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CHAMP B. TANNER November Id, I920-September 22, 1990 BY WILFORD R. GARDNER CHAMP BEAN TANNER the scientist cannot be separatec! from Champ Tanner the incliviclual. There was a transcen- clent integrity to his personality that permeated everything he clicI. He conic! be blunt, canclicI, en c! forthright, but he was never lacking in compassion en c! concern for students, colleagues, friends, en c! family. To know Champ was to know his inimitable wife, Kay, en c! to become acloptec! into their far-flung extended family as a full-fledged member. Champ was born in Idaho Falls, Idaho, on November 16, 1920, of Mormon pioneer stock. His life exemplifiec! the goal-oriented determination, regardless of physical or fi- nancial impediment, that was characteristic of his forbear- ers. His father, a construction engineer, cliec! as a result of saving a coworker from cirowning in an accident early in Champs life, leaving his wiclowoc! mother to orovicle for Champ en c! his two younger brothers. His mother prover! . . to be a remarkable woman, en c! there was little cloubt that both nature ant! nurture were strong determinants in Champ's life course. She eventually obtainer! a position as professor of English at Brigham Young University when women professors at any institution were rare en c! when their work was never so highly valued as that of male col- leagues. She became a legenciary en c! belovec! mentor and, 377
378 B I O G RA P H I C A L EMOIRS much too late, was honoree! as one of the most outstanding teachers ever to serve on that faculty. Her high stanciarcis en c! appreciation for the English language were not lost on Champ, whose impatience with verbosity in convoluter! writing macle thesis writing a fearec! task among his students. Champ was one of a large en c! talentec! group of young men who were iclentifiec! early in their student careers by Tommy Martin, a seconc! belovec! en c! legenciary figure at Brigham Young University. Tommy not only encouraged his students to go on to graduate school, preferably in soil science, but also manager! to fins! graduate assistantships for most of them at outstanding eastern universities. Champ proved to be one of the best of this distinguished group. Professor Martin was very concernec! that the military ciraft wouIc! require that Champ enter the service en c! thus inter- rupt Champ's education at a critical point. Following a fa- therly interview, he suggestec! that Champ en c! his sweet- heart, Catherine Cox (she is never caller! Catherine, always Katie or Kay), get marries! immecliately. He suggestec! they conic! clo so secretly en c! continue to live in separate clomi- ciles until an appropriate time came to reveal the marriage to their parents. Champ's marital status wouIcI, Martin hoped, help to keep him out of the clutches of the military. As tempting as this suggestion was, it was not immecliately ac- ceptecI. Champ en c! Katie were marries! in the fall of 1941, prior to completion of his studies at Brigham Young Uni- versity in the spring of 1942, without regarc! for any inten- tions of the local ciraft board. It was uncler Professor Martin's influence that the prom- ising freshman chemistry student traveled immediately to North Carolina State University to carry out graduate stud- ies in soil science. Champ's description of his clays at North Carolina is clifficult to recluce to the page. His aciviser was of the school of thought that did not want students to make
CHAMP B. TANNER 379 any move prior to full consultation en c! permission. Champ, who had worked all of his young life to help support the family, was unaccustomed! to sitting arounc! awaiting instruc- tions, especially when such instructions often seemec! trivial en c! pointless. It took less than a year for him to clecicle that military service was not only inevitable but much preferred to his present status. By this time he hac! aireacly enTistec! in the Army Reserve, en c! in the fall of 1942 he was caller! to active duty with the Signal Corps. The training Champ re- ceivec! in the Corps in the rapidity cleveloping fielc! of radio en c! electronics was to serve him well in his later research career. In ~ 944 he was commissioner! an of fleer en c! as signec! cluties as an automotive officer. He was clischargec! in August 1946. Champ was clisincTinec! to return to North Carolina, but, to the goof! fortune of soil science, he aireacly hac! in hanc! an open offer of an assistantship at the University of Wis- consin. One of Champ's classmates at Brigham Young hac! gone to Wisconsin for graduate school. This frienc! was so effusive in his praise of Champ that Emil Truog, the friencl's major professor at Wisconsin, wrote Champ while he was still in the service en c! offerer! him an assistantship, to be taken up at such time as he was free. THE FORMATIVE YEARS Emil Truog was one of that fabler! class of professors of the era known best as benign tyrants. Truog was then chair- man of the Department of Soils. Though his own training at Wisconsin culminates! with the degree of master's of sci- ence in chemistry, by the time of his retirement he hac! mentorec! over 175 graduate students, most of whom took Ph.D. degrees. Truog was an acknowlecigec! giant in the budding field of soil science. He was insightful, demand- ing, creative, opinionated, and compassionate. After his ex
