Dennis Robert Hoagland
Dennis Robert Hoagland (April 2, 1884 – September 5, 1949) was an American chemist and plant scientist working in the fields of plant nutrition, agricultural chemistry, and physiology. He was Professor of Plant Nutrition at the University of California at Berkeley from 1927 until his death in 1949. He is commonly known for his pioneering work on water culture and the development of the Hoagland solution. Biography. Private life. Dennis Hoagland was the son of Charles Breckinridge Hoagland (1859 – 1934) and Lillian May Hoagland (1863 – 1951). He spent his first eight years in Golden and during his later  childhood he lived in Denver. He attended the Denver public schools and in 1903 entered Stanford University. In 1920, Dennis R. Hoagland married Jessie A. Smiley. She died suddenly of pneumonia in 1933. He was left with the responsibility of bringing up three young boys named Robert Charles, Albert Smiley, and Charles Rightmire. Career. Hoagland graduated from Stanford University (1907) with a major in chemistry. In 1908 he became an instructor and assistant in the Laboratory of Animal Nutrition at the University of California at Berkeley, an institution with which he would be associated for the remainder of his life . He worked in the fields of animal nutrition and biochemistry. In 1910 he was appointed assistant chemist in the Food and Drug Administration of the U.S. Department of Agriculture until 1912, when he entered the graduate school in the department of agricultural chemistry with Elmer McCollum at the University of Wisconsin, receiving his master's degree in 1913. In the fall of that year he became assistant professor of agricultural chemistry and in 1922 associate professor of plant nutrition at Berkeley. Work. Brief overview. During the World War I, Hoagland tried to substitute the lack of imports of potassium-based  fertilizers from the German Empire to the United States with plant extracts from brown algae, inspired by the ability of kelps to absorb elements from seawater selectively and to accumulate potassium and iodide many times in excess of the concentrations found in seawater (Hoagland, 1915). Based on these findings he investigated the ability of plants to absorb salts against a concentration gradient and discovered the dependence of nutrient absorption and translocation on metabolic energy using innovative model systems under controlled conditions (Hoagland, Hibbard, and Davis, 1926). During his systematic research and inspired by a principle of Julius von Sachs, he  developed the basic formula for the Hoagland solution, whose composition was originally patterned after the displaced soil solution obtained from certain soils of high productivity (Hoagland, 1919), and established the essentiality of molybdenum for the growth of tomato plants (Arnon and Hoagland, 1940; Hoagland, 1945). Hoagland was able to show that various plant diseases are caused by a lack of trace elements such as zinc (Hoagland, Chandler, and Hibbard, 1931, ff.), and that boron, manganese, zinc, and copper are indispensable for plant growth (Hoagland, 1937). He took special interest in soil-plant interrelationships addressing, for example, the physiological balance of  soil solutions and the pH dependence of plant growth, in order to gain a better understanding on the availability of plant nutrients in soils and solutions (Hoagland, 1916, 1917, 1920, 1922; Hoagland and Arnon, 1941). Hoagland and his associates, including his research assistant William Z. Hassid, thus contributed to the understanding of fundamental cellular physiological processes in green plants that are driven by sunlight as the ultimate form of energy (Hoagland and Davis, 1929; Hoagland and Steward, 1939, 1940; Hoagland, 1944, 1946). Hoagland's solution. The term "Hoagland solution" was first mentioned by Olof Arrhenius in 1922 with reference