I would check the water right before it is used next watering but let the soil dry a bit more then usual then water so you can catch the drainage water out the bottom then check the ph of that water and see if it dips or rises or stays the same. You want the run off to be around 6.8 to 7 max. If it dips down below say 6.5 or above 7 then you can get nutrient lock out which is kinda what this looks like.If its low then water with higher ph water say 8 once or twice then back to 7 ph and keep testing runoff if its coming out high then you drop the starting ph to 6 ish for bit.Flush good with either to raise or lower to that 6.8 ish sweet spot.