Editing Justus von Liebig (section)

===Plant nutrition===
By the 1840s, Liebig was attempting to apply theoretical knowledge from organic chemistry to real-world problems of food availability.  His book ''Die organische Chemie in ihrer Anwendung auf Agricultur und Physiologie'' (''Organic Chemistry in its Application to Agriculture and Physiology'') (1840) promoted the idea that chemistry could revolutionize agricultural practice, increasing yields and lowering costs. It was widely translated, vociferously critiqued, and highly influential.<ref name=Brock/>

Liebig's book discussed chemical transformations within living systems, both plant and animal, outlining a theoretical approach to agricultural chemistry. The book's first part focused on plant nutrition; the second was on chemical mechanisms of putrefaction and decay.<ref name=Brock/>{{rp|148}} Liebig's awareness of both synthesis and degradation led him to become an early advocate of [[Conservation (ethic)|conservation]], promoting ideas such as the recycling of [[sewage]].<ref name=Brock/>{{rp|250–270}}

Liebig argued against prevalent theories about role of [[humus]] in plant nutrition, which held that decayed plant matter was the primary source of carbon for plant nutrition. Fertilizers were believed to act by breaking down humus, making it easier for plants to absorb. Associated with such ideas was the belief that some sort of "vital force" distinguished reactions involving organic as opposed to inorganic materials.<ref name=Hill>{{cite book|author-last1=Hill|author-first1=Jane F.|chapter=Translator's Introduction|author-last2=de Saussure|author-first2=Theodore|title=Chemical research on plant growth|date=2012|publisher=Springer|location=New York|isbn=978-1-4614-4136-6|chapter-url=https://books.google.com/books?id=cVVJAAAAQBAJ&pg=PR15|access-date=7 November 2014}}</ref>

Early studies of photosynthesis had identified carbon, hydrogen, oxygen, and nitrogen as important, but disagreed over their sources and mechanisms of action. Carbon dioxide was known to be taken in and oxygen released during photosynthesis, but researchers suggested that oxygen was obtained from carbon dioxide, rather than from water. Hydrogen was believed to come primarily from water. Researchers disagreed about whether sources of carbon and nitrogen were atmospheric or soil-based.<ref name=Hill/>{{rp|xv–xxi}} [[Nicolas-Théodore de Saussure]]'s experiments, reported in ''Recherches Chimiques sur la Végétation'' (1804), suggested that carbon was obtained from atmospheric rather than soil-based sources. It also indicated that water was a likely source of hydrogen. He also studied the absorption of minerals by plants, and observed that mineral concentrations in plants tended to reflect their presence in the soil in which the plants were grown. However, the implications of De Saussure's results for theories of plant nutrition were neither clearly discussed nor easily understood.<ref name=Hill/>{{rp|xxii–xxvii}}

Liebig reaffirmed the importance of De Saussure's findings, and used them to critique humus theories, while regretting the limitations of De Saussure's experimental techniques. Using more precise methods of measurement as a basis for estimation, he pointed out contradictions such as the inability of existing soil humus to provide enough carbon to support the plants growing in it.<ref name=Hill/>{{rp|xxix}} By the late 1830s, researchers such as [[Karl Sprengel]] were using Liebig's methods of combustion analysis to assess manures, concluding that their value could be attributed to their constituent minerals.<ref name=Brock/>{{rp|106}} Liebig synthesized ideas about the mineral theory of plant nutrition and added his own conviction that inorganic materials could provide nutrients as effectively as organic sources.<ref name=Brock/>{{rp|148}}

In his theory of mineral nutrients, Liebig identified the chemical elements of nitrogen (N), phosphorus (P), and potassium (K) as essential to plant growth. He reported that plants acquire carbon (C) and hydrogen (H) from the atmosphere and from water (H<sub>2</sub>O). In addition to emphasizing the importance of minerals in the soil, he argued that plants feed on nitrogen compounds derived from the air. This assertion was a source of contention for many years, and turned out to be true for legumes, but not for other plants.<ref name=Brock/>{{rp|181}}

[[File:Minimum-Tonne.svg|thumb|right|Liebig's barrel]]
Liebig also popularized Carl Sprengel's "theorem of minimum" (known as the [[Liebig's Law of the Minimum|law of the minimum]]), stating that plant growth is not determined by the total resources available, but by the scarcest available resource.  A plant's development is limited by the one essential mineral that is in the relatively shortest supply.  This concept of limitation can be visualized as "Liebig's barrel", a metaphorical barrel in which each stave represents a different element. A nutrient stave that is shorter than the others will cause the liquid contained in the barrel to spill out at that level. This is a qualitative version of the principles used for determining the application of fertilizer in modern agriculture.

''Organic Chemistry'' was not intended as a guide to practical agriculture. Liebig's lack of experience in practical applications, and differences between editions of the book, fueled considerable criticism. Nonetheless, Liebig's writings had a profound impact on agriculture, spurring experiment and theoretical debate in Germany, England, and France.<ref name=Brock/>{{rp|165}}

One of his most recognized accomplishments is the development of [[nitrogen]]-based [[fertilizer]]. In the first two editions of his book (1840, 1842), Liebig reported that the atmosphere contained insufficient nitrogen, and argued that nitrogen-based fertilizer was needed to grow the healthiest possible crops.<ref name=Brock/>{{rp|120}} Liebig believed that nitrogen could be supplied in the form of [[ammonia]], and recognized the possibility of substituting chemical fertilizers for natural ones (animal dung, etc.)

He later became convinced that nitrogen was sufficiently supplied by precipitation of ammonia from the atmosphere, and argued vehemently against the use of nitrogen-based fertilizers for many years. An early commercial attempt to produce his own fertilizers was unsuccessful, due to lack of nitrogen in the mixtures.<ref name=Brock/>{{rp|121–124}} When tested in a farmer's field, Liebig's manure was found to have no appreciable effect.<ref>Matter-of-Fact (July 1847) [https://babel.hathitrust.org/cgi/pt?id=hvd.32044048680383&view=1up&seq=214 Liebig's System of Manuring], [[The Cultivator]] series 2, volume 4, page 208 via [[HathiTrust]]</ref>

Liebig's difficulties in reconciling theory and practice reflected that the real world of agriculture was more complex than was at first realized. By the publication of the seventh German edition of ''Agricultural Chemistry'' he had moderated some of his views, admitting some mistakes and returning to the position that nitrogen-based fertilizers were beneficial or even necessary.<ref name=Brock/>{{rp|179}}He was instrumental in the use of [[guano]] for nitrogen.<ref>Charles Mann (2011) [https://archive.org/details/1493uncoveringne00mann 1493: Uncovering the New World Columbus Created], p. 214, Knopf</ref> In 1863 he published the book "Es ist ja die Spitze meines lebens" in which he revised his early perceptions, now appreciating soil life and in particular the biological N fixation.[http://www.vanmansvelt.nl/home/historic-and-actual-awareness-of-soil-fertility-in-agriculture-russia-western-europe-usa-draft-of-a-survey/] 
Nitrogen fertilizers are now widely used throughout the world, and their production is a substantial segment of the chemical industry.<ref name=Travis2013>{{cite journal|last1=Travis|first1=Anthony S.|title=Dirty Business|journal=Chemical Heritage Magazine|date=Spring 2013|volume=31|issue=1|page=7|url=https://www.sciencehistory.org/distillations/magazine/dirty-business|access-date=20 March 2018}}</ref>