The seed needs an adequate and constant supply of moisture, but should not be laying in a pool of water. Dry soil mixes tend to filter out of the bottom of the trays and leave air spaces. Therefore, the remaining soil in these cells will not come in contact with the water and dry cells will result. Sometimes, it is necessary to add water or dry bags of mix to adjust to proper soil moisture. Anytime the moisture is adjusted, through mixing is essential.
Soil density is important because it affects the water and air relationship in the soil.
There proper amount of water and air is necessary for germination. Over packed soil wicks to much water, leaving no airspace in the soil. The result is inhibited or slowed germination. If trays are filled too lightly, the soil will not wick enough water for germination. Compacted soil is the main cause of the spiral root problems.