The micronutrient research covered in this essay is that concerning soil fertility and plant nutrition. These nutrients, once called minor or trace elements, are copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn), which occur as cations in the soil; and boron (B) and molybdenum (Mo), which occur in anionic form. Chlorine (Cl), another micronutrient, has never been a problem for crop production, other than excesses being detrimental to tobacco quality, and will not be discussed. Also, other research has been conducted on Fe and Mn in soil genesis and morphology, especially by students of S. W. Buol, that will not be covered in this presentation. The following is a chronology of research by micronutrient. Specific citations are not given in the text, but a list of references is appended for the reader's reference.
L. G. Willis, who was with the Agronomy Department from 1925 to 1945, was a soil chemist with an interest in micronutrients. In 1928, he reported manganese deficiencies in oats and soybeans, primarily in spots in the coastal plain. He associated this problem with very high soil pH, and thus this observation was likely the first evidence of "overliming." Although overliming is still noticed today, it may have been more prevalent then as lime was first spread by hand from wagons or piles, and the highest pH was always closest to the spot from which it was spread.
In the late 1950s and through the 1960s, there were a number of investigations on manganese. Adolph Mehlich added some detail on the effect of pH on lime-induced manganese deficiencies. Dr. Mehlich was in the Agronomy Department working in soil chemistry from 1938 to 1970, and in 1953, developed the Mehlich-1 extractant. This extractant has been called the "North Carolina extractant" and the "double acid extractant," and its original publication was by Nelson, Mehlich, and Winters.
During this period, Sanchez and Kamprath also looked at the effects of lime and organic matter on manganese availability. W. L. Rivenbark conducted some excellent soil chemistry research on rates and mechanisms of manganese retention and release in soils.
Also in this period, F. R. Cox worked on a soil test interpretation for manganese. The Soil Testing Division, North Carolina Department of Agriculture (NCDA) would, on request, determine manganese in the Mehlich-1 extract at that time. In 1968, Cox used both extractable manganese and soil pH and developed a yield response prediction and manganese soil test interpretation for soybeans. The yield response prediction was later transformed into an "availability index" by NCDA, and manganese levels have been measured and interpreted routinely since.
In the 1980s, H. J. "Rick" Mascagni conducted a host of field studies on manganese in corn and soybean. He reported on sources of materials, rates of fertilization, plant response, residual effects, and soil test interpretation. His work provides the latest calibration of a manganese availability index, which included both manganese concentration in the soil and soil pH, for the current extractants. At this time the Mehlich- 3 extractant was in use, and Rick's work with manganese was the first direct calibration work for that extractant.
Also during the 1980s, work by G. S. Miner showed how the acidifying effect of starter fertilizer could affect manganese uptake in corn and tobacco. The only recent work on manganese has been that by Mikkelsen who used hydrophilic polymers to improve uptake of manganese.
Manganese has been the most extensively studied micronutrient for several reasons. The soils in the coastal plain are inherently low in manganese, especially the more poorly drained ones, since manganese can be reduced and leached in the soil-forming process. Interveinal chlorosis is a clear symptom of manganese deficiency, and its association with overliming shows a strong association with or dependency on pH. Extractable concentrations at the critical level, which varied from 3 to 9 with Mehlich-1 depending on pH, were sufficient to be measured readily. For these reasons, considerable knowledge has been accumulated on soil reactions, rates of application, sources of materials, residual effects, and soil test interpretation for manganese.
In 1930, when also serving as department head and vice-director of the experiment station, C. B. Williams noted that crops grown on muck soils often responded to copper application. This observation was researched in detail by L. G. Willis who moved his laboratory from the university campus to the research station at Castle Hayne in 1935 to be near the problem. Much of his research centered on the aspect that copper may be a catalyst in oxidation-reduction processes in soils.
