Effect of rainwater on soil: Rain water helps to increase the ______ to some extent. Consider natural nitrogen fixation in the atmosphere and deposition to soils. Fill the blank with the most appropriate soil property.

Given data

• Blank asks what rainwater increases to some extent.

Concept / Approach
Lightning and atmospheric processes convert N₂ to oxides of nitrogen that form nitrates/nitrites, which are deposited by rain, slightly increasing soil nitrogen and thus fertility.

Elimination
Rain does not generally raise heavy metals or salinity; pH effects vary with local conditions and are not universally increased.

Final Answer
Soil fertility (nitrogen content).