Purchaes plants and planted with organic potting soil in a container
There are lots of possible reasons for this. Here's an article that explains:
https://www.gardeningknowhow.com/edible/herbs/parsley/why-does-parsley-turn-yellow.htm
Or is 12% better? 35% is too hard to come by and you seem to have to dilute it by 1 part to 10 anyway, so doesn't that just dilute it down to 3.5% anyway? I am in Austin, TX and my parsley plants, random locations all over the yard, so it's not a "soil/fungus thing", are beautiful one day, then the very next day wilt, die, have squishy roots. I suspect the rust fly, but don't think they are IN central Texas?
I would use the 1 tablespoon per cup of water of 3% solution.
https://www.gardeningknowhow.com/edible/herbs/parsley/diseases-of-parsley.htm
I’m not certain as to why it does that.
Your parsley may have been overwatered. Roots rot when there isn't good soil drainage, forcing them to sit in wet conditions. These two articles will help:
https://www.gardeningknowhow.com/edible/herbs/parsley/why-does-parsley-turn-yellow.htm
https://www.gardeningknowhow.com/edible/herbs/parsley/parsley-plant-is-wilting.htm
Initially the plant grows well with green colour but after sometime, the leaves changes colour.. the roots and stem seem to look healthy, however, the leaves only changes colour..
This article should help:
https://www.gardeningknowhow.com/edible/herbs/parsley/why-does-parsley-turn-yellow.htm
My parsely has developed thick stalks. Is this normal? Should I trim them down? What causes this?
It is bolting! Whether this was caused by environmental conditions or its age, It is all downhill from here, unfortunately.
Starting over will be the best solution, with staggered planting.
This article will help, further:
https://www.gardeningknowhow.com/edible/herbs/parsley/bolting-parsley-plants.htm
leaves but then it stopped growing... Any idea what the problem might be. Thanks for your help I live in algeria north Africa weather is 20 degrees Celsius now and dry . thank you
Did you root the parsley first in water before planting it? It may be focusing on growing roots and not leaves. Give it some more time and keep it watered.
https://www.gardeningknowhow.com/edible/herbs/parsley/propagate-parsley.htm
https://www.gardeningknowhow.com/edible/herbs/parsley/diseases-of-parsley.htm
https://www.gardeningknowhow.com/edible/herbs/hgen/rooting-grocery-store-herbs.htm
https://www.gardeningknowhow.com/edible/herbs/parsley/tips-on-how-to-grow-parsley.htm