Gravity causes water to collect at its lowest point. Water rises by capillary action as soil dries. Seeds that are sown at bare soil surface are dried out by sun and wind. Water evaporates as it reaches the surface. Seeds are rolled into soil.
Top dressing protects seed from sun and wind and allows air circulation.
Rising water stops at a boundary between soil and top dressing. In the next figure, the rain or irrigation has stopped. The soil has begun to dry out. Capillary action begins as the tiny channels formed by the fine soil particles regain their porosity. Water rises toward the surface of the soil.