It’s called Liebig’s barrel.
The contents of the barrel represent a crop’s yield potential, while each wooden slat represent a different nutrient required by the plant. The barrel is meant as a visual representation of Liebig’s Law of the minimum – a crucial concept when calculating the nutrient needs and yield potential of any field or garden.
Discovered by Carl Spengrel in 1828, and refined by Justus von Liebig, Liebig’s Law states that plant growth is ultimately determined, not by the total availability of resources, but rather by the most limiting factor.
In other words, the yield potential of all crops will be determined by the least available nutrient, as represented by the shortest slat in Liebig’s barrel. Even if high levels of nitrogen, phosphorus and potassium (i.e. NPK, the major nutrients) are present in the soil, the plant’s potential will still be capped by the least available nutrient.
Liebig’s barrel is particularly relevant to the lesser attended nutrients in the soil. While, NPK levels are carefully monitored by farmers and gardeners, the lesser known micronutrients are sometimes forgotten.
Micronutrients – as their name implies – are still needed by the plant. However, they are required in much smaller quantities than NPK. The eight most important micronutrients include: Boron, Chloride, Copper, Iron, Manganese, Molybdenum, Nickel, & Zinc. These nutrients play a supporting but vital role in plant growth; some nutrients like copper and manganese are directly involved in photosynthesis while others, like boron and nickel, are integral to chemical processes within the plant that produce various enzymes or break down compounds.
Because they are needed in such minute amounts they maintain the lowest bar on Liebig’s Law, it is still possible to produce modest yields with micronutrient deficiencies as highlighted by Liebig’s Barrel.
While these micronutrients are often naturally available in soils, successive years of farming can deplete the natural levels of these nutrients which means they too must be replenished. While organic nutrients like carbon and nitrogen can be supplied via natural process (e.g. legumes fix nitrogen from air), most micronutrients are metals (e.g. iron, copper, nickel) which means there is no natural way to replenish these nutrients. If the soil needs more copper, somebody has to put it there, mother nature won’t do it.
Most micronutrients bond with clay particles in a process known as cation exchange, which holds the nutrients in the soil and makes them available for plant uptake. As a result, micronutrient deficiencies are most common in slightly sandy soil where they leach into the environment.
In terms of plant health, micronutrients play a supporting role in plant development. However, adequate quantities of these nutrients positively affect the plant in a number of ways. Boron plays a key role in plant pollination, extending the flowering period, which directly boosts yield potential.
When applying Liebig’s Law to crop yield potential, the major nutrients (i.e. NPK) are rarely limiting factors. In fact, according to Liebig’s law, high levels NPK fertilizer may be wasted if plant growth is inhibited by a dearth of micronutrients.
By assuring that plants have access to all the micronutrients they need, farmers and gardeners alike will be able to increase the quality and quantity of their crops. For growers who are looking maximize their yield potential, micronutrients may hold the key.
Steenland, Ann., Zeigler, Margaret. (2018) “Global Agricultural Productivity Report” (PDF). Global Harvest Initiative. Washington, D.C.