Which of the following is most likely to cause a soil pH increase?

Irrigation can lead to an increase in soil pH under certain conditions, particularly in areas where the irrigation source is alkaline or has a higher pH level than the existing soil. When water with a higher pH is applied, it can gradually elevate the soil pH by leaching out acidic components and bringing in more alkaline minerals, such as calcium and magnesium, which are often dissolved in the irrigation water. This process may not be universal for every irrigation scenario but can occur, especially in regions where the water management practices involve the use of well water or surface water that is not neutral.

In contrast, fertilization typically affects soil pH by introducing acidic or neutral salts, such as ammonium-based fertilizers, which can lower soil pH over time. Heavy rainfall generally contributes to soil acidification by leaching away basic cations and allowing more acidic components to dominate the soil chemistry. Lastly, the addition of organic matter usually results in a decrease in soil pH as organic materials decompose, producing organic acids that lower the pH. Therefore, irrigation is the most likely practice from the options provided to cause an increase in soil pH.