Liebig - Humus Theory
he usual estimate is that it is 16 subjects that are necessary for all plants function. Roughly, nutrients are divided into macro and micro nutrients. For macro-nutrients counted Nitrogen, Phosphorus, Potassium, Magnesium and Sulfur (N, P, K, Mg and S). Among the micro-nutrients it can be mentioned such as Bor, Copper and Zinc. In the beginning of 1800s, people start to take interest in the natural cycle. It was well known that a field became un-fertile after a few years of cultivation. Harvests got worse and worse. The only thing that seemed to help was to leave the fields in fallow for some years. After that it was OK to grow again. For what reason ?
One explanation came from a declaration, drafted in the ”Vitalistic Humus Theory" of a German scientist by the name of Albrecht Thaer. His theory was, in brief, that plants feed on decaying plants and animals. Life and death is part of an organic cycle, claimed Thaer. Because at the harvest, you took away materials from the field, where there then was a mismatch. You have to supply a bit "dead" in the form of humus, decaying plant parts,etc to the field/land/soil, for it to be able to create new life (soil-life) to grow on.
Around 1850 the "Vitalistic Humus Theory" was challenged by another German chemist Justus von Liebig. He claimed that Thaer was talking nonsense and that everything depended on the chemistry. It was not the humus substances that revitalized arable land, but simple inorganic minerals. If a field gave poor harvest, it was due to the absence of key chemical elements, such as phosphorus and potassium. Liebig formulated what he called ”Law of the minimum". Through chemical analysis of the soil, you could see what was missing. If these key chemical elements were added again, the field will receive new vigor/fertility. Liebig also launched the idea of adding synthetically produced fertilizers, especially potassium, phosphorus and nitrogen.
Liebig's "Thin" illustrates how the lack of a single plant nutrient, reducing the crop's ability to absorb other essential nutrients. This law is usually popularly illustrated with a barrel where each "board" represents a plant nutrient. The water/level in the barrel - symbolizing total credit made of nutrients - flows of course out on the shortest board, no matter how long the other boards are. We must therefore strive to fertilize with all substances the plant needs and in a correct proportion as possible.