Look up "avocado salt burn." They are sensitive to salts in synthetic fertilizers and need to be watered thoroughly to flush out any excess.

When you water, do you let water drain through the bottom holes?

Yes, any remaining water is drained through the bottom. How can i know if theres too much salts right now? I put one spoon of thefertilizer in 500ml of water about once a week

Which fertilizer are you using? Teaspoon or tablespoon? One teaspoon per 500ml is way too much for most synthetic fertilizer and every week with strength will burn any plant. Organic fertilziers are weaker but even then it seems a bit too much at that frequency. To correct it flush the soil with copious amounts of fresh water several times like at least 3-4 times. Let it drain out completely and leave it alone for till soil dries out quite a bit. Water again as needed and do not add any fertilizer for another 2 weeks . After that add fertilizer at maintenance strength with each watering. Read the label on the fertilizer container carefully to find out what is maintenance strength. Usually it is about 1/4 of full strength. When you start seeing lots of growth you can start bumping up the concentration of your fertilizer.

I ment a teaspoon,

There isnt any specific instructions on the label of the bottle, so i think the use of it is part of the problem now after youve explained it.. what will you define as "full strength"?

And thank you for helping out

If there's no room on the label for instructions, you can usually find them online on the product's website. I second the recommendation to follow the instructions carefully. These ferts can be highly concentrated.

If you tell me which fertilizer it is I can tell you the dosage.

For example full strength of Foliage Pro 9-3-6 fertilizer is 1 tsp/gallon (gallon = 4 liters approx). Miracle Grow 24-8-16 is about 1/2 tsp per gallon. These are full strength values. The important part is those 3 numbers for N-P-K in your fertilizer. N= Nitrogen, P = Phosphorus, K = Potassium. For a good fertilizer those three numbers should be in 3:1:2 ratio. In my examples both those fertilizer has the same 3:1:2 ratio. But the Miracle Gro is more than twice as strong as the Foliage Pro (just compare the value for N which is 24 for MG and 9 for FP). Probably all this is little too detailed for you but I think it is better if you become comfortable with these things and you will have lot more success.

Now in your case you had been adding 1 tsp per 500ml and that is equal to 8tsp per gallon. Way too much for any fertilizer I can think of. Those excess salts in soil starts killing the roots and also show up as leaf burns.

Got it.

The label says 5ml /1 liter, rougly a tsp aint it?

Seems like i did use too much..

Should i cut down the burned leaves?

No need to cut the burned leaves. They still have green in them and helping the plant through photo-synthesis. Some of the lower ones will likely drop off on their own.

If the label says 5ml/liter then you put in 2x the amount which is not big a deal. In this the combination of strength and frequency affected the plant. Can you post the make/brand or a pic of your fertilizer or may be a link to it on the web? I am curious as to what kind of fertilizer it is. Seems like it may be an organic fertilizer.

Also at this stage of your avocado it still has a good portion of the seed left. Means it can get enough food out of it. So go real easy on the fertilizer. Let the seed get consumed.

I highly recommend that you repot into a fast-draining mix, probably in May (if your zone allows the avocados to be outdoors at that time). With a porous mix, there's much less chance of fertilizer build-up and subsequent root burn, and much less chance of root rot.

Josh

Will do it, can you give me an example for porous mix? Some product i can use as reference.. i guess here in israel the terminology at the garden stores is a bit different

Etain,

if you can find a "fine-grade Orchid Mix" that is primarily bark, that will make an excellent mix. I make a mix of fir bark, perlite, potting soil, and other grit like lava rock (Scoria). I realize that in Israel your products might be more dififcult to find. Here's an example of a bark-based mix - free draining, durable.

etain: You may also want to look at this thread and get and idea what is a porous well draining soil mix: CONTAINER SOILS - WATER MOVEMENT and RETENTION XXII

After that you may have a better handle how to source the components and how to adapt to what is available in your country. If you find a source of bark similar to what greenman/Josh posted you will have no problem making your own porous soil mix.