to  a Hoagland publication of 1919. The composition of the solution published by Hoagland in 1920 was applied to investigate plant growth parameters of barley in comparison with Shive's solution. The growth of Alfalfa in a modified Hoagland solution was investigated at various pH values in the 1920s. Around the 1930s Hoagland investigated diseases of certain plants, and thereby, observed symptoms related to existing soil conditions. In this context, he undertook water culture experiments with the hope of delivering similar symptoms under controlled conditions. For these experiments the Hoagland solution (0) including macronutrients, iron, and the "A-Z solutions a  and b" or the trace element supplementary solution, respectively, was developed to investigate diseases of the strawberry in California. The Hoagland solution (0) described by Hoagland and Snyder (1933) has been modified several times over time, for example, by Hoagland and Arnon (1938, 1950). These modified solutions are now commonly referred to as the "Hoagland solution". Hoagland's research was supported by the plant pathologists H. E. Thomas and W. C. Snyder, and influenced by another pioneer of plant nutrition and hydroculture, William Frederick Gericke. Gericke's groundbreaking results in this field inspired him to expand his research on the  subject finally resulting in the Hoagland solutions (1) and (2). The composition of macronutrients of the Hoagland solutions (0) and (1) can be traced back to Wilhelm Knop's four-salt mixture (cf. Table (1)) and the respective salt and element concentrations to Dennis Hoagland (cf. Table (2)). Knop's solution, in contrast to Hoagland's solution, was not supplemented with trace elements (micronutrients), with the exception of iron, because the chemicals were not particularly pure in Wilhelm Knop's day. Micronutrients were, without knowing it, already present as impurities in the macronutrient salts. More highly purified chemicals and more  sensitive methods for analysing trace concentrations were developed from 1930 and onwards. Table (1). Knop's four-salt mixture (1865) Table (2). Concentration of macronutrients in Hoagland's solution (0, 1, 2) and in Knop's solution Hoagland's students included Daniel Israel Arnon who modified the composition of macronutrients of the Hoagland solution (2) (cf. Table 2) and the concentration of micronutrients (B, Mn, Zn, Cu, and Mo) of the Hoagland solutions (1) and (2) (cf. Table (3)) as a result of joint efforts, and Folke Karl Skoog. In contrast to the Murashige and Skoog medium, neither vitamins nor  other organic compounds are provided as additives for the Hoagland solution, but only essential minerals as ingredients. It is concluded that the promotion of growth of tobacco callus cultured on White's modified medium is due mainly to inorganic rather than organic constituents in aqueous tobacco leaf extracts added. Table (3). Concentration of essential micronutrients in Hoagland's solution (0, 1, 2) In addition, 9 µM ferric tartrate (CHFeO) is added to the Hoagland solution formulations (0, 1, 2), corresponding to a concentration of 18 µmol/L Fe. Solution (2) contains ammonium and nitrate salts and may sometimes be preferred  to solution (1) because the ammonium ion delays the development of undesirable alkalinity (Hoagland and Arnon, 1950) (cf. Table 2). Awards and honors. Hoagland became a Fellow of the American Association for the Advancement of Science (AAAS) in 1916 and member of the National Academy of Sciences in 1934. In recognition of his many discoveries, the American Society of Plant Physiologists elected Dennis Hoagland as president and awarded him the first Stephen Hales Prize in 1929. In 1940, together with Daniel I. Arnon, he received the AAAS Newcomb Cleveland Prize for the work "Availability of Nutrients with Special Reference to  Physiological Aspects". In 1944 he