Justus Liebig (1803-73) carried out numerous analyses in plants and soils and he also came to the conclusion that plants feed from inorganic nutrients. According to his statement the first sources of plant nutrients are exclusively of inorganic nature. Liebig integrated his statement in a greater context as he speculated that plants in nature play a unique role in transferring chemical elements into organic plant matter. This organic matter when brought back into the soil is decomposed - in those days microbes were not yet known - and inorganic elements are released which again are taken up by plants as nutrients. To our knowledge Liebig was the first assuming a cyclic process in nature. According to this concept the limiting process of producing food for mankind was not the humus, and animals did not compete with man for arable land. He claimed that ''the first sources of the plant nutrients are provided exclusively by inorganic nature'' and he further wrote that ''as the principle of agriculture must be seen that soils must receive in full measure what was taken from it.'' These were the inorganic plant nutrients and this was the start for producing and applying chemical fertilizers.
According to Liebig's concept, crop yield per hectare in Germany was increased by a factor of c. 3 in the last half of the nineteenth century. In addition, infertile soils, particularly sandy soils, were rendered fertile by the application of lime, phosphate, and potassium. This not only was the case in Germany but also in other European countries and in North America which applied Liebig's principle. Thus area and quality of soils limiting food production were increased.
In the beginning Liebig propagated that N fertilizer application was not necessary since according to his investigations ammonium N was taken up by plant leaves. This assumption contrasted with results of the French scientist Boussingault (1802-87) who according to his investigations held the view that fertilizing inorganic N was of great importance. Only later Liebig became convinced - particularly by the Rothamsted field trials - that inorganic nitrogen also should be fertilized. Liebig, however, immediately realized that the amount of potential inorganic fertilizer N, mainly guano, is limited and would run out quickly when farmers apply it in various countries. Liebig was interested when he heard that electrical discharges (lightning) oxidized the molecular nitrogen in the air and he speculated whether this could be a process by which nitrogen fertilizer could be produced. Liebig realized that nitrogen was a limiting factor in plant nutrition and he emphasized that farmers should carefully handle their farmyard manure and slurry thus avoiding N losses.
Liebig propagated the law of the minimum, originally going back on Carl Sprengel's idea that the limiting factor is controlling the crop yield. In the nineteenth century, nitrogen was limiting crop growth in many cases. Liebig's recommendation to fertilize soils with lime, phosphate, and potassium, however, had indirectly also a positive impact on soil nitrogen because lime, phosphate, and potassium are particularly required for the growth of leguminous species. The numerous attempts to grow clover mostly failed because soil pH was too low and potassium and phosphate were not available in sufficient quantities. These limiting factors were overcome by the application of lime, phosphate, and potassium, and leguminous crops thrived vigorously. These crop species living in symbiosis with N2-fixing bacteria increased the level of available nitrogen in soils. Thus a further crop production limitation was overcome. This promoted farming enormously since farmers had more forage. They could feed their animals better and also increase the number of animals which produced more farmyard manure and slurry in the rotation which was brought back on fields and meadows.
In those days microbial dinitrogen fixation was not yet known until 1886. Liebig assumed that the broad leaves of leguminous species were able to absorb the ammonium from the atmosphere which actually is true but the quantities are too low for obtaining satisfactory yields. Liebig recommended a rotation with two cereals followed by legumes or other dicotyledonous species (beets, potatoes). By this strategy infertile soils were rendered fertile and thus the area of fertile soils was enlarged. By increasing soil pH through liming, microbial activity also was promoted since most microbes require a neutral soil pH. Hence the decomposition of organic matter and thus the microbial production of ammonium and nitrate in soils was stimulated by which N nutrients are selectively taken up by plant roots. In addition, N2 assimilation by free-living N2 fixers and by microbes such as Rhizobium species living in symbiosis with higher plants profited from soil liming.
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