Hoagland graduated from Stanford University 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 field of animal nutrition and biochemistry until 1912, when he entered the graduate school in the department of agricultural chemistry at the University of Wisconsin, receiving his master's degree in 1913. The following year he became Assistant Professor of agricultural chemistry and in 1922 Associate Professor at Berkeley.
Private life
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.
Work
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. He investigated the ability of plants to absorb salts against a concentration gradient and discovered the dependence of nutrient absorption on metabolic energy. During his systematic research he developed the basic formula for the Hoagland solution and established the essentiality of molybdenum for the growth of tomato plants. Hoagland was able to show that various plant diseases are caused by a lack of trace elements such as zinc. Further work dealt with plant-soil interactions and the pH dependence of plant growth. A Hoagland hydroponic and soil culture solution provides every nutrient necessary for plant growth being appropriate for the growth of a large variety of plant species. The solution described by Hoagland and Snyder has been modified several times, for example, by Hoagland and Arnon, notably with the number and concentrations of micronutrients and the addition of iron chelates. Hoagland's research was influenced by the plant pathologists H. E. Thomas and W. C. Snyder, and another pioneer of plant nutrition and hydroponics, William Frederick Gericke. Gericke's positive results in this field inspired him to expand their research on hydroponics finally resulting in the Hoagland solutions and. The composition of macronutrients of the Hoagland solution can be traced back to Wilhelm Knop's four-salt mixture and the respective salt and element concentrations to Dennis Hoagland. Knop's solution in contrast to Hoagland's solution was not supplemented with trace elements because the chemicals were not particularly pure in Wilhelm Knop's day. Hoagland's students included Daniel Israel Arnon who developed the Hoagland solution as a result of joint efforts, and Folke Karl Skoog. In contrast to the Murashige and Skoog medium the Hoagland solution contains neither vitamins nor organic compounds but minerals for plant nutrition. It is concluded that the promotion of growth of tobaccocallus cultured on White's modified medium is due mainly to inorganic rather than organic constituents in aqueous tobacco leaf extracts added.
Nowadays the most common solutions for plant nutrition and plant tissue cultivation are the formulations from Hoagland and Arnon, and Murashige and Skoog. Their basic formulas are being replicated by modern manufacturers to commercially produce liquid concentrated fertilizers for plant breeders and the life sciences. Even their names are used as a brand for innovative products, e.g., basal salt mixtures. Hoagland's great research merit was to develop the Hoagland solution, thereby, creating the basis for a modern balanced plant nutrition that is still valid today. His fundamental research contributions are reflected in the following bibliography. Though some sources claim the opposite, Gericke's and Hoagland's recipes for plant nutrition were developed independently of one another. Even if Hoagland and Arnon were never awarded the Nobel Prize for their outstanding research work, the Hoagland solution, although never been applied for a patent, is still one of the most important inventions of modern times.
