Lowering soil pH by using compost is a safe effective way to manage many of the problems found with alkaline and desert soil. When I received this question from Liz in Arizona, I decided it was time to do a special page on how compost can lower soil pH. Here is Liz's question:
Compost is a great answer for lowering soil pH. It doesn't exactly lower soil pH, what it does is buffer the pH making the soil function as if it had a lower pH. Compost can effectively move your pH down a point without having any negative effect on the living soil community.
It's interesting that you are growing a hydrangea in one of your pots. Hydrangeas are a natural pH indicator. If you have blue hydrangeas you have acidic soil. If they are pink you have alkaline soil. Here are a couple of pictures showing a pink and a blue hydrangea.
In the this photo of the pink hydrangea if you look closely at the leaves you can see they are somewhat yellow with dark green veins. Yellow leaves with dark green veins are showing classic symptoms of iron deficiency or iron chlorosis. This is common with plants growing in alkaline soil as at high pH values iron tends to become insoluble and therefore not available to plants. We know the soil pH is alkaline as the flower is pink.
In this photo the plant has dark green leaves. The veins are even a little lighter in color. This plant is able to access enough iron as soil conditions are somewhat acidic. We know this because this hydrangea flower is blue.
Yellow leaves with dark green veins is an indicator of an iron deficiency or iron chlorosis. Given that you have alkaline soil, your yellow leaves are likely caused by the soil pH rather than an actual deficiency of iron. In alkaline conditions the iron becomes insoluble so the plants can't access it.
Adding iron won't necessarily help, but if you want to try that make sure it is a chelated iron amendment. The chelated iron fertilizer has less chance of binding with other elements such as calcium and becoming unavailable to your plants.
For a total biological approach to lowering soil pH follow these two steps.
The Ideal Soil Conditioner for Lowering Soil pH - Compost
Liz mentioned in her question that she uses soil conditioner every time she plants. It's important to use the right soil conditioner but you can not go wrong if you use compost. Here's why
Check the pH of Your Irrigation Water
Liz mentioned that she was feeding her plants a half strength acid fertilizer. What I think might be happening is that the water is so alkaline the fertilizer is neutralized or possibly even made alkaline.
On the farm I used to have our river water, which was our irrigation supply, had a pH of close to 8. Check the pH of your water - it may be making that acid fertilizer alkaline.
Bottom Line for Lowering Soil pH
Use lots of compost as your soil conditioner. Look to soil biology, such as mycorrhizal fungi for the biggest part of your answer for both your regular soil and your containers.