Work continued on copper in the 1950s. J. T. Pesek studied the equilibria of copper with various soil colloids and evaluated the results with plant uptake. P. C. Butler determined the complexing of a number of heavy metals, including copper, by organic matter.
In the early 1960s, there was a resurgence of work on copper that included field evaluation. R. P. Patterson evaluated the response of wheat and soybeans to copper on high organic soils, and R. E. Hanes found copper responses on mineral soils as well. S. E. Younts reported on this work and established the rates of copper recommended on soils varying in organic matter for the recommendations that exist today. In the late 1960s, D. W. Eaddy, who later became the Director of the NCDA Agronomic Division, included copper absorption when studying the relation between drainage class and certain chemical properties of organic matter.
It has been difficult to analyze for copper by routine soil testing methods because of such low concentrations extracted. Adolph Mehlich, who later worked with the NCDA Agronomic Division, developed a hydrochloric acid extractant especially for copper in 1975. Later, in 1984, he added a complexing agent to an acetate-based solution to create the Mehlich-3 extractant, a more universal extractant. Although concentrations of copper were still very low, analyses were now more accurate in the Agronomic Division because of better glassware cleansing with an aluminum chloride solution, another of Dr. Mehlich's contributions.
Makarim evaluated the need for copper with several soil extractants, including the Mehlich 3, in 1983, and his work provided the background for the establishment of the current critical levels and soil test interpretation for copper. His work at that time did not include the Modified Olsen, which is a more universal extractant than Mehlich 3 in that it may be used on high pH soils. In 1988, Rohman determined copper, manganese, and zinc critical levels with the Modified Olsen extractant.
Although progress has been made in most aspects of copper nutrition and its role in the fertility of soils, the concentration extracted is still extremely low. Its critical value has been placed at only 0.5 mg/dm3, so the accuracy of copper analysis in the extractant is quite variable.
J. R. Piland, who was with the Department from 1929 to 1968 and was also the first person in charge of the Service Laboratory for the College of Agriculture, was best known for his work with boron. In 1937, he reported how borax improves the seed set of alfalfa. He worked with a number of legumes and reported on his work in the mid-1940s.
Piland's research is responsible for the boron recommendation on alfalfa that continues today, but it also set the stage for further work by C. M. Wilson and A. C. McLung about 1950 on response to boron and on born behavior in soil. In the mid-1950s, T. B. Hutcheson, advised by W. G. Woltz, reported on boron fertilization of flue-cured tobacco, and A. C. McLung reported on boron in relation to foliar and fruiting abnormality of peach.
Further work was done on boron in the mid-1960s, too. F. R. Cox found one form of concealed damage in peanuts, "hollow heart," related to a lack of boron. Boron, a highly mobile anion, is not evaluated in the soil testing process, but Cox's work led to a general recommendation for boron on peanuts.
E. W. Stoller evaluated the possible role of boron in nucleic acid metabolism in peanut. J. F. Luke studied the effects of high rates of boron in the field, one of the first studies on a potential toxic effect for a micronutrient. In the late 1960s, Zeiger and Shelton reported on internal bark necrosis, a symptom of manganese toxicity in apples, and recently Miner evaluated several sources of boron for cotton and soybean.
In 1977, a regional study on diagnosis and correction of zinc deficiency chaired by F. R. Cox was published as Southern Cooperative Series Bulletin 222. This reference provides a comprehensive account of the status of zinc in the soils of the Southeast and serves as a primary reference for zinc soil test evaluation, fertilization, and residual effects for North Carolina. This work—as well as others by Edwards, Junus, Lins, Borkert, and Cox—showed the dependence of zinc availability on soil pH.
The effect pH on zinc availability is not as striking as with manganese, but it is still a factor to be considered in the soil test evaluation. Even though the effect has not been quantified in detail, it is being accounted for in the zinc soil test interpretation given by the Agronomic Division. The Division changes the critical level for each soil class—mineral, mineral-organic and organic—as the optimum pH for each class differs.
