BTW, I am assuming you are talking about growing in what most people consider soil, but some might be using soil-less media and not even realize the difference. For the most part, feeding and watering the plant are the same and so I’m not really going to go into the exact differences here at this time. You might be able to find my other many comments on the subject spread around the forum on some ideas and descriptions about the differences. And even though I started in soil like almost everyone does, I have more hands on experience with a very specific type of hydro DWC, or deep water culture. And so, often, I like to let @latewood address many of the more varied soil growing techniques and additives and such, as he has nearly a lifetime of hands on experience in the horticultural industry. As I tagged him to this convo, I’m sure he can add anything he feels might be left out or could be expanded upon.
There are many different techniques to feeding and watering in soil or soil-less media. The end result is basically the same. Some people do not really measure what is building up in the soil and use a technique to relatively reliably prevent anything from getting too far from ideal.
I’m going to describe these two versions of probably the most common technique, and most people are already using one of these and don’t even realize that this is often what it is called. And often a minor variation of these that prevents over feeding – like using water only for certain intervals between feedings – are why you can get away with growing in soil and not necessarily checking the pH or TDS.
This is commonly called Drain To Waste, and it is also a sort of manual ‘Flood and Drain’ or even ‘Eb and Flow’, terms you might hear often in cannabis growing, especially in hydroponics. I should mention there is finer point, at least when it comes to hydroponics, with “Drain To Waste” as compared to “Flood and drain also known as Eb and Flow”, and in hydroponics, the “drain” could return to a aerated reservoir and recirculated through the growing media in an “eb and flow” or “flood and drain” manner.
Remember, the end result achieved is basically the same. This desired result is a stable pH and a stable mixture of balanced and properly concentrated dosage of nutrients (balanced and proper dosage for the period of growth the plant is in, i.e. veg vs. flower), and that this balance is available to the roots, regardless of whether it is in soil or hydro. Also, a point to note, even though soil-less media is used generally the same as soil, it is technically a form of hydroponics.
In general, Drain To Waste, is the same way you water a house plant. You pour in the top of the potted container until some comes out the holes in the bottom of the container, and you throw away what settles in the saucer at the bottom.
In both methods, the plant is only watered, or fed, when the soil in the pot is nearly totally dried out as determined by comparing the weight of the container with soil saturated with water, and with totally dry soil, and then only watering again to saturation when the container feels very very light.
Now as far as the two different ways, one would involve closely monitoring what is building in the soil by continuously monitoring the EC/TDS/PPM and pH of the liquid in the saucer, also known as the “run-off”.
Ideal pH targets for soil are 6.5, and for hydro it is 5.8 and as these are targets, maybe as much as 0.5 above or below is not necessarily anything to worry about as long as they aren’t at the extremes for very long. Also, no need to micro manage just because it is off by only a small point, no need to adjust the pH until it is a lot closer to the extreme of 0.5 above or below ideal.
As far as EC/TDS/PPM numbers are concerned, recommendations can vary greatly, depending on style of grow, brand of nutrients and maybe even strain and can be as low as about EC 0.6 which equals 300 PPM in the USA standard conversion, to as high as maybe even EC 3.1 or nearly 1600 PPM. These numbers would be pretty much the same for soil or the pure water and nutrient mixture in hydro.
I personally like about an EC of 1.0 or 500 PPMs minimum with a healthy adult plant, and not much higher than about EC 1.5 or 750 PPM for hydro, at least in my DWC undercurrent system, maybe about EC 1.8 or 900 PPM max. For soil I might go a bit higher, but not by much. And certainly not higher than EC 2.5 or about 1250 PPM and it would need a good flush at the end, IMO, and this high would highly depend on the type of nutrients and the NPK values.
When monitored In this way, additional nutrients would only be added when the soil run-off drops below a certain level, say you are trying to maintain an average of about 900 PPMs in the soil, and only add nutrients when the run-off drops below 800, and so plain proper pH’ed water would be used in between until the run off was significantly lower than the target. In this case, you only water until enough run-off liquid is in the saucer to do your tests.
The other way DTW is done, is by mostly only feeding and watering at the same time, when the plant absolutely needs to be watered, as determined by weight as described above. But in this case, an amount of water and nutrients is mixed to the pre-determined EC/TDS/PPM, this amount of water and nutrient mix is maybe about one and a half times the volume of soil so that you pour the entire contents through the soil and should get at least 30% to maybe as much as 50% run-off, This will usually makes sure you never have too many nutrient salts staying and building up in your soil as the excessive run-through washes out as well as replaces nutrients. Some monitoring of the run-off would still be recommended to make sure nothing is getting too far off track in the soil as determined by measuring the run-off.
Usually too many alkali minerals like limescale calcium in hard water – possibly building up – that tends to not only throw off and raise your TDS, but also raises the pH. Too much biological decomposition in the soil can also cause a downward pH more often towards the acidic.
Of course in all methods, you want your EC/TDS/PPM to drop to near zero the last couple of weeks of flower.
Hope this helps,