Growth and Development of Horticultural Crops (1+1)

Limitation of growth factors

a) Liebig law of minimum

• This concept was formulated by German chemist Justus von Liebig, often called the “father of the fertilizer industry”. This is an example of Liebig’s Law of the Minimum, which states that plant growth will continue as long as all required factors are present (e.g. light, water, nitrogen, phosphorus, potassium etc.). When one of those factors is depleted, growth stops. Increasing the amount of the “limiting” component will allow growth to continue until that component (or another) is depleted.
• The nutrient most typically “limiting” algae growth in lakes is phosphorus. If phosphorus concentrations can be maintained, then algal growth would be controlled. In Mark Twain Lake, (the largest reservoir in northern Missouri), light is the factor that most often limits algae. (http://www.lmvp.org)
• A deficiency or absence of any one necessary component, when all others are present, renders the soil barren for crops for which that nutrient or factor is needed. It is also called as "barren concept".
b) Blackman's law of limiting factors (1905)

• The law states that “When a process is conditioned as to its rapidity by a number of separate factors, the rate of process is limited by the pace of the slowest factor”. To explain the principle of limiting factor, Blackman gave the following illustration which is also shown graphically in Fig. 3


Fig 3. Graphic representation of Blackman’s law of limiting factors
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• Suppose a leaf is exposed to a certain light intensity which allows the leaf to utilize 5 mg of CO2 per hour in photosynthesis. The photosynthesis will not occur if the CO2 is totally absent in the atmosphere. If one mg of CO2 is available, the rate of photosynthesis is limited due to CO2 factor. If the CO2 concentration is increased in the atmosphere from 1 mg to 5 mg / hour, the rate of photosynthesis increases along the line AB. Thus, the increase in photosynthetic rate will be proportionate with the increase in CO2 concentration up to 5 mg. Any further increase in amount of CO2, will not increase the rate of photosynthesis and the rate becomes constant along the line BC. This is because the light factor (low intensity) has now become the limiting factor.
  
• Addition of CO2 with increase in light intensity increases the rate of photosynthesis along the line BD. Under these conditions increase of CO2 will not increase the rate of photosynthesis. The rate becomes constant along the line DE. Here also the light intensity becomes the limiting factor. Further, increase in light intensity from the medium to high, increases the rate of photosynthesis along the line DF by adding CO2. When the rate attains maximum at F, further, increase in CO2 will not increase the rate of photosynthesis, which becomes constant along the line FG. It indicates that sunlight again becomes the limiting factor. Besides, light and CO2, other factors such as temperature, water, etc may also become limiting under certain conditions.
c) Mitscherlich: law of diminishing returns

• Mitscherlich (1909), a Soil Scientist observed that when plants had adequate amounts of all but one limiting element, the growth response is proportional to the limiting element, however plant growth increased with additional increments, of a limiting factor, but not in direct proportion. The law of diminishing return states that the "increase in any crop produced by a unit increment of a deficient factor is proportional to the element of that factor from the minimum". The response is curvilinear, instead of linear, as Blackman suggested.