Zinc deficiencies were noted early by Willis but have never been a severe problem in the state. Symptoms occur primarily in corn grown after another crop and are not as severe in crops grown after corn. Therefore, locating sites and conducting zinc research difficult.
There has been little research reported on molybdenum in North Carolina, but in the late 1970s and early 1980s, two papers by Karimian evaluated the extractability of molybdenum and a means to predict molybdenum availability. Routine soil test extractants do not remove sufficient molybdenum for analysis, so an oxalate extraction, as for amorphous aluminum and iron, was used. The status of aluminum and iron, and notably the pH, gave an indication of availability. Legumes grown on soils that need lime are now known to be likely to respond to molybdenum.
In the mid- and late-1950s, micronutrient research centered on plant nutrition was conducted. M. E. Harward and his student, D. P. Moore looked at some of the interactions of aluminum, iron and manganese and those of copper, iron and molybdenum. This started a plant nutrition focus in soil science research that continues today.
There were a number of plant physiology papers on micronutrients in the 1970s. Edwards and Kamprath evaluated zinc accumulation by corn as affected by temperature and light. The effect of temperature on manganese accumulation was studied both by Rufty with tobacco and Ghazali with soybean. Cox summarized some of these effects in Southern Cooperative Series Bulletin 281.
In the 1980s, L. D. King had a number of papers on the heavy metal content of municipal and industrial sewage sludge and swine manure lagoon sludge. Some of these materials are quite high in copper and zinc and cause a buildup of the elements in the soil, increasing the threat of toxicity. His student L. M. Hajjar studied the residual effect of sewage sludge on metal content of tobacco and peanut. The several reports by King during this period gave a foundation for later studies on micronutrient accumulation and potential toxicity to the environment.
Research in the 1990s has continued to center on the environmental threat of toxic levels of micronutrients. Fontes published several papers on physiological effects of zinc toxicity and reported a unique symptom of zinc toxicity in soybean. Borkert evaluated the effects of high concentrations of both zinc and copper; he also evaluated a range in pH on several crops grown in the greenhouse. Miner looked at soil factors affecting plant concentrations of these elements in sludge-amended soils. When concentrations of heavy metals are high, knowledge of their solubility becomes important. Hesterberg modeled this chemistry for copper and zinc.
These are some of the highlights of micronutrient research that have occurred over the last 70 years. In this period, we have not only gained a lot of knowledge on the subject but also applied it to the betterment of agriculture and our environment in general. North Carolina has indeed been a leader in micronutrient research.
Baird JV, Cox FR, Eaddy DW. 1973. Micronutrients for North Carolina crops. Raleigh (NC): North Carolina Agricultural Extension Service. Circular 533.
Barnes JS, Cox FR. 1973. Effects of copper sources on wheat and soybeans grown on organic soils. Agron J 65:705-8.
Borkert CM, Cox FR. 1999. Effects of acidity at high soil zinc, copper, and manganese on peanut, rice, and soybean. Commun Soil Sci Plant Anal 30:1371-84.
Borkert CM, Cox FR, Tucker MR. 1998. Zinc and copper toxicity in peanut, soybean, rice, and corn in soil mixtures. Commun Soil Sci Plant Anal 29:2991-3005.
Butler PC. 1957. A study of heavy metal complexing by soil organic matter and its effect on the uptake of heavy metals by plants [dissertation]. Raleigh (NC): North Carolina State University.
Colwell WE. 1943. A biological method for determining the relative boron contents of soils. Soil Sci 56:71-94.
Colwell, WE, Cummings RW. 1944. Chemical and biological studies on aqueous solutions of boric acid and of calcium, sodium, and potassium metaborates. Soil Sci 57:37-50.
Corey RB, King LD, Lue-Hing C, Fanning DS, Street JJ, Walker JM. 1987. Effects of sludge properties on accumulation of trace elements by crops. In: Page AL, et al, editors. Land application of sludge: food chain implications. Chelsea (MI): Lewis Publishers. p 25-51.
