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Hector Ybanez

Document Type

Thesis - University Access Only

Award Date


Degree Name

Master of Science (MS)

Department / School


First Advisor

Paul E. Fixen


The absorption of nutrients by an imbibing and elongating root depend on (i) the distribution and the rate of root growth, (ii) the characteristics of the root system which influence the uptake kinetics, and (iii) the characteristics of the soil which dictate the ability of the soil to replenish nutrients in solution and the amount of nutrient maintained in the liquid phase. Therefore, the development of a mechanistic model that incorporates the above processes in an organized fashion requires a profound understanding of the nutrient uptake process. Claassen and Barber developed such a model (patterned after that of Nye and Marriott, 1969), whereby equations describing nutrient flux were combined with equations describing root growth, in order to describe nutrient uptake. The Claassen and Barber model was modified by Cushman and Barber in 1981. The Cushman and Barber model has been utilized to evaluate phosphorus uptake under control and field conditions. This model has also been used to calculate the optimum volume of soil to fertilize provided that the relationships between applied P and soil solution P, and applied P and resin extractable P are known. A mechanistic nutrient uptake model such as that of Cushman and Barber has the potential of providing a great insight to questions of fertilizer rate, placement of fertilizer, and the effects of tillage and weather on nutrient uptake. Experimental data have suggested that phosphorus uptake is proportional to root length. This proportionality that exists between root length and phosphorus uptake is coupled by the supply of P to the plant root. Therefore, it can be conceptualized that total root length can be predicted from a mechanistic nutrient uptake model provided that the soil supply parameters and phosphorus uptake values are known. Therefore the objectives of this research were to: (1) Determine the phosphorus adsorption characteristics of three selected soils from South Dakota whereby, the relationships of applied P and P in solution, applied P and resin extractable P, and soil test extractable P and solution P were quantified. (2) Develop a modified version of the Cushman and Barber model (hereafter referred to as CTU, acronym for a Continuous Time Uptake model). (3) Predict effective total root length with CTU for a given set of measured phosphorus uptake values and phosphorus soil supply parameters (whereby CTU is linked to a derivative free nonlinear model fitter, Dud).

Library of Congress Subject Headings

Phosphatic fertilizers

Phosphates -- Mathematical models

Corn -- Roots -- Physiology



Number of Pages



South Dakota State University