published his "Lectures on the Inorganic Nutrition of Plants" subtitled "Prather Lectures at Harvard University" which he was invited in 1942 to give at Harvard University. In 1945 he was elected member of the American Academy of Arts and Sciences. The "Dennis R. Hoagland Award", first presented by the American Society of Plant Biologists in 1985, and "Hoagland Hall", which is home to the Atmospheric Science program as well as the Environmental Health and Safety office at the UC Davis, are named in his honor. Perception. Nowadays the most common solutions for plant nutrition and  plant tissue cultivation are the formulations from Hoagland and Arnon (1938, 1950), and Murashige and Skoog (1962). Their basic formulas are being replicated by modern manufacturers to produce liquid concentrated fertilizers for plant breeders and average consumers. Even the names of Hoagland and Murashige and Skoog are used as a brand for innovative products, e.g., "Hoagland's No. 2 Basal Salt Mixture" or "Murashige and Skoog Basal Salt Mixture", which are commonly used as standard chemicals in plant science. In fact, several nutrient recipes refer to a standard name although they have little to do with the original formula. As  described by Hewitt several recipes have been published under the name of "Hoagland", and to this day confusion may arise from a loss of memory about the original composition. Dennis Hoagland was considered a leading scientist in his field and his research merit was to initiate the solution named after him, thereby, creating the basis for a modern balanced plant nutrition that is still valid today. His findings are relevant to the sustainable use of natural resources such as soil, water and air, water and nutrient use efficiency in crop production and the production of healthy plant foods. The Hoagland  solution was also used for plant production experiments on the International Space Station. Dennis Hoagland was posthumously characterized as a man of judgment, of tolerance, and of discernment, one who abhorred hypocrisy and admired honesty, and of the quality out of which great human structures are built. Hoagland's fundamental contributions and widely cited publications are of historical relevance for plant nutrition research, which is reflected in the following bibliography. Bibliography. The Determination of Aluminum in Feces. With C. L. A. Schmidt. J. Biol. Chem., 11(4) :387-391. Studies of the Endogenous Metabolism of the Pig as Modified by Various  Factors. (I.-III.). With E. V. McCollum. J. Biol. Chem., 16(3) :299-315, 317-320, 321-325. The Destructive Distillation of Pacific Coast Kelps. J. Ind. Eng. Chem., 7(8) :673-676. Organic Constituents of Pacific Coast Kelps. J. Agr. Res., 4(1) :39-58. The Complex Carbohydrates and Forms of Sulphur in Marine Algae of the Pacific Coast. With L. L. Lieb. J. Biol. Chem., 23(1) :287-297. Acidity and Adsorption in Soils as Measured by the Hydrogen Electrode. With L. T. Sharp. J. Agr. Res., 7 :123-145. The Effect of Hydrogen and Hydroxyl Ion Concentration on the Growth of Barley  Seedlings. Soil Sci., 3(6) :547-560. Relation of Carbon Dioxide to Soil Reaction as Measured by the Hydrogen Electrode. With L. T. Sharp. J. Agr. Res., 12(3) :139-148. The Freezing-Point Method as an Index of Variations in the Soil Solution Due to Season and Crop Growth. J. Agr. Res., 12(6) :369-395. The Chemical Effects of CaO and CaCO on the Soil. Part I. The Effect on Soil Reaction. With A. W. Christie. Soil Sci., 5(5) :379-382. The Relation of the Plant to the Reaction of the Nutrient Solution. Science, 48(1243) :422-425. Notes on Recent Work  Concerning Acid Soils. With L. T. Sharp. Soil Sci. 7(3) :197-200. Note on the Technique of Solution Culture Experiments with Plants. Science, 49(1267) :360-362. The Effect of Certain Aluminum Compounds on the Metabolism of Man. With C. L. A. Schmidt. Univ. Calif. Pub. Path., 