380 B I O G RA P H I C A L EMOIRS perience at North Carolina en c! in the military, the thought of a martinet as a major professor hell! no fears for Champ. Though Truog shower! tremendous concern for all his stu- dents, Champ probably most nearly exemplified Truog's own philosophy about science. One of Champ's favorite Truogisms was: "Here you are swatting at mosquitoes while an elephant is trampling all over you!" Truog also preached that if you wantec! to think clearly you shouIc! "get up with the bircis." Champ went Truog one better, something not easy to clot He was almost always up before the bircis en c! into the office well before daybreak, summer or winter. This habit was initially essential since Champ en c! his now grow- ing family liver! at Badger Village, a student housing com- plex near Baraboo, Wisconsin, some 35 miles from campus. One hac! to be up early to catch the bus to campus, en c! Champ never forsook this early habit. . . During his graduate student days at Wisconsin, Champ en c! Kay clemonstratec! the determination with which they facet! life en c! its adversities. An epidemic of poliomyelitis swept the country in 1950-51 and hit many college commu- nities especially hard. Champ contractor! the virus, en c! he en c! Kay battler! it with both tremendous determination en c! optimism. Champ never fully recovered the use of his stom- ach muscles, but he never allowed! the consequences to cle- ter him from whatever physical task was at hancI. Despite his disability, the Army insisted that his reserve cIassifica- tion status shouIc! remain "Erosion Control Specialist." The military unclerestimatec! Champ's determination en c! even- tually capitulates! to reality en c! a superior force en c! recIas- sifiec! him. Champ completed his Ph.D. degree in soil physics with M. L. Jackson (NAS, 1986) as his scientific mentor and E. E. Miller of the physics department as his adviser in physics en c! lifelong friend. Champ was certain that his illness wouic!
CHAMP B. TANNER 381 prevent completion of his graduate studies. Truog and the department saw it otherwise and assured Champ that his only responsibility was to overcome to the extent possible the illness that had beset him. As a result, Champ devel- oned a, 1 ov~,1 t,v to, t,h ~ .~oi 1.s ~ en~,rt,m en t, an ~ to t,h ~ I In iver.si t,v ~ _ ~ r. · . ~ . · A A ~ ~ . ~ ~ . . ~ . ~ ot Wisconsin that was unwavering. lie telt a cleat that ne could never adequately repay. Nonetheless, over his career, repay it he did, many times over. Champ found as he approached graduation that many institutions were leery of hiring as a faculty member some- one with a possible physical disability. lob offers were few, despite the rapid growth of the field of soil physics. Again, Truog's unerring judgment came to the rescue. Truog had a policy of keeping the best graduate students at Wisconsin as the department built up following World War II. Since Wisconsin at that time was turning out many of the best soils students in the country, this was more than chauvin- ism: it was hard-headed pragmatism. It was almost inevi- table that Truog offer Champ a position at Madison. He was clearly an exceptional individual, and his sense of obli- gation to the university in view of the support given him during his illness made his acceptance of an offer inevi- table. Over the years Champ was to receive many feelers about moving elsewhere, but he never gave any encourage- ment. His loyalty to Wisconsin was unwavering. THE SOIL PHYSICS YEARS Champ's entry into academic life as a faculty member emulated that of his mentor, Truog. When his first gradu- ate student, R. I. Hanks, could not find housing in Madi- son, Champ put the necessary plumbing in an upstairs bed- room and invited John and his wife to stay with them. While . · · . . . · · ~ later students were able to find their own housing, the pat- tern was set. It was a rare visiting scientist in Madison who
382 B I O G RA P H I C A L EMOIRS was not invites! to stay with Champ en c! Kay. Once familiar with their hospitality and Kay's cuisine, they rarely declined subsequent invitations. They clic! this with the full knowI- ecige that, unless they hac! their own transportation from MicicIleton to campus, they would have to rise no later than 5:00 a.m. Even then, they wouIc! fins! Champ waiting for them so he conic! start his clay. Champ's first work in soil physics was along rather cIassi- cal lines. His first clozen or so papers were clevotec! mainly to improver! methods for the characterization of the physi- cal properties of soils en c! soil materials. He clemonstratec! early his flair for improving experimental equipment and techniques to which he turner! his attention. He clevelopec! improver! methods for measuring water retention by soil en c! for