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Phosphorus Movement in Soils: Theoretical Aspects¹

ABSTRACT

The renovation of waste water by land application shows promise because of the potential to use certain waste water nutrients to fertilize agricultural crops and to recycle these nutrients as agricultural products. In order to locate, design, and manage land treatment processes, quantitative descriptions of critical processes are required.

The adsorption of P by soils is a critical process which needs to be considered. This paper compares a new model for P movement in soils with existing adsorption-desorption models developed for chromatography and ion exchange processes. The effect of the model parameters on P movement in soils is also examined.

From the new model considered here, it was found that under P adsorption, a P profile of fixed shape (shock layer) developed and moved through the soil at a speed which could be calculated from the P Langmuir adsorption isotherm, density of the soil, and the P rate of application to the soil.

Model parameters have an effect on the shape of the P profile, but over the range of condition studied here, the S-shape P front (shock layer) was less than 4 cm in thickness. The range of conditions were chosen to cover a range of soils and operating conditions which might be considered during the selection and design of a waste water land treatment process. It should not be construed from this study, that a small shock layer exists for all adsorption cases, or for the case of desorption of P from soils during leaching.

Key Words: P mathematical model • P adsorption • waste water renovation • land disposal of waste water • nonideal flow • shock layer

¹ Contribution from the Department of Electrical Engineering and Systems Science and the Department of Chemical Engineering, College of Engineering, and Department of Crop and Soil Sciences, College of Agriculture, Michigan State University, East Lansing, Michigan 48823. Financial support from NSF-RANN through Grant GI-20 and from the Michigan State Univ. Agr. Exp. Sta. is gratefully acknowledged. Michigan Agricultural Station Journal Article No. 6712.

² Former Assistant Professor in Electrical Engineering and Systems Science, Assistant Professor in Crop and Soil Sciences, Associate Professor in Chemcial Engineering, and Research Assistant in Chemical Engineering, respectively. The senior author is now Assistant Professor, Department of Chemical Engineering, Cleveland State University, Cleveland, Ohio 44115.

Received for publication February 1, 1974.