Dennis Hoagland is commonly known for discovering the active transport of electrolytes in plant cells, using innovative model systems, such as Nitella, under controlled experimental conditions, such as solution culture.

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.

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).

[4] 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 systems 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).

He took special interest in plant-soil 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 and absorption of nutrients in soils and (artificial) solutions (Hoagland, 1916, 1917, 1920, 1922; Hoagland and Arnon, 1941).

He also developed the first successful concept for distinguishing between concentration and total amount of nutrients in a solution (Johnston and Hoagland, 1929).

[10] Around the 1930s Hoagland and his associates[5] investigated diseases of certain plants, and thereby, observed symptoms related to existing soil conditions such as salinity.

In this context, Hoagland undertook water culture experiments with the hope of delivering similar symptoms under controlled laboratory conditions.

Table 2) and the concentration of micronutrients (B, Mn, Zn, Cu, Mo, and Cl) of the Hoagland solutions (1) and (2) (cf.

[5] 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.

[20] Hoagland concluded that solutions of radically different concentrations and salt proportions did not affect the yield of a crop to any important extent.

[21] As an example, Hoagland's solution (2) led to increased growth of fig trees in high-tunnel and open-field conditions, respectively.

For example, water culture led to highest biomass and protein production of hydroponically grown tobacco plants compared to other growth substrates, cultivated in the same environmental conditions and supplied with equal amounts of nutrients.

The authors' differing views are illustrated by the following quotations: "Its commercial application is justifiable under very limited conditions and only under expert supervision" (Hoagland and Arnon, 1938, 1950, The Water Culture Method for Growing Plants Without Soil); "Indeed, it is obvious that since hydroponics requires a larger expense per unit of area than does agriculture, either yields must be larger, or there must be other compensations, if the method is to succeed commercially.

Not surprisingly, the history of hydroponics has proved Gericke right in his claims about the commercial use of this technique as a useful complement to conventional agriculture.

[25] In recognition of his many discoveries, the American Society of Plant Physiologists elected Dennis Hoagland as president in 1932[26] and awarded him the first Stephen Hales Prize in 1929.

[27] 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".

[34] The basic formulas of Hoagland and Arnon are being replicated by modern manufacturers to produce liquid concentrated fertilizers for plant breeders, farmers, and average consumers.

The Hoagland and Knop medium was specially formulated for plant cell, tissue and organ cultures on sterile agar.

For example, 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.

[1][17] The Hoagland solution is not only used on earth, but has also proven itself in plant production experiments on the International Space Station.

[39] Hoagland's fundamental scientific contributions and widely cited publications are of historical relevance to research in modern plant physiology and soil chemistry, which is reflected in the following bibliography.

The Freezing-Point Method as an Index of Variations in the Soil Solution Due to Season and Crop Growth.

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.

Phytopathology, 25(5) :522-522 Absorption of Potassium by Plants and Fixation by the Soil in Relation to Certain Methods for Estimating Available Nutrients.

Upward and Lateral Movement of Salt in Certain Plants as Indicated by Radioactive Isotopes of Potassium, Sodium, and Phosphorus Absorbed by Roots.

Methods of Sap Expression from Plant Tissues with Special Reference to Studies on Salt Accumulation by Excised Barley Roots.

Crop Production in Artificial Culture Solutions and in Soils with Special Reference to Factors Influencing Yields and Absorption of Inorganic Nutrients.