The Law of the Minimum and the Law of Return have created plenty of debate in the days since they were proposed. These two laws are regarded as representing two opposing perspectives on soil nutrients and the health of the plants and animals that depend on them.
Liebig’s Law of the Minimum
Stated in its most basic terms, the Law of the Minimum is as follows:
Plant growth and yield is determined by the most limiting factor.
To phrase this principle differently, we could say that the total available resources do not dictate plant growth and yield. Therefore, continuing to apply more and more of an already abundant nutrient is a waste, because it will not result in a larger harvest.
To illustrate the principle, the Law of the Minimum is sometimes restated in the form of the old adage, “A chain is only as good as its weakest link.” Likewise, the Law of the Minimum is also illustrated by “Liebig’s Barrel.” Imagine a barrel made up of staves of unequal length. If the barrel is slowly filled with water, the water level cannot rise above the top of the shortest stave.
The Law of the Minimum was first developed by botanist Carl Sprengel in the early 1800s. However, German professor Justus von Liebig often receives credit for the maxim because he was the one who popularized it.
Liebig is also known as the “Father of the Fertilizer Industry.” This is largely due to the fact that he promulgated the theory (radical at the time) that plants do not derive nutrients from organic materials, but only from mineral sources. Furthermore, since nitrogen, potassium, and phosphorus were often the most limiting factors, Liebig’s law meant that fertilization could focus almost exclusively on the application of NPK, revolutionizing the fertilizer industry. (Note, however, that Liebig himself also extended his maxim beyond soil nutrients to include water and carbon dioxide as limiting factors for plant growth.)
In the field, researchers have noted several violations of Liebig’s Law of the Minimum. Violations occur when organisms adapt genetically to the presence of the limiting factor. In nature, there is a constant tension between limiting factors and adaptation to those factors. This is known as the Law of the Minimum Paradox.
Likewise, Liebig’s assertion that plants can only take up nutrients derived from mineral sources is now known to be false.
Howard’s Law of Return
Stated in its most basic form, the Law of Return is as follows:
Whatever is taken from the soil must be returned.
The Law of Return was formulated by Albert Howard in 1943. It has served as the foundation of a great deal of organic and sustainable thinking.
The idea behind the Law of Return was that all organic materials, particularly those regarded as waste products, should eventually be returned to the soil to enrich it.
Howard vehemently disagreed with Liebig’s perspective that plants need only water, carbon dioxide, and mineral nutrients to thrive. While Howard did acknowledge the importance of mineral nutrients, he believed that plants could indeed draw additional nutrients from organic materials and would further benefit from the fungal and bacterial soil communities such materials could support.
In short, rather than feed the plants themselves as Liebig did, Howard sought to feed the soil.
Can the Two Laws Coexist?
The work of Liebig and Howard proved to be extremely polarizing in the agricultural world. Proponents of Liebig fell solidly into the chemical-based fertilizer camp, while advocates of Howard pioneered organic agriculture. A middle ground was seldom sought in the decades after Howard’s rise to fame.
Yet both laws are true to some degree. In fact, Howard himself, while a harsh critic of Liebig, nevertheless termed Liebig’s work as “a great advance.” The problem lies in the focus.
To be economical, an extensive soil improvement program typically must focus on the limiting factor to some degree. Identifying and correcting limiting factors can make an enormous improvement in plant yield fairly quickly.
However, there will always be a limiting factor, and the limiting factor will change over the life cycle of any given plant, which means that solely chasing the limiting factor is an exercise in futility in the long run. Furthermore, a narrow focus on the limiting factor encourages application of single nutrients in isolation without a regard for the whole picture. This is why Howard wrote that “artificial fertilizers were born out of the abuse of Liebig’s discoveries.”
Furthermore, Liebig emphasized the chemistry of plant health, while Howard prioritized the biology. Both aspects are necessary for a balanced approach.
A Third Factor
The Law of the Minimum Paradox brings up an interesting consideration not fully factored into either perspective. Ultimately, the full potential of a plant is determined by its genetics. It is true that how well a plant lives up to its full potential is determined by environmental factors, such as soil nutrients. However, a plant with genetics adapted to these factors can outperform one bred for higher yield (typically a commercial hybrid with a short history) under the same conditions.
With this in mind, we could roughly formulate maximal plant health and yield as follows:
Adapted Plant Genetics + Minerals + Soil Community = Optimal Harvest
To reap the most abundant harvest, we must start with plants that have been selected for generations for their adaptation our circumstances (including climate, hours of sunlight, soil type, pest and disease pressure, etc.). Then we must provide our plants with the minerals they need to thrive and the soil microbial communities they need to access and use these minerals.
A holistic viewpoint is usually the surest path to the best results.
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