Question

In an average soil the percentage of organic matter is about
A. 20
B. 30
C. 45
D. 5

Answer 1

Hint:- Organic matter consists of organic compounds for Carbon-containing compounds. The source of organic compounds in the soil is the decaying plants and animals. The excretory products of animals and plants are also organic in nature. The major organic compounds are lipids, carbohydrates, proteins, nucleic acids. The cells of living organisms are made up of these organic compounds and they decompose into the soil after the death of the organism thereby making the soil rich in organic matter.

Complete solution: Soil contains different types of materials. It is a heterogeneous mixture. It consists of several organic and inorganic matters like minerals, water, soil, living and dead plants, animals, etc. The ratio of different components in the soil varies according to the climate, availability of water, precipitation rate, etc. of the area. A major part of the soil consists of minerals. Minerals like phosphorus, nitrogen, and potassium are seen in greater amounts while calcium, sulfur, magnesium are seen in lesser amounts. Minerals are formed by the weathering of rocks. Soil also contains water. The amount of water in the soil depends upon the topographic and climatic conditions of the area. Soil is porous. The porosity is due to the air trapped between the soil particles. Organic matter is seen in a comparatively lesser percentage in average soil. Organic matter comprises mainly the dead and decaying remains of plants and animals. They are called organic matter because they are carbon-containing compounds. The breakdown of the complex organic substances is done by detritivores like earthworms, and microorganisms like bacteria and fungi. The complex organic substances undergo fragmentation, humification, and mineralization. The organic substances are finally converted into inorganic nutrients by certain microbes, therefore the percentage of the organic substance is very less in average soil. They have been seen to be approximately 5 percent in the soil. Minerals are seen at approximately 45 percent and the remaining percentage comprises air and water.

The correct answer is option (D), 5 percent.

Note:- Decomposition is the process of breaking down complex organic substances into simpler inorganic substances like carbohydrates, nutrients, minerals, and water. The raw materials like decaying plants and animals are called detritus. Detritus undergo fragmentation, which is the breaking down process. Then it becomes humus, which is a dark-colored, colloidal substance which is difficult to be decomposed. The humus finally undergoes mineralization which is the conversion into minerals.