Cox FR. 1965. Factors affecting the Mn content and yield of soybeans. NC Soil Sci Soc Proc 8:57-67.
Cox FR. 1968. A new soil test for manganese. Research and Farming 25(12):8.
Cox FR. 1968. Development of a yield response prediction and manganese soil test interpretation for soybeans. Agron J 60:521-524.
Cox FR. 1972. Micronutrient. In: Sanchez PA, editor. A review of soils research in tropical Latin America. Raleigh (NC): North Carolina State University Soil Science Department. p 244-63.
Cox FR. 1973. Micronutrient. In: Un Resumen de las Investigaciones Edafologicas en las America Latina Tropical. Raleigh (NC): North Carolina Agricultural Experiment Station. Technical Bulletin 219. p 199-215.
Cox FR. 1975. Zinc needs for corn in North Carolina. NC Soil Sci Soc Proc 18:37-8.
Cox FR. 1979. Factors that influence the need to apply micronutrients in North Carolina. Raleigh (NC): North Carolina State University Soil Science Department. Soil Science Information Series.
Cox FR. 1979. Methods of applying micronutrients. India/FAO/Norway seminar on micronutrients in agriculture; 1979 Sep 1721; New Delhi (India). [no place]: United Nations Food and Agriculture Organization.
Cox FR. 1979. Micronutrient toxicities. India/FAO/Norway seminar on micronutrients in agriculture; 1979 Sep 1721; New Delhi (India). [no place]: United Nations Food and Agriculture Organization.
Cox FR. 1979. The need for micronutrients in North Carolina. NC Soil Sci Soc Proc 22:31-45.
Cox FR. 1982. Effects of environment on manganese uptake and utilization. In: Anderson OE, Mortvedt JJ, editors. Athens (GA): University of Georgia. p 30-7. (Southern cooperative series; bulletin 281).
Cox FR. 1987. Micronutrient soil tests: Correlation and calibration. In: Brown JR, editor. Soil testing: sampling, correlation, calibration, and interpretation. Madison (WI): Soil Science Society of America. Special Publication 21. p 97-117.
Cox FR. 1990. A note on the effect of soil reaction and zinc concentration on peanut tissue zinc. Peanut Sci 17:15-7.
Cox FR. 1990. Trace elements and crop productivity. Proc Philadelphia Soc Promoting Agric 1989-1990:77-83.
Cox FR. 1992. Residual value of copper fertilization. Commun Soil Sci Plant Anal 23:101-12.
Cox FR, Adams F, Tucker BB. 1982. Liming, fertilization and mineral nutrition. In: Pattee HE, Young CT, editors. Peanut science and technology. Yoakum (TX): American Peanut Research and Education Society. p 139-63.
Cox FR, Gilliam JW. 1971. Adjustment of soil test interpretation for manganese due to elimination of charcoal during extraction. Commun Soil Sci Plant Anal 2:433-8.
Cox FR, Kamprath EJ. 1972. Micronutrient soil tests. In: Mortvedt JJ, editor. Micronutrients in agriculture. Madison (WI): Soil Science Society of America. p 289-317.
Cox FR, Reid PH. 1964. Calcium-boron nutrition as related to concealed damage in peanuts. Agron J 56:173-6.
Cox FR, Reid PH. 1964. Manganese status and needs of the southern region. Plant Food Review 10:17-20.
Cox FR, Reid PH. 1965. Manganese boosts soybean yields. Research and Farming 24(1):6.
Cox FR, Wear JI. 1977. Diagnosis and correction of zinc problems in corn and rice production. Raleigh (NC): North Carolina State University. (Southern cooperative series; bulletin 222).
Eaddy DW. 1968. Relation between drainage class and certain chemical properties of soil organic matter [dissertation]. Raleigh (NC): North Carolina State University.
