You should pay them market rates to replace them (ie, much higher than you currently are). Yes, you should also foster their need for growth and learning, but t

You should pay them market rates to replace them (ie, much higher than you currently are). Yes, you should also foster their need for growth and learning, but that's because it's an investment in their productivity. Investing in engineer's productivity, and paying them what they're worth, turns out that's all it takes.