Question: When should I fertilize my plants, and what should I use?
Answer: Have your soil tested every couple of years. Your county Extension office has the necessary sampling materials and instructions. The soil test will show which nutrients are deficient. Don't be surprised if nitrogen is the main concern. It's not uncommon, especially in clay soils, that an all-nitrogen fertilizer will be recommended for almost all of your plants. Ideally it will have at least half of its nitrogen in slow-release form.