Development of the humus theory of plant nutrition in the 18th century

The understanding of how plants obtain nutrients has been shaped over centuries. At the turn of the 17th and 18th centuries, when chemistry did not yet exist as a science, agronomists sought answers to questions of soil fertility after the invalidity of the water theory was proven. This vacuum was filled by the humus theory of nutrition. The term "humus" itself was used as far back as Roman times to denote soil in general, and later came to refer to soil organic matter, its fractions, and complex compounds obtained through the action of chemical reagents.

• Period of the concept "oleum unctuosum" — from 373–328 BC to 1709–1785 AD.
• Publication of "Systema Naturae" — 1740.
• First scientific definition of humus — 1761.
• Publication of the work on plant nutrition — 1804.

From 373–328 BC to 1709–1785, the notion of "oleum unctuosum" ("grease-like oil"), which was equated to soil or the "fat of the earth," prevailed among naturalists. Later, in the period 1707–1778, a classification of soils analogous to the classification of plants was proposed. In the book "Systema Naturae" (1740), only sand and clay were classified as primary earths, while all "vegetable earth" was considered a product of the decay of plants and animals, which multiplies daily but eventually reverts to one of the types of sand.

In the book "Fundamentals of Agricultural Chemistry" (1761), the concept of "humus" was first assigned to decomposed organic matter. At the same time, the hypothesis of plant nutrition by humus arose, which explained the high productivity of soils rich in humus. However, the theory erroneously assumed the direct assimilation of this complex organic substance by plant roots.

In the work "Fundamenta agriculturae chemical" (1766), it was stated that the ash parts of plants obtained through chemical analysis are not identical to those contained in the soil and are prepared by the plant from water and air. The "fatty" substance of humus was given primary importance; it was used to explain the effect of manure and humus. Soil salts, in particular saltpeter, were attributed only the value of solvents for soil "fat." Based on the principle "nutritio non fieri potesta rebus heterogeneis, sed homogeneis," scientists concluded that only organic substances of the soil are nutritive for plants (nutritiva), while other parts, such as chalk or salt, play only an auxiliary role (instrumentalia), facilitating the dissolution of fatty substances.

Fractionation of humus and the discovery of photosynthesis

The development of the humus theory was facilitated by research conducted during the period 1767–1845. In the scientific work "Chemical Researches on Plant Life" (1804), special attention was paid to humus (terreau). It was proven that humus is a heterogeneous substance and consists of various complexes (extractive substances, fats, salts) that can easily be separated. Humus is capable of absorbing oxygen, which, by combining with soil carbon, produces carbonic acid. The extract from humus contains the same ash substances as the plants themselves and enters them in small quantities along with other soil substances.

Also during this period, the doctrine of the aerial nutrition of plants was developed. It was experimentally established that the source of carbon dioxide is not the soil, but the atmospheric air. During the decomposition of carbon dioxide by plants, not only carbon is assimilated, but also the elements of water — hydrogen and oxygen.

The processes of respiration and aerial nutrition are distributed as follows:

• In the dark, the plant absorbs oxygen and releases carbonic acid during respiration.
• In the light, the plant assimilates the carbon of carbonic acid and releases oxygen.
• Atmospheric air serves exclusively as the source of carbon dioxide for nutrition.
• Upon the decomposition of carbon dioxide, the plant assimilates carbon, as well as hydrogen and oxygen, which are components of water.

The understanding of the role of organic matter in harvest formation has been built over centuries. The agronomy of the late 18th and early 19th centuries was based on the theory of humus plant nutrition, detailed in the book "Principles of Rational Agriculture" by a German scientist (1752–1828). According to this concept, soil fertility directly depends on humus reserves, because, excluding water, it was recognized as the only source of nutrition for crops. It was assumed that mineral compounds of the soil do not nourish plants by themselves, but only help them assimilate humus.

Soil classification and the practical legacy of the theory

Parallel to the development of the nutrition theory, work was underway on the systematization of land. One of the first soil classifications in Western Europe was created. It was based on three key practical criteria:

• texture (particle-size distribution);
• humus content;
• lime content.

The new approach quickly gained popularity among practitioners, as the book "Principles of Rational Agriculture" provided a scientific explanation for many techniques that had previously been used blindly. Nearly 200 years after the publication of these works, the author of the theory is still considered the founder of scientific agriculture in Germany. Many of his conclusions remain valuable for modern crop production, and the researcher himself also went down in history as the creator of the German agricultural education system.