I certify that I have read this study and that in my opinion it 

 conforms to acceptable standards of scholarly presentation and is 

 fully adequate, in scope and quality, as a dissertation for the degree 

 of Doctor of Philosophy. 



W. G. Blue, Chairman 

 Professor of Soil Science 



I certify that I have read this study and that in my opinion it 

 conforms to acceptable standards of scholarly presentation and is 

 fully adequate, in scope and quality, as a dissertation for the degree 

 of Doctor of Philosophy. 



T. L. Yuan 



Professor of Soil Science 



I certify that I have read this study and that in my opinion it 

 conforms to acceptable standards of scholarly presentation and is 

 fully adequate, in scope and quality, as a dissertation for the degree 

 of Doctor of Philosophy. 



r& J\ajULll~~> 



$~\% • Sartain 

 Associate Professor of 

 Soil Science 