2(20) :215-244. Table of pH, H , and OH Values; Corresponding to Electromotive Forces Determined in Hydrogen Electrode Measurements, with a Bibliography. With C. L. A. Schmidt. Univ. Calif. Pub. Phys., 5(4): 23-69. Relation of Nutrient Solution to Composition and Reaction of Cell Sap of Barley. Bot. Gaz., 68(4) :297-304. Relation of  the Concentration and Reaction of the Nutrient Medium to the Growth and Absorption of the Plant. J. Agr. Res., 18(2) :73-117. The Effect of Several Types of Irrigation Water on the pH Value and Freezing Point Depression of Various Types of Soils. With A. W. Christie. Univ. Calif. Pub. Agr. Sci., 4(6) :141-158. Optimum Nutrient Solutions for Plants. Science, 52(1354) :562-564. Effect of Season and Crop Growth on the Physical State of the Soil. With J. C. Martin. J. Agr. Res., 20(5) :396-4O3. Relation of the Soil Solution to the Soil Extract. With J. C. Martin  and G. R. Stewart. J. Agr. Res., 20(5) :381-395. The Soil Solution in Relation to the Plant. Trans. Far. Soc., 17(2) :249-254. Soil Analysis and Soil and Plant Interrelations. Calif. Agr. Exp. Sta. Cir., 235 :1-8. Soil Analysis and Soil and Plant Interrelations. Citrus Leaves, 2(6) :1-2, 16-17. The Feeding Power of Plants. With A. R. Davis and C. B. Lipman. Science, 57(1471) :299-301. The Composition of the Cell Sap of the Plant in Relation to the Absorption of Ions. With A. R. Davis. J. Gen. Phys., 5(5) :629-646. Effect of Salt on  the Intake of Inorganic Elements and on the Buffer System of the Plant. With J. C. Martin. Calif. Agr. Exp. Sta. Tech. P., 8 :1-26. Further Experiments on the Absorption of Ions by Plants, Including Observations on the Effect of Light. With A. R. Davis. J. Gen. Phys., 6(1) :47-62. The Absorption of Ions by Plants. Soil Sci., 16(4) :225-246. A Comparison of Sand and Solution Cultures with Soils as Media for Plant Growth. With J. C. Martin. Soil Sci., 16(5) :367-388. The Effect of the Plant on the Reaction of the Culture Solution. Calif. Agr . Exp. Sta. Tech. P., 12 :1-16. The Electrical Charge on a Clay Colloid as Influenced by Hydrogen-Ion Concentration and by Different Salts. With W. C. Dayhuff. Soil Sci., 18(5) :401-408. Suggestions Concerning the Absorption of Ions by Plants. With A. R. Davis. The New Phytologist, 24(2) :99-111. Physiological Aspects of Soil Solution Investigations. Calif. Agr. Exp. Sta. Hilg., 1(11) :227-257. Some Phases of the Inorganic Nutrition of Plants in Relation to the Soil Solution: 1. The Growth of Plants in Artificial Culture Media. Sci. Agr., 6(5) :141-151. Some Phases of the Inorganic Nutrition of  Plants in Relation to the Soil Solution: 2. Soil Solutions as Media for Plant Growth. Sci. Agr., 6(6) :177-189. Effect of Certain Alkali Salts on Growth of Plants. With J. S. Burd and A. R. Davis. (20) Abstract. Nature and Promise of Soil Solution. (21) Abstract of Papers Read Before Pan-Pacific Scientific Congress, Australia. The Influence of Light, Temperature and Other Conditions on the Ability of Nitella Cells to Concentrate Halogens in the Cell Sap. With P. L. Hibbard and A. R. Davis. J. Gen. Phys., 10(1) :121-146. The Investigation of the Soil from the Point of  View of the Physiology of the Plant. 4th Int. Conf. Soil Sci. Rome, 1924, 3 :535-544. The Synthesis of Vitamin E by Plants Grown in Culture Solutions. With H. M. Evans. Am. J. Phys., 80(3) :702-704. Recent Experiments Concerning the Adequacy of Artificial Culture Solutions and of Soil Solutions for the Growth of Different Types of Plants. With J. C. Martin. Proceedings and Papers of the First Int. Cong. Soil Sci., 3 :1-12. Resume of Recent Soil Investigations at the University of California. Mo. Bull. Calif. Dept. Agr., 16(11) :562-568. First International Congress of Soil Science , Fourth Commission, Soil Fertility. (Summary.) Soil Sci., 25(1) :45-50. The Influence of One Ion on the Accumulation of Another