Agronomy DVANCES I N VOLUME 74 Advisory Board Martin Alexander Ronald Phillips Cornell University University of Minnesota Kenneth J. Frey Kate M. Scow Iowa State University University of California, Davis Larry P. Wilding Texas A&M University Prepared in cooperation with the American Society of Agronomy Monographs Committee Jerry M. Bigham Jerry L. Hatfield David M. Kral Linda S. Lee Diane E. Stott, Chairman David Miller Matthew J. Morra John E. Rechcigl Donald C. Reicosky Wayne F. Robarge Dennis E. Rolston Richard Shibles Jeffrey Volenec Agronomy DVANCES IN VOLUME 74 Edited by Donald L. Sparks Department of Plant and Soil Sciences University of Delaware Newark, Delaware San Diego San Francisco New York Boston London Sydney Tokyo This book is printed on acid-free paper. ∞  C 2001 by ACADEMIC PRESS Copyright  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. The appearance of the code at the bottom of the first page of a chapter in this book indicates the Publisher’s consent that copies of the chapter may be made for personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc. (222 Rosewood Drive, Danvers, Massachusetts 01923), for copying beyond that permitted by Sections 107 or 108 of the U.S. Copyright Law. This consent does not extend to other kinds of copying, such as copying for general distribution, for advertising or promotional purposes, for creating new collective works, or for resale. Copy fees for pre-2001 chapters are as shown on the title pages. If no fee code appears on the title page, the copy fee is the same as for current chapters. 0065-2113/01 $35.00 Explicit permission from Academic Press is not required to reproduce a maximum of two figures or tables from an Academic Press chapter in another scientific or research publication provided that the material has not been credited to another source and that full credit to the Academic Press chapter is given. Academic Press A Harcourt Science and Technology Company 525 B Street, Suite 1900, San Diego, California 92101-4495, USA http://www.academicpress.com Academic Press Harcourt Place, 32 Jamestown Road, London NW1 7BY, UK http://www.academicpress.com International Standard Book Number: 0-12-000792-4 PRINTED IN THE UNITED STATES OF AMERICA 01 02 03 04 05 06 SB 9 8 7 6 5 4 3 2 1 Contents CONTRIBUTORS . . . . . . . . . . . . . . . . . . .