From Ecology Action
We received this question from a Biointensive educator: People sometimes ask why we don't use compost tea to boost the fertility in flats instead of pricking them out into flats of new fertile soil. What are the disadvantages to this? Also, a common question is why do we not just plant the kale in a deep flat on larger centers to being with, rather than starting in a shallower flat and progressing to a deeper one. I usually say because there is more fertility in a fresh 6 inch deep flat than if the kale had been growing there since it was a seed and also the transplanting stimulates root growth. Is there anything else I should be telling them?
In GROW BIOINTENSIVE agriculture, the method of growing plants from seed is to plant the seeds in a shallower flat, allowing them to germinate, and then pricking the resulting seedlings out to a deeper flat where they are allowed to grow until it is time to transplant them into the garden bed. Many people wonder why this is...for example, why not put lettuce on 1 1/2" centers in a 3"-deep flat initially, rather than broadcasting them in one flat close together, and then pricking them out onto 1 1/2" centers in a second 3"-deep flat? The answer is, because it works better!
The efficacy of the pricking out technique was tested several times here at Ecology Action by a GB Certified Teacher candidate, who did not believe that all the labor involved in pricking out seedlings was actually necessary. However, when this person compared final yields for lettuce planted according to the GB standard method of seeding, pricking out to a deeper flat, and then finally transplanting to a bed vs. seedlings grown without the intermediate pricking out step, in each comparison, the lettuce yield for the GB standard method was double. Why? There are probably several factors involved, but the short answer is: I do not know, but it works. If you are working with people who still don't believe it, tell them that they can always check this out by testing it both ways themselves.
As to the compost tea, I have never comparatively tested the two possibilities, but I can see why people ask. Your point about there being more fertility in the deeper flat is relevant. Also, I believe that by placing the seedlings into a deeper flat at the pricking out stage, you are providing more air in the new soil, allowing them to grow better, and allowing the roots to spread and grow naturally. If the seedlings were instead kept in one flat and watered with compost tea, they would receive additional nutrients from the tea, certainly; however, since the soil in the seedling flat mix is already very fertile the additional nutrients in the compost tea may not provide significant benefits, and if the plants stay in a single flat the whole time, when they achieve the size necessary for the transplanting stage, the soil will be comparatively compacted; this will affect the ability of the plant roots to grow and spread once they are in the deeper soil of the garden bed.