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.
If seeds are sown and lie on a bare soil surface, in the absence of precipitation, sun and air movement dry the seeds even if they were previously watered. Moisture, that a germinating seed needs constantly, cannot reach the seed. Water that migrates to the surface evaporates. Under these conditions germination can fail completely. To properly germinate, seeds should be surrounded by moist soil.