Young beetroot plants growing in expanded clay balls. As they grow, the larger beetroot are harvested early to leave room for the rest of the beetroot plants to fill out.
I have grown many successful crops of beetroot by sowing them directly into a bed of expanded clay balls, but it is best to soak the seeds overnight in warm water beforehand. They can be sown at almost any time expect for the hottest and coldest times of the year in a greenhouse in a temperate climate. The seeds are generally inserted into the growth medium just above the level of the nutrient solution and spaced every few centimetres.
When the seedlings emerge they can be lifted from among the balls and redistributed more evenly if desired. They grow quickly and in order to make the best use of the space I gradually thin them out as they grow. I can generally obtain several dozen large beetroot from a small area this way, harvesting them several weeks apart.