measuring particle size distribution, air-fi~lec! po- rosity, and permeability. This was a time during which the field of soil physics was exploding rapidly, with many uni- versities cleveloping teaching en c! research programs in this area. While the funciamental physical concepts were in place, experimental techniques for both laboratory en c! field! were generally crucle en c! imprecise. Champ macle significant improvements in every technique he addressed but, more importantly, lair! the foundation for his keen unclerstancI- ing of the physics of soil systems. THE MICROMETEOROLOGY PHASE It was not until he turned his attention to the energy budget of soils, however, that Champ truly shower! his tal- ent for originality in experimentation while focusing on the most basic problems at hancI. The work by Penman at the Rothamstec! Experiment Station in Englanc! hac! lair! the theoretical basis for the unclerstancling of evaporation from crops and soils. Champ was among the leaders of an ever-growing number of researchers attracted to this area
CHAMP B. TANNER 383 of work. A rich collaboration was begun with his colleague, Verner Suomi, en c! a progression of outstanding students as they began to explore the rapidly expanding area of evapo- ration en c! transpiration from plants en c! soil as part of the larger effort on the earth's energy buciget. In characteristic fashion, Champ first examiner! critically the methoclology en c! instrumentation user! in the fielcI. This pattern was to be repeater! several times in his career. He wouIc! almost invariably fins! ways to improve the preci- sion en c! reliability of a measurement. He emphasizec! to his students in the strongest possible terms that instruments hac! to be "kept honest" or they wouIc! give the researcher misTeacling or incorrect results. Manufacturers' calibration curves were never to be trustee! en c! were always to be veri- fiec! or corrected. The amount of water user! by crops hac! become a very controversial issue by this time. It hac! be- come well recognizec! that glass-house measurements clic! not cluplicate external conditions acloquately en c! that only field! measurements were meaningful. Most ciata available were inferred from soil water content measurements. For many reasons such measurements lack precision and, even worse, do not account for drainage from the soil profile. Direct measurements offerer! the best hope of resolving the issues. Over the next clecacle almost every known or pro- posec! experimental technique was investigated. The ratio of vapor flux to heat flux above a plant canopy is a critical quantity in many theories, en c! much effort arounc! the worIc! was focuses! on these flux measurements. Champ was one of the leaders in this effort. Stomatal conductance mea- surements were improved. Net racliation measurements above crops en c! bare soils were aciciressec! en c! improved. In a highly active area of research, Champ's efforts often went beyonc! those of most colleagues. He clesignec! en c! built two very precise weighing Tysimeters. One was a cylindrical
384 B I O G RA P H I C A L EMOIRS metal tank about 5 meters in diameter, en c! the other was a rectangular tank, about 2 by 3 meters. Both were over about a meter creep en c! were frolic! with soil packet! to simulate a soil profile. A suction drainage system combiner! with a very precise weighing mechanism permitted measurements of evapotranspiration over periods of time as brief as 15 minutes. All the more remarkable was the fact that Champ kept these installations functioning for over fifteen years, despite the problems of winterizing the equipment to en- sure survival through the bitter Wisconsin winters. As a legacy of his polio, pleurisy was a constant threat as he worker! unclergrounc! beneath the lysimeters. Nevertheless, Champ always gave every cletaiT his personal attention. On the other enc! of the measurement scale was the ecicly correlation method, in which the heat content of incliviclual wine! eciclies is correlatec! with the movement of incliviclual eciclies. This requires high-speec! wine! velocity en c! thermal measurements. Virtually every aspect of evaporation en c! transpiration receiver! the Champ Tanner touch, en c! a large cacire of well-trainee! students began to be gracluatecI. Champ was a leader in setting up a joint program with a number of Midwestern universities to provicle fielc! instrumentation en c! experiments for biologists. By 1965, workers in the field had worked out the general physics of water Toss from cropped surfaces and were begin- ning to explore some of the more esoteric issues. Champ felt that he hac! pusher! the problems of transport in the Tower atmosphere about as far as he could. There were many unsolved problems, but the complexity of the plant canopy convincer! him that something more than straight- forwarc! transport equations wouic! be requires! to clear with this situation. Simply coupling the stomata! resistance with a canopy resistance term worker! remarkably well in many cases, but he found it a very unsatisfactory approach.
