Dennis Robert Hoagland (April 2, 1884 – September 5, 1949) was an American chemist and leading plant and soil scientist who pioneered work in plant nutrition, soil chemistry, agricultural chemistry, biochemistry, and physiology. He was professor of Plant Nutrition at the University of California, Berkeley, from 1927 until his death in 1949.
Dennis Hoagland is commonly known for discovering the active transport of electrolytes in plant cells using innovative model organisms such as Nitella. Under controlled experimental conditions, he succeeded in analyzing ions in a virtually uncontaminated vacuolar solution. Using hydroculture or solution culture, Hoagland was able to show that various plant diseases are due to a lack of trace elements. He demonstrated their importance for the nutrition and metabolic processes of plants by investigating the absorption of zinc and copper in fruit trees.
He gained deep insights into the complexity of interactions between plant and soil by establishing the measurement of soil pH and demonstrating the importance of soil solution, oxygen, carbon dioxide, temperature and light for plant growth and development. The application of innovative techniques such as the hydrogen electrode and freezing-point depression played a central role in measuring plant and soil reactions. His extensive empirical work on soil and fertilizer issues revealed the power of certain soil types to extract potassium from the solution.
Between 1919 and 1950, Hoagland and his associates, particularly Daniel Israel Arnon, based on their experiments with soil solutions, formulated an artificial, completely inorganic nutrient medium, universally known as Hoagland solution, that continues to be used worldwide for hydroponic plant cultivation. Although Hoagland received numerous awards and honors for his scientific expertise and outstanding human qualities, the heated public debate with his colleague William Frederick Gericke over the use of modern water culture in commercial agriculture left a bitter aftertaste.
Biography
Private life
Dennis Hoagland was the son of Charles Breckenridge 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. There 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 (Schmidt and Hoagland, 1912, 1919), 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 (McCollum and Hoagland, 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
Historical overview
During 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 giant kelp 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. Innovative model organisms and techniques, used under rigidly controlled experimental conditions, thus, enabled the identification and isolation of individual variables in the measurement of plant-specific parameters (Hoagland, Hibbard, and Davis, 1926).
During his systematic research, mainly by solution culture technique, and inspired by a principle of Julius von Sachs and the work of Wilhelm Knop, 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)1. His research also led to new discoveries on the need and function of trace elements required by living cells, thus, establishing the essentiality of molybdenum for the growth of tomato plants, for example (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 normal plant growth (Hoagland, 1937).
He took special interest in plant-soil interrelationships addressing, for example, the physiological balance of soil solutions and the pH dependence of plant growth, to gain a better understanding on the availability and absorption of micronutrients and macronutrients in soils and (artificial) 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 was a founder of the Annual Review of Biochemistry and a proponent of the Annual Review of Plant Physiology and the Annual Review of Medicine which first appeared in 1950, after his death.
Hoagland's and Knop's solutions
Dennis Hoagland was the first to develop a new type of solution based on the composition of the soil solution (Hoagland, 1919)1. He also developed the first successful concept for distinguishing between concentration and total amount of nutrients in a solution (Johnston and Hoagland, 1929). The term Hoagland solution (HS) was first mentioned by Olof Arrhenius in 1922 with reference to the Hoagland publication of 19191 where he defined an optimum nutrient solution as "the minimum concentration which gave maximum yield and beyond there was no further improvement".Hewitt E. J. (1966). Sand and Water Culture Methods Used in the Study of Plant Nutrition. Farnham Royal, England: Commonwealth Agricultural Bureaux, pp. 547. Technical Communication No. 22 (Revised 2nd Edition) of the Commonwealth Bureau of Horticulture and Plantation Crops. The respective 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 and his associates investigated diseases of certain plants, and thereby, observed symptoms related to existing soil conditions such as salinity or nutrient deficiency. In this context, Hoagland undertook water culture experiments with the hope of delivering similar symptoms under controlled laboratory conditions. For these experiments the Hoagland solution (0), including macronutrients, iron, and the supplementary solutions A and B (trace elements), was newly developed to investigate certain diseases of the strawberry in California (Hoagland and Snyder, 1933).
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, who is featured in a historical video by Arthur Clarence Pillsbury. Gericke's groundbreaking results in applying the principles of water culture to commercial agriculture inspired him to expand his research on the subject finally resulting in the Hoagland solutions (1) and (2) (Hoagland and Arnon, 1938, 1950). The composition and concentration of macronutrients of the Hoagland solutions (0) and (1) can be traced back to Wilhelm Knop's four-salt mixture and the molar ratio of the components to experimental results of Hoagland and his associates (cf. Tables (1) and (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.
Knop's solution Table (1). Knop's four-salt mixture (1865) Macronutrient saltsQuantities in solutiong/L KNO3 0.25 Ca(NO3)2 1.00 MgSO4•7H2O 0.25 KH2PO4 0.25
Macronutrients Table (2). Composition and full concentration of macronutrients in Hoagland's solution (0, 1, 2) and in Knop's solution Macronutrients Hoagland's solution (0, 1) Hoagland's solution (2) Knop's solution Quantities in solutionμmol/Lμmol/Lμmol/L K+ 6,000 6,000 4,310 Ca2+ 5,000* 4,000** 6,094 Mg2+ 2,000 2,000 1,014NO 15,000 14,000 14,661NH – 1,000 –SO 2,000 2,000 1,014PO 1,000 1,000 1,837
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, Mo, and Cl) 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. Murashige and Skoog 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.
Micronutrients Table (3). Composition and full concentration of essential micronutrients in Hoagland's solution (0, 1, 2) Micronutrients Hoagland's solution (0) Hoagland's solution (1, 2) Quantities in solutionμmol/Lμmol/L B(OH)4− 9.88 46.25 Mn2+ 1.97 9.15 Zn2+ 0.34 0.77 Cu2+ 0.22 0.32 MoO – 0.50* or 0.11** MoO 0.18 – Cl− 3.93 18.29
As an additional micronutrient, 9 μM ferric tartrate (C12H12Fe2O18) is added to each of the Hoagland solution formulations (0, 1, 2), corresponding to a concentration of 18 μmol/L Fe3+. Solution (2) contains ammonium and nitrate salts and may sometimes be preferred to solution (0, 1) (cf. Table 2) because the ammonium ion delays the development of undesirable alkalinity (Hoagland and Arnon, 1938, 1950). However, it is toxic to most crop species and is rarely applied as a sole nitrogen source.
Disputed hypotheses