Do you know what is the best soil for an avocado plant? Would it vary depending on the breed? What would be a good recipe for this soil (for growing indoors)?
Soil needs do vary a little between the three basic types, the Guatemalan, the Mexican and the West Indian; however, all three will tolerate most types of soil, but not wet feet; they prefer a light/ sandy potting compost (specially formulated for indoor plants) that is free-draining and will not take too long to dry a little between waterings (when it should be slightly damp to the touch but never wet). Garden soil, of course, is unsuitable for indoor plants, as it may not be sufficiently free-draining or balanced in nutrients, and also contains organisms that could thrive indoors and cause disease. A standard multipurpose potting soil, available from your local garden center should meet your avocado's needs, but I would try to avoid a peat-based one, as it loses moisture very quickly and, if allowed to dry out, is quite difficult to wet again.
As a long time avocado grower and fan of guacamole I was going to tell you that you can grow it in most soils. As with any plant proper watering and food will make a big difference. Since you are growing indoors I would suggest a quality potting soil that will have what the plant needs in the indoor environment. I would give it as much light as possible, preferably sunlight. Water only when the soil is dry and then water thoroughly. Feed during the growing season and do not fertilize over the winter. Can't guarantee any fruit, but you should have a nice plant to grow.
Most tropical plants enjoy a lighter soil mix to grow in. I started a navel orange tree over ten years ago and it is STILL thriving for me. I use two parts regular potting soil to one part sand or perlite. So far that does the trick for me. I had started an avocado a few years ago and it lasted for almost a year and then lost its leaves and died. I suspect back then I used a heavier potting mix. You need to allow oxygen to reach the roots so DON'T pack it down hard as well and add that sand or perlite. Hope this helps!