CHAMP B. TANNER 385 During the 1960s en c! early 1970s Champ was one of the major ciriving forces behinc! an exciting experiment in co- operative research between the University of Wisconsin en c! the Agricultural Research Service (ARS) of the U.S. De- partment of Agriculture. This joint effort resultec! in the establishment of the Hydrology Research Group, staffed by Champ, E. E. Miller, G. W. Thurtell, en c! W. R. Gardner from the university en c! P. A. C. Raats, C. Dirksen, en c! R. Amerman from the USDA. This assembly of scholars at- tractec! an outstanding group of visiting scholars, postclocs, en c! graduate students. Seminars were not to be missecI, as almost every facet of any subject of interest to any partici- pant conic! leac! to stimulating en c! enlightening debates. The entire group was singularly productive. Though it was an unquestionable scientific en c! eclucational success, it was · . · . . . too fragile to survive tne mlnctless ranctom motion cnarac- teristic of the Washington bureaucracy, en c! cluring one of many reorganizations of the ARS it was simply clissolvecI. THE PLANT PHYSIOLOGY PHASE Partly because it was not clear how to push the transport problems forwarc! en c! partly as a result of an extremely stimulating sabbatical spent with John Passioura in Austra- lia, Champ turner! his attention from the plant environ- ment to the response of the plant to its environment. Once again, he started with the literature, reading critically virtu- ally every paper publisher! in English on plant-water rela- tions, making notes as though he were reviewing them for publication. Within a few months it wouIc! be hare! to argue that any plant scientist hac! as thorough a knowlecige of the literature of plant response to water stress en c! of the weak- nesses in the experiments as clic! Champ. No physical mea- surement was ever too clifficult for Champ to attempt, en c!
386 B I O G RA P H I C A L EMOIRS he was soon into the business of buiTcling thermocouple psychrometer en c! plant pressure chambers. While Champ lover! unclerstancling physical systems for unclerstancling's sake, he always hac! a pragmatic enc! in view. His work with the lysimeters resultec! in ciata clesignec! to improve irrigation efficiency in order to recluce the leach- ing of fertilizer to grounc! water. Not content to work with easy plants, Champ chose to work with potatoes, an impor- tant crop in the central Wisconsin sane! plains. He clic! not stop at measuring the water status of the plant leaves but set himself the task of observing clirectly the turgor of the potato tubers. Loss of turgor at a critical perioc! coup! re- sult in misshapen en c! less valuable potatoes. Measuring tu- ber elongation in situ clic! not ciaunt Champ, despite the new! for minimal disturbance. The task of observing with precision minute ciroplets of exudate forcer! out of the tu- bers in laboratory pressure chambers in order to measure their turgor was approaches! with confidence. With the en- couragement en c! acivice of Arthur Kelman, Champ attacker! the question of the relation between the plant water status or the water status of the tuber and certain tuber diseases. Champ also stucliec! the water relations of alfalfa, another clifficult plant structure with which to clear. One of his fa- vorite experiments clealt with the effect of direct solar ra- cliation on onion umbels cluring flowering. This problem appealed to him very much because, geometrically, it was a sphere sitting atop a cylincler. Where else in the plant king- clom conic! one fins! an experimental arrangement so con- clucive to simulation? One of those simulations consistec! of a Styrofoam sphere coverer! with different densities of se- quins, in order to achieve variable roughness. He en c! his students fount! the actual heat transfer from the onion um- bel to the atmosphere to be greater than the theoretical, but, more importantly, they showed that the "sun scald"
CHAMP B. TANNER 387 observer! in the onion seec! fielcis in Idaho resultec! when solar racliation en c! wine! were from the same direction. This project brought Champ almost as much fun as his favorite project on heat transfer. Always open to challenging prob- lems whatever their context, he colIaboratec! with colleagues from animal science to work out the temperature clistribu- tion en c! evaporative heat exchange in the scrotal system of the boar. Uncler pressure, he wouIc! confess that his ap- proach was less "hancis-on" than was normal for him. Champ was also a keen student of science. F. H. King hac! hell! the first chair of agricultural physics in the Uniter! States, en c! Champ was fully conversant with all his work en c! felt a strong kinship with King. The two careers spanner! a century of soil physics in the Uniter! States, en c! between them there were few important problems in the field! that they clic! not aciciress en c! clic! not bring more physical sci ence to. Champ hac! a unique ability to synthesize information from an extremely diverse set of experiences, theories, specu- lations, en c! observations. His career contributions are prob- ably best summer! up in the 1983 review paper with Sinclair. In simplistic terms they showed that the production of total dry matter by a plant was directly proportional to the water transpired and inversely proportional to a mean saturation cleficit of the atmosphere. While C3, C4, en c! CAM plants all clifferecI, their transpiration efficiency, countless genera- tions of plan t bre ecling, aciverten t or in aciverten i, hac! serve c! to change these efficiencies hardily at all. While this over- simplifies the actual situation, the conclusions pointer! out clearly the directions that future research must take, if the relation between crop water use en c! crop growth was to be alterec! in clesirec! directions. The heatec! controversy that hac! characterizec! soil physics en c! crop physiology for cle- cacles was now resolvecI.
