The amount of nutrients, such as carbon, nitrogen, phosphorus and calcium present in the soil at any given time, is referred as :

The correct answer is Standing state.

Step-by-Step Explanation:

1. Understanding the Concept of Standing State:
In an ecosystem, the soil acts as a reservoir of various nutrients that are essential for the growth and development of plants and other organisms. The total amount of inorganic nutrients, such as carbon, nitrogen, phosphorus, calcium, and others, present in the soil or ecosystem at any given time is defined as the standing state (or standing state of nutrients). This state is dynamic and varies in different kinds of ecosystems and also on a seasonal basis.

2. Analyzing the Other Options:
- Climax / Climax community: These terms refer to the final, stable, and self-sustaining community in an ecological succession that is in near equilibrium with the climate of the area. It represents a biotic stage, not the nutrient content of the soil.
- Standing crop: This term refers to the total amount of living biomass or organic matter present in an ecosystem at a given time, usually expressed in terms of fresh or dry weight per unit area. While "standing state" refers to non-living (inorganic) nutrients, "standing crop" refers to the living (organic) matter.

3. Conclusion:
Since the question specifically asks for the amount of nutrients (inorganic substances like carbon, nitrogen, phosphorus, calcium) present in the soil at any given time, the correct term is Standing state.