Question

To the plant, the soil is not the source of:
(a) N
(b) K
(c) Fe
(d) H

Answer 1

Hint: The element for which soil is not the source is considered one of the most important nutrients and comes under the category of macronutrients i.e. it is required in large quantities by the plants. It is the third most abundant element in the cells.

Complete Answer:
Hydrogen (H) is the element that is not derived from the soil but instead by the splitting of water during photosynthesis. This process occurs inside the thylakoid of chloroplast with the help of manganese and chlorine. It leads to the evolution of ${ O }_{ 2 }$ gas, and the hydrogen is utilized by the plant to form many organic compounds required by it. Hydrogen along with carbon, oxygen, and nitrogen make up most of the constituents ($99\%$) of a cell.

Additional information: Let us look at the other nutrients mentioned in the options.
Nitrogen (N): It is the nutrient that is absorbed by the plants from the soil in the greatest amount. It is mainly absorbed in the form of nitrate ions ${ { NO }_{ 3 } }^{ - }$ and is required by all parts of the plant. It is greatly required by the meristematic tissues and metabolically active cells as well. Nitrogen makes up many organic compounds such as proteins, nucleic acids, vitamins, and hormones.
Potassium (K): It is absorbed in the form of potassium ion ${ K }^{ + }$ and is most needed by meristematic tissues, root tips, and shoot tips. Its major function includes the anion- cation balance of the cell thus affecting the opening and closing of stomata.
Iron (Fe): It is absorbed in the form of ferric ions ${ Fe }^{ 3+ }$ and is required in larger amounts as compared to other micronutrients. Due to the presence of two oxidation states, it is mostly used in redox reactions. It also acts as a catalyst for the synthesis of chlorophyll.
So, the correct option is ‘(d) H’.

Note:
- Macronutrients are defined as nutrients required by plants in quantity more than 10 mmole ${ Kg }^{ - 1 }$ and it consists of 9 elements.
- Micronutrients are defined as nutrients required by plants in a very small amount i.e. less than 10 mmole ${ Kg }^{ - 1 }$ and consist of 8 elements.
- Carbon and oxygen are obtained from ${ CO }_{ 2 }$ gas.