Changing soil pH
Soil pH is generally corrected using iron sulphate (decrease) or lime (increase). There are some organic soil improvers on the market which are claimed to stabilise the pH on neutral. Make sure the soil additive is well worked and watered into the soil.
To increase the pH by one point, apply dolomite lime at 100g/m2 in sandy soils, and up to 250g/m2 in clay. The effect is noticeable after two to three months.
To decrease the pH, iron sulphate is available from nurseries and hardware stores. Follow label instructions for the correct rate. Compared to lime, the effect is immediate.