Plant Nutrition: Meaning, Elements, Forms and Nutrition

After reading this article you will learn about:- 1. Meaning of Plant Nutrition 2. Essential Elements in Plant Nutrition 3. Forms of Plant Nutrients 4. Classification of Plant Nutrients.

Meaning of Plant Nutrition:

“Plant nutrition” is defined as the supply and absorption of chemical compounds required for plant growth and metabolism. It is the process of absorption and utilization of essential elements for plant growth and reproduction.

“Nutrients” may be defined as the chemical compounds required by an organism. The mechanisms by which nutrients are converted to cellular material or used for energetic purposes are known as “metabolic processes”. The term “metabolism” encompasses the various reactions occurring in a living cell in order to maintain life and growth. Nutrition and metabolism are thus very closely inter-related.

The essential nutrient elements required by higher plants are exclusively of inorganic nature. This exclusive requirement of higher plants for inorganic nutrients basically distinguishes these organisms from man, animals and a number of micro-organisms which additionally need organic foodstuffs.

For an element to be considered an essential plant nutrient, three criteria as proposed by Arnon and Stout (1939) must be satisfied.

These are:

1. A deficiency of the element makes it impossible for the plant to complete its life cycle.

2. The deficiency is specific for the element in question.

3. The element is directly involved in the nutrition of the plant as for example as a constituent of an essential metabolite or required for the action of an enzyme system.

Nicholas advanced the term “functional or metabolic nutrient” to include any mineral element that functions in plant metabolism, whether or not its action is specific. This definition avoids the confusion that sometimes occurs when the more rigid criteria of essentiality are imposed.

Several terms namely, deficient, insufficient, toxic and excessive are commonly used to describe levels of nutrient elements in plants.

Deficient:

When an essential element is at a low concentration that severely limits yield and produces more or less distinct deficiency symptoms. Extreme deficiencies will lead to death of the plant.

Insufficient:

When the level of an essential plant nutrient is below that required for optimum yield or when there is an imbalance with another nutrient symptoms of this condition are rarely observed.

Toxic:

When the concentration of either essential of other elements is sufficiently high to inhibit plant growth to a great extent. Severe toxicity will result in death of plants.

Excessive:

When the concentration of an essential plant nutrient is sufficiently high to result in a corresponding shortage of another nutrient.

Essential Elements in Plant Nutrition:

Previously there were 16 essential elements for the plant nutrition, but recently the number of essential elements for plant nutrition has been increased to 20, although more than 90 elements can be absorbed by plants.

Recently the following nutrient elements are known to be essential for higher plants:

Sodium, Si, and Co has not been established as essential elements for all higher plants. Chlorine has been found to be an essential element for all plants and that has been established by Broyer et al. (1954). Vanadium has now also been established as an essential element for some micro-organisms.

Forms of Nutrient Elements Absorbed by Plants:

Plants absorb nutrients from the soil as:

(i) Single nutrient element, uncombined with other nutrients and

(ii) Essential nutrient element, combined with other elements to form nitrate, phosphate, sulphate etc.

(i) Absorb as a Single Nutrient Element:

(ii) Absorb as a Combined Form:

Thus, mineral nutrients are available to the plant both in ionic and molecular forms, the proportions depending on the nature of soil solutions and the conditions.

Classification of Plant Nutrients:

The plant nutrients may be divided into macro-nutrients (Primary and Secondary nutrients) and micro-nutrients. Macro-nutrients are found and needed in plants in relatively higher amounts than micro-nutrients.

This above division of the plant nutrients into macro- and micro-nutrients is somewhat arbitrary and in many cases differences between the contents of macro-nutrients and micro-nutrients are considerably less well defined. As for example, the Fe or Mn content of plant tissues is sometimes nearly as high as the content of S or Mg.

From a physiological point of view it is difficult to justify the classification of plant nutrients into macro-nutrients and micro-nutrients depending on element concentration in plant tissues. Therefore classification of plant nutrients according to biochemical behaviour and physiological function seems more appropriate. So the classification of plant nutrients is shown below in Table 20.2.