Hello, I have bought a PH sensor for measuring soil PH on a plant pot. However as searching in the internet it can be used only for liquids... So, can I take useful information If I try to measure the PH of the water with which I irrigate the plant pot? What I mean is, does the water's PH change? Wil I see something varying? For my experiments I use mint plant pot. For the construction Arduino MEGA 2560 R3 and this type of PH sensor.. https://www.ebay.com/itm/263478680179which seems very nice sensor!
Measuring the water is not enough.
The soil itself has a certain pH value. When the soil or roots are too wet, they can start to rot, that causes bacteria to lower the pH value. Water can contain chemicals such a Chlorine and Fluoride (I'm happy that is not the case in my country). Perhaps you apply fertilizer that influences it. It is a magical bio-world in your plant pot 
You could take a sample from the bottom of the pot, make that wet and then use the pH sensor.
You might have trouble with just a cheap amplifier module. You can try a cheap module with a small plant pot, but a galvanic isolation solves a few problems.
https://www.tindie.com/products/rezahussain/dormant-labs-ph-module-v2/
Tip: You only need this part of the URL: https://www.ebay.com/itm/263478680179
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Which type of fertilizer should I use?
Have you looked at how soil pH is typically determined in a laboratory or field setting? I suspect you have not, but should have. The first problems is that you need to define a standard soil sample size. The sample must then be mixed with water and stirred for a sufficient time for ion equilibration. Then you measure the pH. Depending on the type of pH electrode, certain ions may or may not interfere with the measurements. You could manually dig out soil samples and check the pH, but that has problems on a continuous basis, because you would be continuously disturbing the soil layers. on a fairly large scale.
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It is ok for me just to change somehow the pH of the water with I irrigate the mint in order to get measurements....
It is ok for me just to change somehow the pH of the water with I irrigate the mint in order to get measurements....
If I understand this comment, you are looking at the cart before the horse. You can measure the pH of the feed water all you want, but unless you are saturating the soil continuously with a defined buffer solution, the general pH of the soil will not be the same as the feed water. Look up the concept of "pH buffering" to understand how you can force a pH of a solution to stay within a narrow specified limit. However, adding a buffer to the soil via feed water will invariably add different types of salts to the soil, which likely will not be a good idea.
Periodic manual pH soil sampling is about the best you can do. You then can carefully add lime or sulfur, or other agents to modify the soil acidity.
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I add them in the water before irrigation?
I'm not at all sure whether that sensor is appropriate for continuous monitoring. pH sensors are rather tricky.
Not sure why you would be happy at all! 
Chlorination is important to ensure that the water is free of dangerous bacteria or protozoa such as Giardia Lamblia.
Fluoridation is important and incontestably proven to improve community dental health of children and adults - which is not just a matter of comfort, but actually related to such medical conditions as heart disease. Not everybody reliably uses fluoride toothpaste and I was told by my dentist that I had developed decay after moving to an area whose water supply was not fluoridated (subsequently corrected) due to my habit of vigorously flushing my mouth after brushing since the benefit of fluoride in toothpaste relies on actually swallowing a proportion. 
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I really think you need to step back and do a search and a lot of reading on soil pH, measuring soil pH, and modifying soil pH. Your questions suggest that you have put in, at most, superficial effort in really understanding the physical and chemical issues involved in what you want to do. The problem may seem simple to you, but I assure you that it is not. A number of replies have already suggested very broad and fundamental difficulties for measuring soil pH easily.
Once you research the problem, I think you will understand the real nature of what you are hoping to do, and you will have to decide if the project is too complex to automate without a long, dedicated effort electronically, mathematically, and mechanically. There are indeed ways to monitor the pH of soil in an automated fashion, but they are not as straightforward as sticking a pH probe into the soil or the feed water.
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How to measure soil pH (scroll down to the section on using pH test strips, but use your pH meter instead):
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After many days searching I resulted in measuring the pH in water. If I let the device inside the water, and gather measurements every 1 hour without removing the sensor from the water, will I face any problems? So, for 2 weeks the pH sensor will be inside the water...
Assuming you have somehow bypassed the issue of directly measuring the water in soil, your next problem is calibration of the pH electrode. For accurate results, a two point calibration is always performed, usually bracketing the expected pH range. For semi-quantitative results, you can get away with a single calibration centered near the expected pH values.
pH values with every type of pH electrode drift. The electrode requires periodic re-calibration. Just how bad the drift is can only be determined experimentally. Two weeks is an extremely long time to not expect drift. pH electrodes are usually calibrated with standardized buffers. You can buy buffer calibration kits that bracket specific pH ranges (for instance, Amazon). It may be possible to buy the buffer components like citric acid and sodium citrate. or sodium hydrogen phosphate and phosphoric acid to mix your own buffers according to standard preparation formulas.
Despite what you may find on some internet sites. using water as a calibration "standard" is a very bad idea. If you are not careful, you could be one pH unit or more off
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If you read all the text in the link you gave in the first post, there is NO mention of continuous PH readings. Especially when they specify correct readings for up to 1 minute of time. So do not leave the probe in any water for more than a minute.
Paul
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Sounds like a job for a servo!
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