Soil pH management outdoor

Your plants are not looking bad IDK if this is a full on 911. There could be errors in PH measurement or it could just be low in that one spot. Soil PH swings a bit, if you want to raise it long term you have to amend things into it that when the bacteria break it down it becomes more alkaline vs acidic. eg. to lower PH mixing in sulphur compounds into the soil helps as they turn it into sulphuric acid. to raise PH you wanted to be mixing in things with higher amounts of calcium like dolomite lime. These are not the only options but some of the main one. Many garden centres sell sulphur and lime for altering soil PH. The rain water is naturally a bit more acidic on the regular. Wood Ash is a good way to add in K and some micros as well as raise PH turning it more alkaline. The rate of use can be googled but start small as its harder to take it away vs add more in. Again. I don't see a massive problem with this grow based on your single pic., If you notice some issues later one it could be something you look into. their is also the big issues of how you are getting readings of PH in the soil. many methods of doing this, I'm sure some are more accurate then others but soil manages its own PH quite well. Wood Ash is the cheapest most abundant option, can get it at any fire pit. granted you don't want them burning garbage just wood and not pressure treated/finished wood as that shit is toxic and can add in excess arsenic to the soil. Good Luck!