The oriental lily is the largest group of lillies when it comes to commercial propagation in Holland at the moment. This type of lilies is incredibly popular with flower growers over the entire globe. Rightfully so, they are the most colourful and most graceful lilies of all. Unlike most other lilies they prefer to grow in slightly acidic soil.
When the acidity in the soil is not good, the plant is not able to absorb enough nutrition to grow well. Once you know your soil is not acidic enough for oriental lilies, you can solve the problem by adding a good amount of peat in the planting hole and you will see the lilies perform much better. Please do make sure to use peat and not potting soil. Potting soil is rather neutral and does not raise the acidity.