You're getting a lot of responses that answer some of the questions you're asking, but not really addressing the whole picture. There are a lot of techniques and tools you could use to figure out when and how much fertilizer to add, but all of it misses the point. Fertilizer is a band-aid, addressing (usually inadequately) shortcomings of a perverted system, and so are pest-/herb-/fung-icides.

The last question you asked is the key: gardens and farms don't need to replace their soil because they are "feeding" it with organic matter. The organic matter is animal manure, plant litter, or compost (which is... aged animal manure or plant litter). Bacteria and fungi eat that organic matter, larger organisms eat the bacteria and fungi, and plants operate in relation to all of them to uptake the nutrients they unlock from the previously living material. In natural systems, those dead plants feed the next generation in a relatively closed loop.

I've developed something of an obsession with all this recently as I learn through my own garden, and my goal this year is not to use any fertilizers (though I may if I think it's needed), no herbicides or fungicides, and only very little & very selectively applied organic pesticides.

Container gardening can be a bit of a special case, because it's difficult to have a complete ecosystem on a balcony, but integrating organic matter will replenish that too provided you have the soil organisms to do the work. Which is fairly likely.

In gardens and farms you'll usually amend the soil not only with fertilizers but also with as much organic material as you can sensibly get. This way a healthy soil ecosphere is created that will take care of a lot of the soil depletion and pests issues.

On a balcony usually the containers are too small and too shallow to establish this kind of natural system. Also the environmental conditions are often varying too much. The containers might dry out at some time and be over watered at other times.

It is similar to keeping a very small aquarium healthy. The smaller it gets, the more delicate and instable it becomes. Thus to keep it simple, balcony and container gardeners are advised to exchange the pot soil on a regular basis.

Whan you can do to avoid wasting the "spent" soil, is simply to rejuvenate it. Mix it together with a large amount of fresh and healthy compost and other components to give it a boost. No need to throw it away.

If you want to make sure that pests do not survive and accumulate in this process, simply collect the spent container soil into a large compartment and heat it up to 90°C in the oven for an hour or so, sterilizing it in the process.

The other alternative would be to use hydroponics and specifically Kratky Hydroponics on the balcony and window shelf. I'm growing some beet root plants this way atm. Very easy and low maintenance.

As has been said, you can't really go wrong with adding organic matter to your soil. Probably that means lots of manure if you can get it and then mulch.

A really simple way to see how good or bad your soil is starting out is to just dig up a couple of lumps of earth and see if you find earthworms. If you do, then you're off to a good start!

The simple answer is to "feed your soil". Basically give back the nutrients lost, and modify the soil to promote growth of soil bacteria.

You can look up what plants you had in there before and what nutrients those usually consume the most of. Then you can look up another plant that specifically generates those nutrients, or you can add them via a fertilizer. Companion plants help get rid of pests that attack certain plants and also increase production of needed nutrients.

Before planting, you can aerate the soil (basically mix it up) while simultaneously adding organic material (grass clippings, dried leaves, sticks, fallen trees), then water it and tend it like the soil is a plant itself. You can also introduce beneficial organisms like earthworms and beneficial nematodes.

"Modern" farms don't replace their soil because they're basically growing monocultures and using a sort of "nutrient flamethrower" to completely drench the soil with what that one plant needs, and pesticides to kill anything that a genetically modified plant can't survive. It's unsustainable and harmful to the ecosystem in many cases, but it results in big yields.

There are some fairly simple steps that get overcomplicated due to companies selling products.

1) Figure out what you want to grow

2) Look up what kind of soil pH and what nutrients those plants need

3) Test your soil for pH and those nutrients, where possible

4) Amend the soil with fertilizer to correct measured imbalances

A lot of it is trial and error, also. Take note of what you do and the outcomes and keep doing what works. There are so many local variables generalized advice can never hope to cover them all.

The short answer is experience. Buy some good quality soil and use it in your planters. Learn how the plants grow and thrive - if they slow growth or do not bloom as much it may be the weather or it may be because it's lacking nutrients. You can also simply give them some fertilizer. If it doesn't make much of a difference it was probably not needed.

If possible you should renew your soil, not be replacing it, but by mixing it with good manure or even just a bunch of leaves. I have experience with manure from rabbits and horses - both can be directly mixed with the earth.

>how do you find out whether your soil quality is still good enough?

The plants will tell you. But the answer always, always is: how good is your compost?

Until you get composting right, gardening can be very frustrating. But it all starts with a proper compost. You can fix almost any soil issue with composting techniques.

The curious thing about that, though, is that your compost, if its a house compost (based on output from your household) is only going to be as healthy as you, yourself eat. So that is a factor as well ..

Search ebay/etc for "EC/TDS meter", it's ~10 USD. Cheap ones do work, no worry. Keep your drain water at a stable EC, somewhere between 500 and 2500 uS/cm, and you're golden. If the drain water's EC is low, use irrigation water somewhat richer (+500..1000 uS/cm). Some tap water is already high on minerals, in that case use mainly nitrogen-rich solution.

Btw if your plants look healthy, no action is required.

You can buy simple soil test kits that tell you about your soil's pH, nitrogen, phosphorus, and potash levels. Then you know if/what to replenish. A university or lab near you may also offer a service where you send them a soil sample and you get a detailed report. Probably overkill for a balcony though.

This depends on the species that you are trying to culture. Some plants are avid eaters, other not so much. Orchards solve it using the concept of crop rotation