Liebig suggested imagining a barrel being filled with water as a
metaphor for plant growth: the amount of water held in the barrel
being the amount of growth. Each stave represents one of the factors
or requirements plants need in order to grow such as light, water,
oxygen, nitrogen, phosphorus, copper, boron, etc. Lowering any one
stave of the barrel, no matter which one, lessens the amount of
water that can be held and thus growth is reduced to the level of
the most limited growth factor.

For example, one essential plant protein is called chlorophyll, the
green pigment found in leaves that makes sugar through
photosynthesis. Chlorophyll is a protein containing significant
amounts of magnesium. Obviously, the plant's ability to grow is
limited by its ability to find enough fixed nitrogen and also
magnesium to make this protein.

Animals of all sizes from elephants to single cell microorganisms
are primarily composed of protein. But the greatest portion of plant
material is not protein, it is carbohydrates in one form or another.
Eating enough carbohydrates to supply their energy requirements is
rarely the survival problem faced by animals; finding enough protein
(and other vital nutrients) in their food supply to grow and
reproduce is what limits their population. The numbers and health of
grazing animals is limited by the protein and other nutrient content
of the grasses they are eating, similarly the numbers and health of
primary decomposers living on the forest floor is limited by the
nutrient content of their food. And so is the rate of decomposition.
And so too is this true in the compost pile.