I want to talk about a seemingly large topic, sustainability. I am going to, as the academics say, "recontextualize" the term for you. I'm going to make you rethink the whole thing. Sustainability first and foremost means "don't shit where you eat". Simple enough. From there, it means making decisions that will benefit you not only in the present, but also in the future. Sustainability is an investment mindest. How much can you manage down your needs today, so that you have something set aside for tomorrow? How much work can you do now, that will benefit you later?
What does that mean in tangible terms? Well in terms of a homestead it's pretty clear. For starters, literally don't spread your poop in your garden. Orrr if you want to, its got some great nutrients, you're going to need to compost it one way or another for probably about a year. We can get in to the details later. But not all shit is literal shit. If you move to a piece of land and haven't done a soil and water analysis, you may find the water you are drinking and the food you grow is filled with things like arsenic or lead.
So we've mention you want your food and water supplies to be sustainable. So the first step is making sure they are clean before you use them. But the key after that is making sure you don't ruin them going forward. It's also not just about ruining them, but what about using them up? If the well runs dry, was it sustainable? That is something that again, is helpful to figure out up front. Does it produce less water than you will use, over a given time period, say a year. So part of sustainability is having a plan, thinking ahead. It often means throwing together a little calculation, maybe a spreadsheet, that you can put in daily water requirements and figure out how quickly it replenishes itself vs how much you use. And you will need to account for drinking, showering, perhaps flush toilets, or garden/crop irrigation. How much of a reserve are you comfortable maintaining in case of something like a drought year?
Energy usage falls in to this as well, particularly if you decide to focus on being off grid. If you have solar you need to understand how to size the system, how much energy you need, how much you will be able to generate in various conditions, how much you can store.
You can run out of water if you mismanage it, but can you run out of dirt? It would seem not by looking at it, but if you consider soil health things start to look a little different. Modern, conventional farming is the opposite of sustainable. Before mass pesticides and fertilizers, farmers used to rotate which crop was in a given field every year. Organic farms now use a 5 crop rotation, with each one returning something back to the soil that will be required the following year. This has the side effect of reducing costs, reducing pests and disease because they lose their food source each year. It is the definition of sustainable.
Of course it has to be sustainable for you, not just the land. So you need to figure out the balance of growing the crops you need to survive and thrive, not just what makes the soil happy. It's a balancing act, and its not the simplest way to get started. But once you figure it out it is sustanable over time, something that nuking fields with chemicals is not, setting aside the potential health effects of doing so.
Conclusion
All life is either sustainable, or on its way to extinction. Viruses that quickly kill their hosts before they spread aren't very successful. We sometimes forget because so many of us live such insulated lives, but if you want to live off the land never forget; don't shit where you eat.