By Teo Spengler Fertilizing rhododendron bushes isn’t necessary if the shrubs are planted in fertile soil. If garden soil is poor, or you use certain types of mulch that deplete nitrogen in the soil, feeding rhododendrons is one way to provide the plants with nutrients. Read on to learn how to fertilize rhododendrons. When to Feed a Rhododendron If your soil is fertile and your plants look happy, there’s no urgency to learn about feeding rhododendrons. No fertilizer is always better than too much fertilizer so you might do best leaving healthy plants alone. Be wary of nitrogen deficiencies, however, if you mulch with fresh sawdust or wood chips. As these materials disintegrate into the soil, they use up available nitrogen. If you see your rhododendron growth slowing and the leaves turning yellow, you’ll need to start fertilizing rhododendron bushes with a nitrogen fertilizer. Take care when applying nitrogen fertilizer.