Water reaching the seeds then depends on water passing downward from irrigation or rain. There are several problems with germinating seeds this way. The high water use is not only wasteful, it's impractical. Overwatering will also leach nutrients out of the soil, nutrients that are necessary to starting a good growth. In some areas where soil and water are high in salts, excess water will build up the salts in the soil.
Salty soil hinders the seed's ability to take up water through osmosis.
The fact that capillary action and molecular adhesion are stopped by coarse material can be put to use in helping seeds germinate. The bottom figure shows an ideal condition. The seeds are sown and pressed into the bare soil using a roller. A coarser top dressing is laid over the seeds. Water Works Wonders Water passes down through the top dressing and seeks its lowest level, wetting the seeds as it passes.