Primary productivity can be constrained by whichever resource is least available relative to biological demand.

A limiting nutrient is a nutrient whose scarcity restricts biological production even when other required resources are abundant.

For example, a lake can receive abundant sunlight and contain plenty of dissolved carbon, yet phytoplankton growth may remain low if phosphorus is scarce. Adding more carbon would have little effect, while adding phosphate could increase production strongly.

This idea is related to the law of the minimum: growth is constrained by the most limiting requirement, not by the total abundance of all resources combined.

But real ecosystems can be more complex than one permanently limiting nutrient. Different organisms can be limited by different resources, and limitation can shift seasonally or spatially. Two resources can also co-limit production when adding either one alone has little effect but adding both produces a large response.

Terrestrial productivity is often influenced by nitrogen, phosphorus, water and temperature. Aquatic productivity can depend strongly on nitrogen, phosphorus, iron, light and vertical nutrient transport.

A useful experimental test is nutrient addition. If adding nutrient $X$ increases growth while adding nutrient $Y$ does not, $X$ is evidence for the stronger limitation under those conditions.

The key principle is

$$\boxed{\text{productivity depends on resource availability relative to biological demand}.}$$

Nutrient cycles therefore matter ecologically because they determine not only how much of an element exists, but how much is present in biologically available forms at the place and time organisms need it.