Edwards JE, Kamprath EJ. 1974. Zinc accumulation by corn seedlings as influenced by phosphorus, temperature, and light intensity. Agron J 66:479-82.
Edwards JH. 1973. Effects of light intensity, temperature, and phosphorus nutrition on uptake and translocation of zinc by corn seedlings [dissertation]. Raleigh (NC): North Carolina State University.
Fontes RLF. 1992. Zinc toxicity in soybean as affected by plant iron and sulfur [dissertation]. Raleigh (NC): North Carolina State University.
Fontes RLF, Cox FR. 1993. Response of soybean plants to toxic levels of Zn in different concentrations of S. In: Sociedade Brasileira de Ciencia do Solo, editor. Cerrados: Froteria Agricola no Seculo XXI. Summary of XXIV Congress of the Brazilian Soil Science Society, Goiania (GO). [no place]: [no publisher]. p 191-2.
Fontes RLF, Cox FR. 1993. Zinc-binding peptides as a function of Zn and S in soybeans. Plant Soil 155/156:435-6.
Fontes RLF, Cox FR. 1995. Effects of sulfur supply on soybean plants exposed to zinc toxicity. J Plant Nutr 18:1893-1906.
Fontes RLF, Cox FR. 1997. Rhythmic movement of leaves in soybean plants grown under Zn toxicity at different doses of Fe and S. R Bras Fisiol Veg 9:131-4.
Fontes RLF, Cox FR. 1998. Iron deficiency and zinc toxicity in soybean grown in nutrient solution with different levels of sulfur. J Plant Nutr 21:1715-22.
Fontes RLF, Cox FR. 1998. Zinc toxicity in soybean grown at high iron concentration in nutrient solution. J Plant Nutr 21:1723-30.
Ghazali NJ, Cox FR. 1981. Effect of temperature on soybean growth and manganese accumulation. Agron J 73:363-7.
Gilliam JW, Cox FR, Reid PH. 1968. Effects of adding charcoal during dilute acid extraction of manganese and iron from soils. Soil Sci Soc Amer Proc 32:511-4.
Guertal EA, Abaye AO, Lippert BM, Miner GS, Gascho GJ. 1996. Sources of boron for foliar fertilization of cotton and soybean. Commun Soil Sci Plant Anal 27:2815-28.
Gutierrez R. 1992. Cadmium, copper and zinc in crops grown on sludge-amended soils [MSc thesis]. Raleigh (NC): North Carolina State University.
Hajjar LM. 1985. The residual effect of sewage sludge on metal content of tobacco and peanuts [MSc thesis]. Raleigh (NC): North Carolina State University.
Hanes RE. 1964. Influence of certain soil properties on the need for copper fertilization in the North Carolina Coastal Plain [MSc thesis]. Raleigh (NC): North Carolina State University.
Harward ME, Jackson WA, Lott WL, Mason DD. 1955. Effects of Al, Fe and Mn upon the growth and composition of lettuce. Proc Am Soc Hort Sci 66:261-6.
Hesterberg D, Bril J, del Castilho P. 1993. Modeling Cd, Cu, and Zn solubilities in an acidic loamy sand soil of the Netherlands. J Environ Qual 22:681-8.
Hutcheson TB Jr, Woltz WG. 1956. Boron in the fertilization of flue-cured tobacco. Raleigh (NC): North Carolina Agricultural Experiment Station. Technical Bulletin 120.
Junus MA. 1984. Incorporation of acidity and cation exchange capacity into the zinc soil test interpretation [dissertation]. Raleigh (NC): North Carolina State University.
Junus MA, Cox FR. 1987. A zinc soil test calibration based upon Mehlich-3 extractable zinc, pH, and cation exchange capacity. Soil Sci Soc Am J 51:678-83.
Kamprath EJ, Collins E, Cox F. 1965. Trace elements in North Carolina. Raleigh (NC): North Carolina Agricultural Extension Service. Circular 455.
