uptake when the S requirement of the plant has been met. Such a mechanism is not accounted for by the current model. A modification of the model to account for a possible shutdown mechanism should be considered. As implemented, the model indicated that the main mechanism for S transport to the roots was with water flux. The concentration of S in soil solution appears to be the best index for S availability. The present model does not consider all possible sources of plant S. In addition to the buffer power of the soil, other sources of S including dry and wet deposition and the mineralization of soil organic matter need to be quantified, if a good estimation of S supply through the growing season is to be achieved.