The three terms are often confused. However, they cover different concepts:
Agriculture is the set of activities that transform the environment for the production of animals and plants for human use. Agriculture concerns techniques, including the application of agronomic research.
Traditional agricultural systems, sometimes termed subsistence agriculture, which feed most of the poorest people in the world. These systems are of interest as they sometimes retain a level of integration with natural ecological systems greater than that of industrial agriculture, which may be more sustainable than some modern agricultural systems.
Food production and demand on a global basis, with special attention paid to the major producers, such as China, India, Brazil, the US and the EU.
Various sciences relating to agricultural resources and the environment (e.g. soil science, agroclimatology); biology of agricultural crops and animals (e.g. crop science, animal science and their included sciences, e.g. ruminant nutrition, farm animal welfare); such fields as agricultural economics and rural sociology; various disciplines encompassed in agricultural engineering.
One of the most common yield reducers is because of fertilizer not being applied in slightly higher quantities during transition period, the time it takes the soil to rebuild its aggregates and organic matter. Yields will decrease temporarily because of nitrogen being immobilized in the crop residue, which can take a few months to several years to decompose, depending on the crop's C to N ratio and the local environment.
In the United States, a scientific revolution in agriculture began with the Hatch Act of 1887, which used the term "agricultural science". The Hatch Act was driven by farmers' interest in knowing the constituents of early artificial fertilizer. The Smith-Hughes Act of 1917 shifted agricultural education back to its vocational roots, but the scientific foundation had been built. After 1906, public expenditures on agricultural research in the US exceeded private expenditures for the next 44 years.:xxi
^John Armstrong, Jesse Buel. A Treatise on Agriculture, The Present Condition of the Art Abroad and at Home, and the Theory and Practice of Husbandry. To which is Added, a Dissertation on the Kitchen and Garden. 1840. p. 45.
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Vavilov Nicolai I. (Starr Chester K. editor), The Origin, Variation, Immunity and Breeding of Cultivated Plants. Selected Writings, in Chronica botanica, 13: 1–6, Waltham, Mass., 1949–50
Vavilov Nicolai I., World Resources of Cereals, Leguminous Seed Crops and Flax, Academy of Sciences of Urss, National Science Foundation, Washington, Israel Program for Scientific Translations, Jerusalem 1960
Winogradsky Serge, Microbiologie du sol. Problèmes et methodes. Cinquante ans de recherches, Masson & c.ie, Paris 1949