If you want to use soil as a substrate, it can be done using cheap top soil. This would be known as a Natural Planted Tank. You can find more information on this type of setup and peoples' experiences with it here:
Both Eco-Complete and fluorite are said to grow plants well.
SMS is said to really lower KH levels. Some say that if you use it in a low tech setup then this is less likely to happen, but I don't see how that can be considering that water changes usually replenish KH, so in a low tech tank where you are not doing water changes that frequently, if anything the KH is more likely to get sucked up quicker.
The reality is that you can pretty much grow plants in anything, including but not limited to Laterite capped with chicken grit, pool filter sand over laterite and or root tabs, Schultz Aquatic Soil, kitty litter, etc.,
The difference is that some substrates tend to grow plants better than other, at least based on what most people are reporting. ADA Aquasoil I and II seem to be the talk of the town. However, Tom Barr was kind enough to point out to me that if you use ADA Aquasoil, you have to stay on top of your c02 levels and ensure that the c02 levels remain consistent. You are not likely to do this for a low tech tank, at least not at the rate needed to prevent problems with ADA Aquasoil, so I would not suggest it for a low tech tank, although if you are brave enough, you can always experiment to see what happens.
My suggestion is to go with a Natural Planted Tank and use Top Soil capped with gravel(you cannot get any cheaper than this), or if the thought of using top soil scares you use a tried and proven inert substrate like fluorite(you can use it over root tabs) for a low tech setup.
You might find this link interesting.