388 B I O G RA P H I C A L THE FINAL YEARS EMOIRS Champ was honoree! with a namer! professorship at Wis- consin, appropriately namer! after Emil Truog. He also re- ceivec! the Soil Science Research Awarc! from the Soil Sci- ence Society of America en c! the Outstanding Achievement in Biometeorology Awarc! from the American Meteorology Society. At one time he was an associate editor of journals in three distinct clisciplines. He often expressed the com- plaint that editors seemec! to sent! him only the most cliffi- cult papers to review. He was quite correct in this. If an editor hac! a paper that was certain to rouse the ire of an important en c! contentious scientist, it was a sure bet that Champ wouic! get it for review. His work hours were legenciary. His stanciarcis of science en c! personal integrity were almost unrealistically high. His willingness to debate politics with even the most ardent partisan, coupled with the unfailing generosity and hospi- tality of the Tanner home, meant that an evening at the Tanner home was a never-to-be-forgotten experience. The stories his students now pass on to their students may sound apocryphal to those who clic! not know Champ. But it was impossible to exaggerate where Champ was concerned. He was entirely without guile en c! what you saw was what you got. The Tanners' youngest son, Clarke, a gifted pianist with a promising career aheac! of him, cliec! of leukemia just before he was to accept a music scholarship at Milton College. Despite such heartaches en c! his own physical limi- tations, Champ never lost his zest for life and learning that buoyed! up all those who knew him. At a time in his life when he might well have follower! the tradition of many of his colleagues en c! starter! slowing clown en c! enjoying the fruits of his labors, Champ remainec! en- tirely true to his character. He was elected to the National
CHAMP B. TANNER 389 Academy of Sciences in 1981, the first such soil scientist thus recognized. He took this not so much as a well-cle- servec! honor but as a call to cluty. He worker! conscien- tiously to seek out en c! nominate others deserving of recog- nition. He acceptec! appointment to the Bo arc! on Agriculture of the National Research Council en c! playoc! a very active role on the board. Finally, although he cletestec! paperwork with great fervor, his loyalty to his campus en c! his clepart- ment compelled him to accept the chairmanship of the soils department. He undertook this assignment in the only way he knew how, with thoroughness, candor, en c! selfless- ness. A series of key retirements threatened to tarnish the luster of what hac! been one of the top such departments in the world. Champ set about a vigorous effort to obtain po- sitions en c! fill them with the best scientists available. At the same time he continues! working with his students. Champ found great satisfaction in working with his old- est son, Bert, who, trainee! in geophysics, eventually enterer! the private sector with a small, creative company producing ciata logging en c! processing systems. His micicile son, Myron, trainee! in hydrology, also clirectec! his talents to the private sector. The Tanners hac! two daughters, Taffy en c! Terri, whose own careers have demonstrates! that they inheritec! both the capabilities en c! the stanciarcis of their parents. Both of Champ's brothers, now deceased, were talented engineers. On the occasion of his retirement, Champ's colleagues honoree! him with a Symposium on Biophysical Measure- ments en c! Instrumentation at the annual meeting of the American Society of Agronomy in November ~ 988. Selectec! papers from the symposium were printer! in the journal Theoretical and Applied Climatology (vol. 42, 1990~. Despite the knowledge that his pancreatic cancer was almost cer- tain to prove fatal, Champ maintainer! his work schedule to
390 BIOGRAPHICAL MEMOIRS the limit of his physical ability en c! clic! as much as he conic! to put his personal en c! professional affairs in order. His life's work incluclec! some 150 technical articles, book chap- ters, en c! reports, as well as more than three clozen theses supervised.
