According to Doreen Warriner, a primary problem with subsistence farming economies is inadequate capital accumulation. Capital accumulation "refers to the gathering or amassing of objects of value; the increase in wealth through concentration; or the creation of wealth."
I can only understand her assertion to mean that the surplus of labor in subsistence farming households or communities doesn't amount to enough to form a viable buffer against disaster.
How much capital do you need?
Whether or not a buffer is adequate depends entirely on the context. In a non-competitive primitive society, having one full grain silo mortared into a canyon wall might be enough to get through a bad year. Where there is an ongoing risk of raids by a competing society, an adequate buffer might include a class of warriors supported largely by the women's agricultural efforts.
In contemporary society, it's hard to know what's necessary, and competitors tend to keep pace with each other just to feel secure. During the cold war, two competitive societies poured tremendous amounts of surplus capital into developing expensive nuclear arsenals and accumulating arms just in case. In the Soviet Union, they managed to extract this surplus of labor from a relatively impoverished society. In a hypothetical permanent colony on Mars, basic subsistence would involve a lot more than just growing food and making clothes, so there would presumably need to be very efficient mechanisms in place for taking care of these things so ample additional time could be freed up for troubleshooting life support systems. For the colonists, your surplus doesn't even begin until a lot more in the way of basic needs have been met--your subsistence is dependent on a lot more accumulated capital than a hunter-gatherer, and your surplus labor goes towards ensuring the continuity of this essential capital before you can begin to accumulate a buffer. (Note: this is just an exaggerated version of the difference between hunter-gatherer and agricultural societies--which I delve into more in part 3--in that farmers need more capital than h-gs because the surrounding environment cannot provide for their needs by itself...in other words, they can rely less on ecosystem services.)
A lot of our notions around self-sufficiency and idyllic, rural hamlet economies in the developed world arise in the context of massive capital accumulation occurring all around us. There is no village or inner city in the U.S. that is not impacted by the vast amount of capital accumulation in our society as a whole. For example, a family homesteading in rural Eastern Washington has a truck, a grain mill, a wood stove, etc. Where do these objects come from? They probably didn't make them. Even if they found the grain mill second-hand and fixed it up, it's existence in the pawn shop is still a result of massive capital accumulation in the surrounding economy. It is a relic of that economy and it's value (in this case: cheap) arises in that context--there are enough floating around out there (high supply), and not that many people want them right now (low demand).
As one example of differences in value relative to collective capital accumulation, my neighbor consistently puts old wood out on the curb with a FREE sign on it. Sometimes it sits there for days. I can promise you that wood would be gone in an instant in the highlands of Vietnam, or anyplace in the world where people routinely walk several miles to gather firewood, which are the same places that are poor, and without mass amounts of capital accumulation leaching out of the cities into the countryside. So, in America and all the developed world we are awash in material goods, which makes it easy to take for granted the value of capital accumulation...which we harvest, in the homesteading community, more or less as scavengers. (Quick note: there is nothing wrong with this; it is a very reasonable and ethical choice to make, but it is not a viable model for a sustainable economic structure, and ultimately that's what I'll be exploring here.)
That said, it is also abundantly clear that the distribution patterns of capital accumulation in our society have resulted in gross inequalities that are not the product of differences in how hard people work. Moreover, this has almost always been the case when agriculture yields a surplus. It's never been the productive farmers that call the shots, but those that live off the surplus of their productivity, e.g. the aristocrats taxing heavily for "protection" services. Jeff Vail's Rhizome Theory demonstrates how dependency which begins with the desire to pool risk by coordinating capital accumulation results in (a) heirarchy, which (b) drives unsustainable competition between social actors that (c) results in resource destruction and oppression.
But of course capital accumulation is necessary for a secure society and decent standard of living! So the big question is: how do we accumulate capital without the associated inequalities and deprivations? Here begins the thought experiment.