by Plant Cells with Special Reference to Experiments with Nitella. With A. R. Davis and P. L. Hibbard. Plant Phys., 3(4) :473-486. An Apparatus for the Growth of Plants in Controlled Environment. With A. R. Davis. Plant Phys., 3(3) :277-292. Minimum Potassium Level Required by Tomato Plants Grown in Water Cultures. With E. S. Johnston. Soil Sci., 27(2) :89-109. The Intake and Accumulation of Electrolytes by Plant Cells. With A. R. Davis. Protoplasma, 6(4) :610-626. Fertilizer  Problems and Analysis of Soils in California. Calif. Agr. Exp. Sta. Cir., 317 :1-16. Accumulation of Mineral Elements by Plant Cells. Contrib. Marine Biol., pp.  131–144. Recent Advances in Plant Physiology. Ecology, 11(4) :785-786. Little-Leaf or Rosette in Fruit Trees, I. With W. H. Chandler and P. L. Hibbard. Proc. Am. Soc. Hort. Sci., 28 :556-560. Absorption of Mineral Elements by Plants in Relation to Soil Problems. Plant Phys., 6(3) :373-388. Little-Leaf or Rosette of Fruit Trees, II: Effect of Zinc and Other Treatments. With W. H. Chandler and P. L. Hibbard. Proc. Am . Soc. Hort. Sci., 29 :255-263. Mineral Nutrition of Plants. Annu. Rev. Biochem., 1 :618-636. Some Effects of Deficiencies of Phosphate and Potassium on the Growth and Composition of Fruit Trees under Controlled Conditions. With W. H. Chandler. Proc. Am. Soc. Hort. Sci., 29 :267-271. Little-Leaf or Rosette of Fruit Trees, III. With W. H. Chandler and P. L. Hibbard. Proc. Am. Soc. Hort. Sci., 30 :70-86. Mineral Nutrition of Plants. Annu. Rev. Biochem., 2 :471-484. Nutrition of Strawberry Plant under Controlled Conditions. (a) Effects of Deficiencies of Boron and Certain Other Elements, (b) Susceptibility  to Injury from Sodium Salts. With W. C. Snyder. Proc. Am. Soc. Hort. Sci., 30 :288–294. Absorption of Potassium by Plants in Relation to Replaceable, Non-Replaceable, and Soil Solution Potassium. With J. C. Martin. Soil Sci., 36 :1-33. Methods for Determining Availability of Potassium with Special Reference to Semi-Arid Soils. Trans. 2nd Commission and Alkali Subcommission of the International Soc. Soil Sci. Kjobenhavn (Danmark). Vol. A, pp.  25–31. Little-Leaf or Rosette of Fruit Trees, IV. With W. H. Chandler and P. L. Hibbard. Proc. Am. Soc. Hort. Sci., 32 :11-19. The Potassium Nutrition of Barley  with Special Reference to California Soils. Proc. Fifth Pacific Science Congress, pp.  2669–2676. Little-Leaf or Rosette of Fruit Trees, V: Effect of Zinc on the Growth of Plants of Various Types in Controlled Soil and Water Culture Experiments. With W. H. Chandler and P. L. Hibbard. Proc. Am. Soc. Hort. Sci., 33 :131-141. Comments on the Article by A Kozlowski on "Little Leaf or Rosette of Fruit Trees in California". With W. H. Chandler. Phytopathology, 25(5) :522-522 Absorption of Potassium by Plants and Fixation by the Soil in Relation to Certain Methods for Estimating Available Nutrients. With  J. C. Martin. Trans. Third Inter. Cong. Soil Sci., 1 :99-103. Little-Leaf or Rosette of Fruit Trees, VI: Further Experiments Bearing on the Cause of the Disease. With W. H. Chandler and P. R. Stout. Proc. Am. Soc. Hort. Sci., 34 :210-212. The Plant as a Metabolic Unit in the Soil-Plant System. Essays in Geobotany in Honor of Wm. A. Setchell. Univ. Calif. Press, pp. 219–245. General Nature of the Process of Salt Accumulation by Roots with Description of Experimental Methods. With T. C. Broyer. Plant Phys., 11(3) :471-507. Some Aspects of the Salt Nutrition  of Higher Plants. Bot. Rev., 3 :307-334. The Water-Culture Method for Growing Plants without Soil. With D. I. Arnon. Calif. Agr. Exp. Sta. Cir., 347, pp.  1-39.