CHAMP B. TANNER SELECTED BIBLIOGRAPHY 1956 391 With V. E. Suomi. Lithium chloride dewcel properties and use for dewpoint and vapor-pressure gradient measurements. Trans. Am. Geophys. Union 37:413-20. 1958 With V. E. Suomi. A max-min dewpoint hygrometer. Trans. Am. Geophys. Union 39:63-66. With V. E. Suomi. Evaporation estimates from heat budget mea- surements over a field crop. Trans. Am. Geophys. Union 39:298- 304. 1960 With W. L. Pelton and K. M. King. An evaluation of Thornthwaite and mean temperature methods for determining potential evapo- transpiration. Agron. f. 52:387-95. With W. L. Pelton. Potential evapotranspiration estimates by the approximate energy balance method of Penman. 7. Geophys. Res. 65:3391-3413. With J. A. Businger and P. M. Kuhn. The economical net radiom- eter. 7. Geophys. Res. 65: 3657-67. 1961 A simple aero-heat budget method for determining daily evapo- transpiration. Transactions of the 7th International Congress on Soil Science, vol. 1, pp. 203-9. 1962 With E. R. Lemon. Radiant energy utilized in evapotranspiration. Agron. J. 54:207-12. 1963 Plant temperatures. Agron. J. 55:210-11.
392 B I O G RA P H I C A L 1966 EMOIRS With C. A. Federer. The spectral distribution of light in the forest. Ecology 47:555-60. With C. A. Federer. Sensors for measuring light available for photo- synthesis. Ecology 47:654-57. With M. Fuchs. Infrared thermometry of vegetation. Agron. f. 58:597- 601. 1967 With D. H. Sargeant. A simple psychrometric apparatus for Bowen ratio measurements. 7. Appl. Meteorol. 6:414-18. 1968 With M. Fuchs. Evaporation from unsaturated surfaces: a general- ized combination method. 7. Geophys. Res. 73:1299-1304. With T. A. Black and G. W. Thurtell. Hydraulic load cell lysimeter construction, calibration, and tests. Soil Sci. Soc. Am. Proc. 32:623- 29. 1969 With M. Fuchs, G. W. Thurtell, and T. A. Black. Evaporation from drying surfaces by the combination method. Agron. f. 61:22-26. With E. T. Kanemasu and G. W. Thurtell. The design, calibration, and field use of a stomata! diffusion porometer. Plant Physiol. 44:881-85. With J. M. Norman and G. W. Thurtell. Photosynthetic light sensor for measurement in plant canopies. Agron. f. 61:840-43. With E. T. Kanemasu. Stomatal diffusion resistance of snap beans. 1. Influence of leaf-water potential. Plant Physiol. 44: 1542-46. With S. M. Goltz, G. W. Thurtell, and F. E. Jones. Evaporation mea- surements by an eddy correlation method. 7. Water Resour. Res. 6:440-46. 1970 With G. W. Thurtell and M. L. Wesely. Three-dimensional pressure- sphere anemometer system. J. Appl. Meteorol. 9:379-85.
CHAMP B. TANNER 1972 393 With M. L. Wesely and G. W. Thurtell. An improved pressure-sphere anemometer. Boundary-Layer Meteorol. 2:275-83. 1975 With P. W. Gandar. Comparison of methods for measuring leaf and tuber water potentials in potato. Am. Potato f. 52:387-97. 1976 With P. W. Gandar. Leaf growth, tuber growth, and water potential in potatoes. Crop Sci. 16:534-38. 1983 With T. R. Sinclair. Efficient water use in crop production: research or re-search? In Limitations to Efficient Water Use in Crop Produc- tion, ed. H. M. Taylor, H. R. Jordan, and T. R. Sinclair, pp. 1-28. Madison, Wisc.: American Society of Agronomy.