T I Lgronomy VOLUME 49 Advisory Board Martin Alexander Eugene J. Kamprath Cornell University North Carolina State University Kenneth J. Frey Larry P. Wilding Iowa State University Texas A&M University Prepared in cooperation with the A m ' c a n Society of Agronomy Monographs Committee S. H. Anderson P. S. Baenziger L. P. Bush M. A. Tabatabai, Chairman R. N. Carrow W. T. Frankenberger, Jr. S. E. Lingle R. J. Luxrnoore G. A. Peterson S. R. Yates D V A N C E S IN VOLUME 49 Edited by Donald L. Sparks Department of Plant and Soil Sciences University of Delaware Newark, Delaware ACADEMIC PRESS, INC. Harcourt Brace & Company San Diego New York Boston London Sydney Tokyo Toronto This book is printed on acid-free paper. 8 Copyright 0 1993 by ACADEMIC PRESS, INC. All Rights Reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher. Academic Press, Inc. 1250 Sixth Avenue, San Diego, California 92 101.431 1 United Kingdom Edition published by Academic Press Limited 24-28 Oval Road, London NW1 7DX Library of Congress Catalog Number: 50-5598 International Standard Book Number: 0- 12-000749-5 PRINTED IN THE UNITED STATES OF AMERICA 9 3 9 4 9 5 9 6 9 7 9 8 BB 9 8 1 6 5 4 3 2 1 Contents CONTRIBUTORS .......................................... PREFACE ................................................ vii ix USEOF COMPUTER-ASSISTED TOMOGRAPHY IN STUDYINGWATER MOVEMENT AROUND PLANT ROOTS L . A. G . Aylmore I. I1 . I11. IV . V. VI . VII . VIII . IX . Introduction ............................................. Computer-Assisted Tomography ............................. X-Ray CATScanners ...................................... y-Ray C A T Scanners ...................................... Application of Computer-Assisted Tomography to Soil- Water Studies ................................................. Nuclear Magnetic Resonance Imaging ........................ Dual-Energy Scanning ..................................... Recent and Future Developments ............................ Summary and Conclusions .................................. References .............................................. 2 4 13 22 26 41 43 47 49 50 PHOSPHOGYPSUM IN AGRICULTURE: A REVIEW Isabel0 S . Alcordo and Jack E . Rechcigl I . Introduction ............................................. I1. Uses of Phosphogypsum in Agriculture ....................... 111. Environmental Considerations............................... IV Conclusions ............................................. References .............................................. . 55 65 93 100 102 NUTRIENT CYCLING AND SOILFERTILITY IN THEGRAZED PASTURE ECOSYSTEM R . J . Haynes and P . H . Williams I . Introduction ............................................. 119 121 The Pasture System and Its Effect on Soil Properties ............. I1. I11. Nutrient Return