The factor that influences the availability of plant nutrients is

Soil pH is the single most important factor governing the availability of plant nutrients. The pH scale measures how acidic or alkaline the soil is, and this directly determines whether nutrients dissolve into the soil solution where roots can absorb them or become locked in insoluble compounds that plants cannot access.

Most essential nutrients reach their maximum availability in the pH range of roughly 6.0 to 7.0. When soil becomes too acidic (below about 5.5), elements such as phosphorus react with aluminium and iron to form insoluble compounds, while calcium and magnesium leach away. When soil becomes too alkaline (above about 7.5), iron, manganese, zinc, copper, and boron become increasingly unavailable because they precipitate as insoluble hydroxides or carbonates.

Sunlight, temperature, and relative humidity are climatic factors that influence plant growth processes such as photosynthesis, transpiration, and enzyme activity, but they do not directly control whether nutrients in the soil are chemically available for root uptake. Only soil pH governs the chemical solubility of nutrients already present in the soil.

Examination tip: When a question asks about nutrient availability rather than nutrient supply, the answer almost always involves soil pH, because availability refers to whether nutrients are in a soluble, absorbable form.