Karimian N. 1977. Molybdenum extractability, adsorption, and availability in selected North Carolina soils [dissertation]. Raleigh (NC): North Carolina State University.
Karimian N, Cox FR. 1978. Adsorption and extractability of Mo in relation to some chemical properties of soil. Soil Sci Soc Am J 42:757-61.
Karimian N, Cox FR. 1979. Molybdenum availability as predicted from selected soil chemical properties. Agron J 71:63-5.
King LD. 1981. Effect of swine manure lagoon sludge and municipal sewage sludge on growth, nitrogen recovery, and heavy metal content of fescue grass. J Environ Qual 10:465-72.
King LD. 1986. Agricultural use of municipal and industrial sludges in the southern United States. Raleigh (NC): North Carolina State University. (Southern cooperative series; bulletin 314).
King LD. 1988. Retention of metals by several soils of the southeastern United States. J Environ Qual 17:239-46.
King LD, Hajjar LM. 1990. The residual effect of sewage sludge on heavy metal content of tobacco and peanut. J Environ Qual 19:738-48.
Lindsay WL, Cox FR. 1985. Micronutrient soil testing for the tropics. In: Vlek PLG, editor. Micronutrients in tropical food crop production. [no city] (Netherlands): Martinus Nijhoff/Dr. W. Junk. p 169-200.
Lins IDG. 1987. Improvement of soil test interpretations for phosphorus and zinc [dissertation]. Raleigh (NC): North Carolina State University.
Lins IDG, Cox FR. 1989. Efeito do pH solo e teor de agila sobre a disponsibilidade de zinco para o milho. Bolenimde Pequisa No. 6. Empresa de Pesquisa, Assistencia Technica e Extensao Rural de Mato Grosso do Sul.
Lins IDG, Cox FR. 1989. Effect of soil pH and clay content on the zinc soil test interpretation for corn. Soil Sci Soc Am J 52:1681-5.
Lins IDG, Cox FR, Souza DMG. 1989. Test of a mathematical model to optimize P fertilization for soybean grown on Cerrado soils with different types and contents of clay. R bras Ci Solo 13:65-73.
Lopes AS. 1977. Available water, phosphorus fixation, and zinc levels in Brazilian Cerrado soils in relation to their physical, chemical, and mineralogical properties [dissertation]. Raleigh (NC): North Carolina State University.
Luke JF. 1969. Residual effects of high rates of fertilizer boron on a Norfolk sandy loam [MSc thesis]. Raleigh (NC): North Carolina State University.
Makarim AK. 1981. Critical soil and plant copper levels for wheat, soybeans, and corn [MSc thesis]. Raleigh (NC): North Carolina State University.
Makarim AK, Cox FR. 1983. Evaluation of the need for copper with several soil extractants. Agron J 75:493-6.
Mascagni HJ Jr. 1984. The diagnosis and correction of manganese deficiency in soybeans and corn [dissertation]. Raleigh (NC): North Carolina State University.
Mascagni HJ Jr, Cox FR. 1984. Diagnosis and correction of manganese deficiency in corn. Commun Soil Sci Plant Anal 15:1323-33.
Mascagni HJ Jr, Cox FR. 1985. Calibration of a manganese availability index for soybean soil test data. Soil Sci Soc Am J 49:382-6.
Mascagni HJ Jr, Cox FR. 1985. Critical levels of manganese in soybean leaves at various growth stages. Agron J 77:373-5.
Mascagni HJ Jr, Cox FR. 1985. Effective rates of fertilization for correcting manganese deficiency in soybeans. Agron J 77:363-6.
Mascagni HJ Jr, Cox FR. 1985. Evaluation of inorganic and organic manganese fertilizer sources. Soil Sci Soc Am J 49:458-61.
Mascagni HJ Jr, Cox FR. 1988. Residual effects of manganese fertilization. Soil Sci Soc Am J 52:434-8.
