All the natural resources can be divided into two categories:
(i) Exhaustible natural resources (ii) Inexhaustible natural resources
Exhaustible natural resources are soils, forests, water, coal, petroleum, natural gas, minerals etc. These are consumed or exhausted through continuous use or misuse. Exhaustible natural resources can be further divided into two-
(a) Renewable natural resources
(b) Non-renewable natural resources
Inexhaustible natural resources are those which cannot be exhausted through continuous use or misuse eq. air and sunlight etc.
The natural resources which are consumed/exhausted/depleted through continuous use and can be recovered by very hard efforts taken up for long periods are called Renewable Resources, for e.g. Soils, forests, groundwater etc. In other words we can say that all renewable resources are replenished through natural cycles or manually. For example oxygen in air is replenished through photosynthesis. 'Forest is maintained themselves and manually. Similarly fresh water is available through cycles & manually too.
Most of the removable resources are interdependent to each other. Forests maintained the environment/climate; plants need to check soil erosion & soil is needed for plants. Air and insects are needed for pollination. Wood, fibre, fodder, fruits, vegetables, milk etc. are developed directly or indirectly by recent photosynthetic activity.
Thus the renewal of these resources will continue at as long as photosynthesis continues on this planet. These resources are the life support system which can fulfill all human needs. But its productivity/renewability is limited or depends upon availability of water, nutrients and environmental conditions.
The natural resources are useful to human society in one way or other. Hence we should ensure a continuous yield of useful plants, animals and materials by establishing a balanced cycle of harvest and renewal (Odom - 1971).
Non renewable resources are not replenished able or we cannot get back our coal and petroleum reserves in our life time, if ones they are consumed/exhausted completely. Non-renewable resources are metals (iron, copper, zinc etc.), coal, oil deposits, minerals, stone, mineral, salts (Phosphate, nitrates, carbonates etc.) etc.
Minerals are often called the 'STOCK' resources, because their new materials can only be extracted from the earth's crust once. But even in the transformed state in which they are used, they are not lost to the planet and so are ideally available for reuse. Metals/minerals come from a very slow process of geo-chemical concentration, which took millions of years to form. Therefore, these deposits which occur today can disappear at some point of time in future.
Coal, petroleum and natural gas are called as 'FOSSIL FUELS' because they are formed from dead remains of plants and animals buried in the earth long ago. They are called fuels because they are burnt to give off energy. Coal have a high heating value, hence it is a useful fuel. Since fossil fuels are non-renewable sources of energy, therefore it is essential to explore more and more alternatives. Today we are utilizing solar energy, wind energy, ocean, geothermal & atomic energy as alternative sources of energy.
Minerals, rocks, salts and chemicals etc. are termed as 'a biotic resources', as biological activity is not involved in their formation. Continuous over exploitation of these resources shall exhaust many of our valuable deposits, which took millions of years to form. They cannot be duplicated within the human scale of time.