What Stops Growth Liebigs

Considering the generalized growth and development phenomenon, we have to ask the question: Why does it not continue indefinitely? What stops growth? Why do the trees not grow to the sky, bacteria not fill the oceans, etc.? The answer is that the elements needed to build the biomass of various species are present only in a limiting amount in the environment/the ecosystems. Liebig's law considers these interactions between the organisms and their growth on the one side and the concentrations of nutrients on the other side. The elementary composition varies from plant to plant and from organism to organism and even the same species may have a different composition in different environments and at different times of the year. There are, however, some basic biochemical processes that require a combination of elements in a certain ratio.

The biochemistry and the biochemical reactions for all organisms on Earth are very similar, although they vary slightly from organism to organism. About 20 elements are necessary to build an organism, sometimes with the addition of one to five more elements. Table 1 shows an example of the elementary composition of freshwater plants. The relative quantities of the 19 essential elements in plant tissue are shown. The composition is not strictly stoichiometric, as is that of a chemical compound, but may vary between lower and upper values following the composition of different biochemical compounds in the cells. For instance, phyto-plankton can contain approximately 0.5-2.5% of phosphorus and 5-12% of nitrogen by dry weight.

The required elementary composition of an organism reflects the chemical constraints for growth. Growth implies that the environment delivers the elements - for plants as suitable compounds dissolved in water in contact with the plants, and for animals by the composition of available food. The environment rarely has exactly the same composition as required for growth, which implies that the element less abundant in the environment compared with the need determines the limits to growth. This is expressed in Liebig's law of the minimum (see Figure 1); if a nutrient is at a minimum relative to its use for growth, there is a linear

Table 1 Average freshwater plant composition on wet basis

Element Plant content (%)

Oxygen

80.5

Hydrogen

9.7

Carbon

6.5

Silicon

1.3

Nitrogen

0.7

Calcium

0.4

Potassium

0.3

Phosphorus

0.08

Magnesium

0.07

Sulfur

0.06

Chlorine

0.06

Sodium

0.04

Iron

0.02

Boron

0.001

Manganese

0.0007

Zinc

0.0003

Copper

0.0001

Molybdenum

0.00005

Cobalt

0.000002

Figure 1 Illustration of Liebig's law. The phosphorus concentration is plotted against the yield. At a certain concentration another component will be limiting, and a higher P concentration will not increase the yield. The three levels A, B, and C correspond in this case study to three different potassium concentrations.

Figure 1 Illustration of Liebig's law. The phosphorus concentration is plotted against the yield. At a certain concentration another component will be limiting, and a higher P concentration will not increase the yield. The three levels A, B, and C correspond in this case study to three different potassium concentrations.

relation between growth and the concentration of the nutrient. If the supply of other factors is at a minimum, further addition of the nutrient in question will not, as shown, influence growth.

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