Answer: Commercial soil block mixes may have some kind of binder, but I'm afraid I don't know what it is. Some mixes include peat humus (sometimes called "peat dirt"), which is peat that has decayed to the point where the individual particles are more akin to soil than moss fibers. Others contain up to 1/4 of their volume of screened, pasturized compost. Organic matter has "sticky" properties that holds inorganic particles together.
Some gardeners have been successful using unflavored gelatine as a binder for their soil blocks!
In order for blocks to hold together, be sure that the mix is well moistend by letting it sit for a few hours to absorb as much water as possible. Then pack the blocker firmly. The best way to water soil blocks is to place them on capillary matting so you don't have to water them from above.
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