Primary productivity is defined as the rate of biomass production or the rate of capture of solar energy by the producers (plants) in an ecosystem. It is expressed in terms of weight (gm⁻²) or energy (kcal m⁻²) per unit area over a time period, typically as gm⁻² yr⁻¹ or (kcal m⁻²) yr⁻¹ to compare productivity of different ecosystems.

Primary productivity is divided into two types: Gross Primary Productivity (GPP) and Net Primary Productivity (NPP). GPP is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration. NPP is the remaining biomass after respiration losses (R), represented as: GPP - R = NPP. NPP is the available biomass for consumption by heterotrophs (herbivores and decomposers).

The factors that affect primary productivity include:

1. The plant species inhabiting a particular area
2. A variety of environmental factors
3. Availability of nutrients
4. Photosynthetic capacity of plants

Due to these varying factors, primary productivity differs in different types of ecosystems. The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter. Notably, oceans occupy about 70% of the Earth's surface but contribute only 55 billion tons to productivity, with the rest coming from land.