What fertilizer and mulch are best for already planted shrub roses to make the soil less alkaline? Soil test showed high alkaline content. Thank you.
Generally organic fertilizers don't have a strong effect on pH, except wood ash (very alkaline) and bone meal (alkaline). Mulches don't have much effect either, except oak leaf compost (acid).
There's no reason to avoid sulfur as a soil acidifier. It is a safe, natural material that acts gently over months and years. Get a recommendation of amount from the county agent based on your soil type and pH, or in the mean time one cup per square yard won't hurt.
Someone told me to use dried pine needles to increase soil acidity. I'm trying it around some azaleas, but haven't done it long enough to know if it works or not.
There is nothing you can put in your soil to "increase the acidity" although there are several things you can do to change the soils pH. Temporary "fixes" are sulfur or ammonium sulphate. A somewhat more permanent fix is lots of woody organic matter (peat moss if nothing else is available) that encourages a more fungaly active soil than bacterial.
It is most bestest to add lots of woody organic matter to soils you want to have a lower soil pH than one of the "quick fixes".
Among which of the following increases acidity in soils
superphosphate of lime