When the ground is wet, putting the seed inside, makes the seed germinate well. Realized this, when browntop millet was put and tilled using a fork,same thing was done with cowpea and that also came up well.
But sesame seedball was a failure, and was thinking if seeds experience the same atmosphere as that of underneath the soil, then they should germinate. Seedballs sown at surface, dries up faster. One is to use mulching, but heavy mulching will be a problem for small seeds since they can not come up pushing. So it is important that seedballs when wet, retain moisture well so that they can germinate well.
While searching for the soil used for seedball, came to know that clay soil retains water maximum. Some information from the internet.
The ability for a soil to retain water is partly determined by the size of the soil's particles. The smaller a soil's particles are, the greater the soil's surface area is, and so the more water the soil retains. Clay soil has small, fine particles, which is why it retains the most amount of water. Sand, with its larger particles and low nutritional content, retains the least amount of water, although it is easily replenished with water. Silt and loam, with medium-size particles, retain a moderate amount of water.
The amount of organic matter in soil also affects how much water the soil is able to retain. This is because organic matter has a natural attraction to water. So the more organic matter a soil contains, the greater the affinity it has with water. Clay soil is very rich in organic matter while sandy soil has very little.
Another idea is to use charcoal?
http://www.seedballskenya.com/seedballs/4593024001 - This site says, A Seedball is simply that - a seed inside of a ball of charcoal dust mixed with some nutritious binders. Once soaked, the seedball will help retain and prolong a moist environment around the seed to encourage germination.