* Fertilizer Problems and Analysis of Soils in California. Calif. Agr. Exp. Sta. Cir., 317 :1-16 (Revision). A Comparison of Water Culture and Soil as Media for Crop Production. With D. I. Arnon. Science, 89 :512-514. Upward and Lateral Movement of Salt in Certain Plants as Indicated by Radioactive Isotopes of Potassium, Sodium, and Phosphorus Absorbed by Roots. With P. R. Stout. Am. J. Bot., 26(5) :320-324. Metabolism  and Salt Absorption by Plants. With F. C. Steward. Nature, 143 :1031-1032. Salt Absorption by Plants. With F. C. Steward. Nature, 145 :116-117. Hydrogen-Ion Effects and the Accumulation of Salt by Barley Roots as Influenced by Metabolism. With T. C. Broyer. Am. J. Bot., 27 :173-185. Upward Movement of Salt in the Plant. With T. C. Broyer and P. R. Stout. Nature, 146 :340-340. Minute Amounts of Chemical Elements in Relation to Plant Growth. Science, 91 :557-560. Methods of Sap Expression from Plant Tissues with Special Reference to Studies on Salt Accumulation by Excised  Barley Roots. With T. C. Broyer. Am. J. Bot., 27(7) :501-511. Crop Production in Artificial Culture Solutions and in Soils with Special Reference to Factors Influencing Yields and Absorption of Inorganic Nutrients. With D. I. Arnon. Soil Sci., 50(1) :463-485. Salt Accumulation by Plant Cells with Special Reference to Metabolism and Experiments on Barley Roots. Cold Spring Harbor Symposia on Quantitative Biology, Vol. 8. Some Modern Advances in the Study of Plant Nutrition. Proc. Am. Soc. Sugar Beet Tech., Part 1 :18-26. Water Culture Experiments on Molybdenum and Copper Deficiencies of Fruit Trees. Proc. Am. Soc. Hort . Sci., 38 :8-12. Physiological Aspects of Availability of Nutrients for Plant Growth. With D. I. Arnon. Soil Sci., 51(1) :431-444. Aspects of Progress in the Study of Plant Nutrition. Trop. Agr., 18 :247. Accumulation of Salt and Permeability in Plant Cells. With T. C. Broyer. J. Gen. Physiol., 25(6) :865-880. Metabolic Activities of Roots and Their Bearing on the Relation of Upward Movement of Salts and Water in Plants. With T. C. Broyer. Am. J. Bot., 30(4) :261-273. Composition of the Tomato Plant as Influenced by Nutrient Supply, in Relation to Fruiting. With D. I. Arnon . Bot. Gaz., 104(4) :576-590. General Aspects of the Study of Plant Nutrition. Sci. Univ. Calif., pp. 279–294. The Investigation of Plant Nutrition by Artificial Culture Methods. With D. I. Arnon. Biol. Rev. Cambr. Phil. Soc., 19(2) :55-67. Lectures on the Inorganic Nutrition of Plants. (Prather Lectures at Harvard University). Published by Chronica Botanica Co. Waltham, Mass. Molybdenum in Relation to Plant Growth. Soil Sci., 60(2) :119-123. Potassium Fixation in Soils in Replaceable and Non-Replaceable Forms in Relation to Chemical Reactions in the Soil. With J. C. Martin and R. Overstreet. Soil Sci. Soc. Am. Proc., 10  :94-101. The Nutrition and Biochemistry of Plants, Currents in Biochemical Research. Interscience Publ. Inc. N. Y., pp.  61–77. Little-Leaf or Rosette of Fruit Trees, VIII: Zinc and Copper Deficiency in Corral Soils. With W. H. Chandler and J. C. Martin. Proc. Am. Soc. Hort. Sci., 47 :15-19. Trace Elements in Plants and Animals by Walter Stiles. Rev. Arch. Biochem., 13 :311-312. Fertilizers, Soil Analysis, and Plant Nutrition. Calif. Agr. Exp. Sta. Cir., 367 :1-24. Minute Amounts of "Minor" Elements Essential in Addition to "Regular" Fertilizer. Agr. Chem. Some Problems of Plant Nutrition. With D. I.  Arnon. Sci. Mo., 67(3): 201-209. Fertilizers, Soil Analysis, and Plant Nutrition. Calif. Agr. Exp. Sta. Cir., 367 :1-24 (Revision). Absorption and Utilization of Inorganic Substances in Plants. With P. R. Stout. Chap. VIII of Agricultural Chemistry, ed. by Frear, Van Nostrand. The Water-Culture Method for Growing Plants without Soil. With D. I. Arnon. Calif. Agr. Exp. Sta. Cir., 347, pp.  1-32 (Revision).** Availability of Potassium to Crops in Relation to Replaceable and Non-Replaceable Potassium and to Effects of Cropping and Organic Matter. With J. C. Martin. Soil Sci. Soc. Am. Proc., 15 :272-278. "Courtesy  of The National Academy of Sciences Archives, and without these entries it would not have been possible."