Which pores stay filled longest after rainfall?

• A. Macropores
• B. Mesopores
• C. All equally
• D. Micropores

Answer: (D)

The key idea is how pore size affects drainage after rain. Water in large pores drains out quickly under gravity, so macropores don’t stay filled for long. Water in mid-sized pores drains more slowly, but eventually many of those pores lose their water too. Water in micropores, however, is held tightly by the soil surface through capillary forces and adsorption. That strong attraction keeps water in these tiny openings even after rainfall ceases, so micropores stay filled the longest. (Note: that water is not readily available to plants due to the high suction, but it remains in the pores for a longer time.)