McLung AC, Clayton CN. 1956. Boron in relation to foliar and fruiting abnormality of peach. Plant Dis Report 40:542-8.
McClung AC, Dawson JE. 1951. Some studies on the behavior of soil boron under cropping. Soil Sci Soc Am Proc 15:268-72.
Mehlich A. 1957. Aluminum, iron, and pH in relation to lime induced manganese deficiencies. Soil Sci Soc Am Proc 21:625-8.
Mehlich A. 1978. New extractant for soil test evaluation of phosphorus, potassium, magnesium, calcium, sodium, manganese and zinc. Commun Soil Sci Plant Anal 9:477-92.
Mehlich A. 1984. Mehlich 3 soil test extractant: A modification of Mehlich 2 extractant. Commun Soil Sci Plant Anal 15:1409-16.
Mehlich A, Bowling SS. 1975. Advances in soil test methods for copper by atomic absorption spectrophotometry. Commun Soil Sci Plant Anal 6:113-28.
Mikkelsen RL. 1995. Using hydrophilic polymers to improve uptake of manganese fertilizers by soybeans. Fert Res 41:81-92.
Mikkelsen RL, Camberato JJ. 1995. Use of potassium, sulfur, lime, and micronutrients. In: Rechcigl JE, editor. Environmental aspects of soil amendments and pesticides. Boca Raton (FL): CRC-Lewis Publishing. p 105-32.
Miner GS, Batle EG. 1981. Effects of micronutrients on the control of tobacco mosaic virus. Tobacco Sci 25:22-3.
Miner GS, Galindez AZ, Tucker MR. 1987. Response of flue-cured tobacco to foliar and soil applications of Mn. Tobacco Sci 321:28-31.
Miner GS, Gutierrez R, King LD. 1997. Soil factors affecting plant concentrations of cadmium, copper, and zinc on sludge-amended soils. J Environ Qual 26:989-94.
Miner GS, Taore S, Tucker MR. 1986. Corn response to starter fertilizer acidity and manganese materials varying in water solubility. Agron J 78:291-5.
Moore DP, Harward ME, Mason DD, Hader RJ, Lott WL, Jackson WA. 1957. An investigation of some of the relationships between copper, iron, and molybdenum in the growth and nutrition of lettuce: II. Response surfaces of growth and accumulations of Cu and Fe. Soil Sci Soc Am Proc 21:65-74.
Mortvedt JJ, Cox FR. 1985. Projection, marketing, and use of calcium, magnesium, and micronutrient fertilizers. In: Englestadt OJ, editor. Fertilizer technology and usage. Madison (WI): Soil Science Society of America. p 455-81.
Mortvedt JJ, Cox FR. 1987. Production, marketing, and use of calcium, magnesium, and micronutrient fertilizers. In: Englestad OP, editor. Fertilizer technology and use. Madison (WI): Soil Science Society of America.
Mortvedt JJ, Cox FR, Shuman LM, Welch RM. 1991. Micronutrients in agriculture. 2nd ed. Madison (WI): Soil Science Society of America.
Mortvedt JJ, Mikkelsen RL, Kelsoe JJ. 1992. Crop response to ferrous sulfate in banded gels of hydrophilic polymers. Soil Sci Soc Am J 56:1319-24.
Nelson WL, Mehlich A, Winters E. 1953. The development, evaluation, and use of soil tests for phosphorus availability. In: Pierre WH, Norman AG. editors. Soil and fertilizer phosphorus. New York (NY): Academic Press, Inc. (Agronomy monograph series; 4). p 153-88.
Patterson RP. 1963. The influence of copper and soil pH on the yield and chemical composition of wheat and soybeans growing on soils high in organic matter [MSc thesis]. Raleigh (NC): North Carolina State University.
