Gardening Failure

I failed miserably at my gardening. ='(

I went out and bought some fertilizer in the powder form. It looked like a white-ish/grey-is kind of loose dirt. Very bad description,i know, but that's all i can remember about it.

My plants were growing ok. They were growing slow and sort of looked like they were missing something coz they started looking miserable, so i decided that maybe they just needed a bit of fertilizer. My brother told me that he had a liquid fertilizer that would be good to use, but coz I'm impatient i decided to just go out and buy whatever i passed in the shop. Woopsy!
It wasn't the fertilizer that did the damage, it was the fact that i had no idea what i was doing and didn't really follow the instructions properly.

So...... what i did that i shall NEVER do again is i loosened the dirt in the pots so i could get my plants out as gently as possible and then i mixed the fertilizer in the dirt and then placed the plants back into the dirt. no.. noo...NO NO NO!!! Apparently that was a very very bad idea. All of the plants that i did that to died very very quickly. It was like they were burnt from the root up. Very sad moment for my garden.

The only plants that survived were 4 pots.
1 pot was rocket and spinach and the other 3 pots were my strawberries. Oh how glad i am that i didn't kill my strawberries!
The only thing i did differently with these 4 pots was i dug a hole away from where the actual root would have been and i mixed the fertilizer in just that area, thinking that maybe when I water them the fertilizer will spread slowly through the rest of the pot and less likely to damage anything. Thankfully i was right.

The reason why i did that with these 4 pots and not the rest was because they were the last pots that i did and i sort of didn't think about it till then. The other pots were smaller and i wasn't really putting that much in so i thought it would be ok. But lesson learnt the hard way.

Now i have to start again.
Another thing, I have heard that those black plastic pots are not good to use because they get too hot for the plants roots or something. So that's interesting. Must find out what pots are better to use. Probably terracotta pots are better to use but the big ones are so expensive sometimes. But if they are better to use them maybe its worth it.