Pesek JT Jr. 1950. A study of soil colloidcopper ion equilibria and their relations to the absorption of copper by plants [dissertation]. Raleigh (NC): North Carolina State University.
Piland JR, Ireland CF. 1941. Application of borax produces seed set in alfalfa. J Am Soc Agron 33:938-9.
Piland JR, Ireland CF, Reisenauer HM. 1944. The importance of borax in legume seed production in the South. Soil Sci 57:75-84.
Ritchey KD, Cox FR, Galrao EZ, Yost RS. 1986. Disponibilidade de zinco para as culturas do milho, sorgo e soja em latossolo vermelho-escuro argiloso. Pesq agropec bras 21:215-25.
Rivenbark WL. 1961. The rates and mechanisms of manganese retention and release in soils [dissertation]. Raleigh (NC): North Carolina State University.
Rohman PC. 1985. Evaluation of the Modified Olsen extractant for micronutrients in tropical soils. [MSc thesis]. Raleigh (NC): North Carolina State University.
Rohman PC, Cox FR. 1988. Evaluation of the modified Olsen extracting reagent for copper, zinc and manganese. Commun Soil Sci Plant Anal 19:1859-70.
Rufty TW, Miner GS, Raper CD Jr. 1979. Effect of temperature on growth and Mn tolerance of flue-cured tobacco. Agron J 71:638-44.
Sanchez C, Kamprath EJ. 1959. Effect of liming and organic matter content on the availability of native and applied manganese. Soil Sci Soc Am Proc 23:302-4.
Soepardi G. 1972. The effect of root zone temperature and copper rate on wheat growth and micronutrient uptake [dissertation]. Raleigh (NC): North Carolina State University.
Stolle EW. 1965. Effect of boron nutrition on growth and protein and nucleic acid metabolism in peanut plants [dissertation]. Raleigh (NC): North Carolina State University.
White WC, Fitts JW. 1959. Trace elements for plants: their place in North Carolina. Raleigh (NC): North Carolina Agricultural Extension Service. Folder 171.
Williams CB. 1930. Factors influencing the productivity of muck soils. Raleigh (NC): North Carolina Agricultural Experiment Station. Report 53. p 46-47.
Williams CB, Mann HB, Rea JL. 1934. Crop response to lime and fertilizer and muck soil. Raleigh (NC): North Carolina Agricultural Experiment Station. Bulletin 292. p 129.
Willis LG. 1928. Response of oats and soybeans to manganese on some coastal plain soils. North Carolina Agric. Exp. Stn. Bul. 257.
Willis LG. 1937. Evidences of the significance of oxidation-reduction equilibrium in soil fertility problems. Soil Sci Soc Am Proc 1:291-7.
Willis LG. 1937. Some factors influencing the productiveness of highly organic soils in North Carolina. Volume B. Transactions of the 6th Commission Int. Soc Soil Sci 53:406-9.
Willis LG, Piland JR. 1934. The influence of copper sulfate on iron absorption by corn plants. Soil Sci 37:79-83.
Willis LG, Piland JR. 1936. The function of copper in soils and its relation to the availability of iron and manganese. J Agric Res 52:467-76.
Willis LG, Piland JR. 1937. Some recent observations on the use of minor elements in North Carolina agriculture. Soil Sci 44:251-63.
Wilson CM. 1949. The effect of soil treatment on boron uptake by legumes and boron movement in soils [dissertation]. Raleigh (NC): North Carolina State University.
Wilson CM, Lovvorn RL, Woodhouse, WW Jr. 1951. Movement and accumulation of water-soluble boron within the soil profile. Agron J 43:363-7.
Younts SE. 1964. Response of wheat to rates, dates of application, and sources of copper and to other micronutrients. Agron J 56:266-9.
Zeiger DC, Shelton JE. 1969. Seasonal distribution of manganese in leaves and shoots of Red Delicious apples showing Internal Bark Necrosis (IBN). HortScience 4:213-5.